call_usermodehelper in containers

[Jeff Layton]

We have a bit of a problem wrt to upcalls that use call_usermodehelper
with containers and I'd like to bring this to some sort of resolution...

A particularly problematic case (though there are others) is the
nfsdcltrack upcall. It basically uses call_usermodehelper to run a
program in userland to track some information on stable storage for
nfsd.

One could envision nfsd running on a machine with several containers,
and each would need to track its own database of info on stable
storage. When processing RPCs that come into the network address within
a certain container, we want to ensure that it tracks this info inside
the mount namespace within that container as well.

So, we ideally need to ensure that when this upcall is run, that we run
the correct binary in the container *and* that it does all of its file
I/O within the correct mount namespace. We might also need other
namespace swapping as well.

Stanislav posted a patch a few months ago that tried to address this:

https://lkml.org/lkml/2013/5/22/80

However, it failed to address some problems -- namely that we have to
consider the case where a container may be running with reduced
capabilities. We want to ensure that the UMH upcall inherits the
correct capability set for its container as well.

What's the correct approach to fix this? One possibility would be to
keep a kernel thread around that sits in the correct namespace(s) and
has the right privileges, and then use that to launch UMH programs.
That thread could be spawned whenever someone runs rpc.nfsd inside a
container.

Not very elegant, but it seems like something that would work.

Are there better approaches?

--
Jeff Layton <jla...@redhat.com>
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[Vasily Kulikov]

Hi Jeff,

On Mon, Nov 11, 2013 at 07:18 -0500, Jeff Layton wrote:
> What's the correct approach to fix this? One possibility would be to
> keep a kernel thread around that sits in the correct namespace(s) and
> has the right privileges, and then use that to launch UMH programs.
> That thread could be spawned whenever someone runs rpc.nfsd inside a
> container.
>
> Not very elegant, but it seems like something that would work.
>
> Are there better approaches?

What's the reasoning behind this? I mean, it is not very obvious what
we should keep here. Compare 2 cases:

1) root process with all caps spawns new ns, then drops some of caps;

2) root process with all caps drops some of his caps and then spawns new ns.

From the programmer's POV both cases are valid and lead to absolutely
the same limitations inside of the new namespace. However, from kernel
POV they differ -- if save cap set when ns is created then in (1) we'll
have cap'ed UMH, in (2) we'll have UMH with only several caps. It might
significantly influence on ability of UMH to do its job and ability of
this limited ns to escape from the sandbox.

So, what semantic should UMH privileges have?


Also, an orthogonal addition: you might want to keep only minimum
information about capabilities or something -- keep only cap_t field in
namespace structure without explicit kernel thread for each ns. When UMH is
created, just fill the required caps in it.

Thanks,

--
Vasily Kulikov
http://www.openwall.com - bringing security into open computing environments

[Jeff Layton]

I don't have a particular affinity for either approach.

I think from a safety POV, it's less error prone to spawn the UMH
programs as a descendent of the userland process that originally
started up nfsd. That way we ensure that it inherits all of the
limitations of the original task.

If we try instead to start with a process that is running with all
capabilities and then drop some of them selectively, then it seems
quite possible to miss something.

But...namespaces aren't really my "thing", and I might have the
terminology wrong or be missing some other crux of the issue.

--
Jeff Layton <jla...@redhat.com>

[Greg KH]

On Mon, Nov 11, 2013 at 07:18:25AM -0500, Jeff Layton wrote:
> We have a bit of a problem wrt to upcalls that use call_usermodehelper
> with containers and I'd like to bring this to some sort of resolution...
>
> A particularly problematic case (though there are others) is the
> nfsdcltrack upcall. It basically uses call_usermodehelper to run a
> program in userland to track some information on stable storage for
> nfsd.

I thought the discussion at the kernel summit about this issue was:
- don't do this.
- don't do it.
- if you really need to do this, fix nfsd

thanks,

greg k-h

[Jeff Layton]

On Mon, 11 Nov 2013 16:47:03 -0800
Greg KH <gre...@linuxfoundation.org> wrote:

> On Mon, Nov 11, 2013 at 07:18:25AM -0500, Jeff Layton wrote:
> > We have a bit of a problem wrt to upcalls that use call_usermodehelper
> > with containers and I'd like to bring this to some sort of resolution...
> >
> > A particularly problematic case (though there are others) is the
> > nfsdcltrack upcall. It basically uses call_usermodehelper to run a
> > program in userland to track some information on stable storage for
> > nfsd.
>
> I thought the discussion at the kernel summit about this issue was:
> - don't do this.
> - don't do it.
> - if you really need to do this, fix nfsd
>

Sorry, I couldn't make the kernel summit so I missed that discussion. I
guess LWN didn't cover it?

In any case, I guess then that we'll either have to come up with some
way to fix nfsd here, or simply ensure that nfsd can never be started
unless root in the container has a full set of a full set of
capabilities.

One sort of Rube Goldberg possibility to fix nfsd is:

- when we start nfsd in a container, fork off an extra kernel thread
that just sits idle. That thread would need to be a descendant of the
userland process that started nfsd, so we'd need to create it with
kernel_thread().

- Have the kernel just start up the UMH program in the init_ns mount
namespace as it currently does, but also pass the pid of the idle
kernel thread to the UMH upcall.

- The program will then use /proc/<pid>/root and /proc/<pid>/ns/* to set
itself up for doing things properly.

Note that with this mechanism we can't actually run a different binary
per container, but that's probably fine for most purposes.

--
Jeff Layton <jla...@redhat.com>

[Stanislav Kinsbursky]

12.11.2013 15:12, Jeff Layton пишет:

Hmmm... Why we can't? We can go a bit further with userspace idea.

We use UMH some very limited number of user programs. For 2, actually:
1) /sbin/nfs_cache_getent
2) /sbin/nfsdcltrack

If we convert them into proxies, which use /proc/<pid>/root and /proc/<pid>/ns/*, this will allow us to lookup the right binary.
The only limitation here is presence of this "proxy" binaries on "host".

And we don't need any significant changes in kernel.

BTW, Jeff, could you remind me, please, why exactly we need to use UMH to run the binary?
What are this capabilities, which force us to do so?

--
Best regards,
Stanislav Kinsbursky

[Jeff Layton]

No, the kernel uses them for a lot more than that. Pretty much all of
the keys API upcalls use it. See all of the callers of
call_usermodehelper. All of them are running user binaries out of the
kernel, and almost all of them are certainly broken wrt containers.

> If we convert them into proxies, which use /proc/<pid>/root and /proc/<pid>/ns/*, this will allow us to lookup the right binary.
> The only limitation here is presence of this "proxy" binaries on "host".
>

Suppose I spawn my own container as a user, using all of this spiffy
new user namespace stuff. Then I make the kernel use
call_usermodehelper to call the upcall in the init_ns, and then trick
it into running my new "escape_from_namespace" program with "real" root
privileges.

I don't think we can reasonably assume that having the kernel exec an
arbitrary binary inside of a container is safe. Doing so inside of the
init_ns is marginally more safe, but only marginally so...

> And we don't need any significant changes in kernel.
>
> BTW, Jeff, could you remind me, please, why exactly we need to use UMH to run the binary?
> What are this capabilities, which force us to do so?
>

Nothing _forces_ us to do so, but upcalls are very difficult to handle,
and UMH has a lot of advantages over a long-running daemon launched by
userland.

Originally, I created the nfsdcltrack upcall as a running daemon called
nfsdcld, and the kernel used rpc_pipefs to communicate with it.

Everyone hated it because no one likes to have to run daemons for
infrequently used upcalls. It's a pain for users to ensure that it's
running and it's a pain to handle when it isn't. So, I was encouraged
to turn that instead into a UMH upcall.

But leaving that aside, this problem is a lot larger than just nfsd. We
have a *lot* of UMH upcalls in the kernel, so this problem is more
general than just "fixing" nfsd's.

--
Jeff Layton <jla...@redhat.com>

[Eric W. Biederman]

Broken in the sense that we don't run them in the container yes.

I tried using the keys api for the uid mapping of containers and I wound
up being very disappointed because for testing/debugging I could never
flush any result I had ever returned a key for. Which rather soured me
on the real-world usability of the key based user mode helpers. Perhaps
I was doing it wrong but it seemed like a very brittle interface, that
was intollerant of human failures.

>> If we convert them into proxies, which use /proc/<pid>/root and /proc/<pid>/ns/*, this will allow us to lookup the right binary.
>> The only limitation here is presence of this "proxy" binaries on "host".
>>
>
> Suppose I spawn my own container as a user, using all of this spiffy
> new user namespace stuff. Then I make the kernel use
> call_usermodehelper to call the upcall in the init_ns, and then trick
> it into running my new "escape_from_namespace" program with "real" root
> privileges.
>
> I don't think we can reasonably assume that having the kernel exec an
> arbitrary binary inside of a container is safe. Doing so inside of the
> init_ns is marginally more safe, but only marginally so...

One thing we have done with the core dump helper is because there is
enough information to know the namespaces of the program dumping core
have the root owned and installed helper use setns to get inside the
namespaces so we can have a per namespace core dump policy.

If we can provide enough context to the other helpers that is probably
the easiest way to go.

The question is can we truly pass enough state.

>> And we don't need any significant changes in kernel.
>>
>> BTW, Jeff, could you remind me, please, why exactly we need to use UMH to run the binary?
>> What are this capabilities, which force us to do so?
>>
>
> Nothing _forces_ us to do so, but upcalls are very difficult to handle,
> and UMH has a lot of advantages over a long-running daemon launched by
> userland.
>
> Originally, I created the nfsdcltrack upcall as a running daemon called
> nfsdcld, and the kernel used rpc_pipefs to communicate with it.
>
> Everyone hated it because no one likes to have to run daemons for
> infrequently used upcalls. It's a pain for users to ensure that it's
> running and it's a pain to handle when it isn't. So, I was encouraged
> to turn that instead into a UMH upcall.
>
> But leaving that aside, this problem is a lot larger than just nfsd. We
> have a *lot* of UMH upcalls in the kernel, so this problem is more
> general than just "fixing" nfsd's.

Yes.

So far I don't think we can trigger any of these upcalls from inside of
a user namespace so we aren't in trouble yet. But it is definitely
worth looking at. Because we are basically one person scratching their
itch to get some feature working from being at the point where nfs or
something else that uses the upcalls needs the support.

Eric

[Stanislav Kinsbursky]

12.11.2013 17:30, Jeff Layton пишет:

Ok. So we are talking about generic approach to UMH support in a container (and/or namespace).

Actually, as far as I can see, there are more that one aspect, which is not supported.
One one them is executing of the right binary. Another one is capabilities (and maybe there are more, like user namespaces), but I don't really care about them
for now.
Executing the right binary, actually, is not about namespaces at all. This is about lookup implementation in VFS (do_execve_common).

Would be great to unshare FS for forked UHM kthread and swap it to desired root. This will solve the problem with proper lookup. However, as far as I
understand, this approach is not welcome by the community.

This problem, probably, can be solved by constructing full binary path (i.e. not in a container, but in kernel thread root context) in UMH "init" callack.
However, this will help only is the dentry is accessible from "init" root. Which is usually no true in case on mount namespaces, if I understand them right.

--
Best regards,
Stanislav Kinsbursky

[Eric W. Biederman]

I don't understand that one. Having a preforked thread with the proper
environment that can act like kthreadd in terms of spawning user mode
helpers works and is simple. The only downside I can see is that there
is extra overhead.

Beyond that though for the user mode helpers spawned to populate
security keys it is not clear which context they should be run in,
even if we do have kernel threads.

> This problem, probably, can be solved by constructing full binary path
> (i.e. not in a container, but in kernel thread root context) in UMH
> "init" callack. However, this will help only is the dentry is
> accessible from "init" root. Which is usually no true in case on mount
> namespaces, if I understand them right.

You are correct it can not be assumed that what is visible in one mount
namespace is visible in another. And of course in addition to picking
the correct binary to run you have to set up a proper environment for
that binary to run in. It may be that it's configuration file is only
avaiable at the expected location in the proper mount namespace, even
if the binary is available in all of the mount namespaces.

Eric

[Stanislav Kinsbursky]

15.11.2013 15:03, Eric W. Biederman пишет:

What do you mean by "simple" here? Simple to implement?
We already have a preforked thread, called "UMH", used exactly for this purpose.
And, if I'm not mistaken, we are trying to discuss, how to adapt existent infrastructure for namespaces, don't we?

> Beyond that though for the user mode helpers spawned to populate
> security keys it is not clear which context they should be run in,
> even if we do have kernel threads.
>

Regardless of the context itself, we need a way to pass it to kernel thread and to put kernel thread in this context. Or I'm missing something?

>> This problem, probably, can be solved by constructing full binary path
>> (i.e. not in a container, but in kernel thread root context) in UMH
>> "init" callack. However, this will help only is the dentry is
>> accessible from "init" root. Which is usually no true in case on mount
>> namespaces, if I understand them right.
>
> You are correct it can not be assumed that what is visible in one mount
> namespace is visible in another. And of course in addition to picking
> the correct binary to run you have to set up a proper environment for
> that binary to run in. It may be that it's configuration file is only
> avaiable at the expected location in the proper mount namespace, even
> if the binary is available in all of the mount namespaces.
>

Yes, you are right. So, this solution can help only in case of very specific and simple "environment-less" programs.
So, I believe, that we should modify UMH itself to support our needs. But I don't see, how to make the idea more pleasant for the community.
IOW, when I was talking about UMH in NFS implementation on Ksummit, Linus's answer was something like "fix NFS".
And I can't object it, actually, because for now NFS is the only corner case.

Jeff said, that there are a bunch of UMH calls in kernel, but this is not solid enough to prove UHM changes, since nobody is trying to use them in containers.

So, I doubt, that we can change UMH generically without additional use-cases for 'containerized" UMH.

> Eric

>


--
Best regards,
Stanislav Kinsbursky

[Oleg Nesterov]

On 11/15, Eric W. Biederman wrote:
>
> I don't understand that one. Having a preforked thread with the proper
> environment that can act like kthreadd in terms of spawning user mode
> helpers works and is simple.

Can't we ask ->child_reaper to create the non-daemonized kernel thread
with the "right" ->nsproxy, ->fs, etc?

IOW. Please the the "patch" below. It is obviously incomplete and wrong,
and it can be more clear/clean. And probably we need another API. Just
to explain what I mean.

With this patch call_usermodehelper(..., UMH_IN_MY_NS) should do exec
from the caller's namespace.

Oleg.
---

--- a/include/linux/kmod.h
+++ b/include/linux/kmod.h
@@ -24,6 +24,7 @@
#include <linux/errno.h>
#include <linux/compiler.h>
#include <linux/workqueue.h>
+#include <linux/task_work.h>
#include <linux/sysctl.h>

#define KMOD_PATH_LEN 256
@@ -53,8 +54,14 @@ struct file;
#define UMH_WAIT_PROC 2 /* wait for the process to complete */
#define UMH_KILLABLE 4 /* wait for EXEC/PROC killable */

+// FIXME: IMH_* is not actually a mask
+#define UMH_IN_MY_NS 8
+
struct subprocess_info {
- struct work_struct work;
+ union {
+ struct work_struct work;
+ struct callback_head twork;
+ };
struct completion *complete;
char *path;
char **argv;
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -541,7 +541,6 @@ struct subprocess_info *call_usermodehelper_setup(char *path, char **argv,
if (!sub_info)
goto out;

- INIT_WORK(&sub_info->work, __call_usermodehelper);
sub_info->path = path;
sub_info->argv = argv;
sub_info->envp = envp;
@@ -554,6 +553,24 @@ struct subprocess_info *call_usermodehelper_setup(char *path, char **argv,
}
EXPORT_SYMBOL(call_usermodehelper_setup);

+static int call_call_usermodehelper(void *twork)
+{
+ struct subprocess_info *sub_info =
+ container_of(twork, struct subprocess_info, twork);
+
+ __call_usermodehelper(&sub_info->work);
+ do_exit(0);
+
+}
+
+static void fork_umh_helper(struct callback_head *twork)
+{
+ if (current->flags & PF_EXITING)
+ return; // WRONG, FIXME
+
+ kernel_thread(call_call_usermodehelper, twork, SIGCHLD);
+}
+
/**
* call_usermodehelper_exec - start a usermode application
* @sub_info: information about the subprocessa
@@ -570,6 +587,10 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
{
DECLARE_COMPLETION_ONSTACK(done);
int retval = 0;
+ bool in_my_ns;
+
+ in_my_ns = wait & UMH_IN_MY_NS;
+ wait &= ~UMH_IN_MY_NS;

if (!sub_info->path) {
call_usermodehelper_freeinfo(sub_info);
@@ -594,7 +615,21 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
sub_info->complete = &done;
sub_info->wait = wait;

- queue_work(khelper_wq, &sub_info->work);
+ if (likely(!in_my_ns)) {
+ INIT_WORK(&sub_info->work, __call_usermodehelper);
+ queue_work(khelper_wq, &sub_info->work);
+ } else {
+ // RACY, WRONG, ETC
+ struct task_struct *my_init = task_active_pid_ns(current)->child_reaper;
+
+ init_task_work(&sub_info->twork, fork_umh_helper);
+ task_work_add(my_init, &sub_info->twork, false);
+
+ // until we have task_work_add_interruptibel()
+ do_send_sig_info(SIGCHLD, SEND_SIG_FORCED, my_init, false);
+
+ }
+
if (wait == UMH_NO_WAIT) /* task has freed sub_info */
goto unlock;

[Oleg Nesterov]

On 11/18, Oleg Nesterov wrote:
>
> On 11/15, Eric W. Biederman wrote:
> >
> > I don't understand that one. Having a preforked thread with the proper
> > environment that can act like kthreadd in terms of spawning user mode
> > helpers works and is simple.
>
> Can't we ask ->child_reaper to create the non-daemonized kernel thread
> with the "right" ->nsproxy, ->fs, etc?
>
> IOW. Please the the "patch" below. It is obviously incomplete and wrong,
> and it can be more clear/clean. And probably we need another API. Just
> to explain what I mean.

Or, perhaps UMH_IN_MY_NS should only work if ->child_reaper explicitly
does, say, prctl(PR_SPAWN_UMH_IN_NS_HELPER) which forks the non-daemonized
kernel kthread_worker thread, I dunno.

Oleg.

[Jeff Layton]

On Mon, 18 Nov 2013 19:02:59 +0100
Oleg Nesterov <ol...@redhat.com> wrote:

> On 11/18, Oleg Nesterov wrote:
> >
> > On 11/15, Eric W. Biederman wrote:
> > >
> > > I don't understand that one. Having a preforked thread with the proper
> > > environment that can act like kthreadd in terms of spawning user mode
> > > helpers works and is simple.
> >
> > Can't we ask ->child_reaper to create the non-daemonized kernel thread
> > with the "right" ->nsproxy, ->fs, etc?
> >
> > IOW. Please the the "patch" below. It is obviously incomplete and wrong,
> > and it can be more clear/clean. And probably we need another API. Just
> > to explain what I mean.
>
> Or, perhaps UMH_IN_MY_NS should only work if ->child_reaper explicitly
> does, say, prctl(PR_SPAWN_UMH_IN_NS_HELPER) which forks the non-daemonized
> kernel kthread_worker thread, I dunno.
>
> Oleg.
>

Neat idea.

So is it always the case that tasks in a container have the same
namespace settings and capabilities as the child_reaper?

We'll still have the basic problem for nfsd that we'll need to keep
track of what the child_reaper is when nfsd is started, but I think
that's not too hard to solve.

--
Jeff Layton <jla...@redhat.com>

[Ian Kent]

On Mon, 2013-11-18 at 18:28 +0100, Oleg Nesterov wrote:
> On 11/15, Eric W. Biederman wrote:
> >
> > I don't understand that one. Having a preforked thread with the
> > proper
> > environment that can act like kthreadd in terms of spawning user
> > mode
> > helpers works and is simple.

Forgive me replying to such an old thread but ...

After realizing workqueues can't be used to pre-create threads to run
usermode helpers I've returned to look at this.

>
> Can't we ask ->child_reaper to create the non-daemonized kernel thread
> with the "right" ->nsproxy, ->fs, etc?

Eric, do you think this approach would be sufficient too?

Probably wouldn't be quite right for user namespaces but should provide
what's needed for other cases?

It certainly has the advantage of not having to maintain a plague of
processes waiting around to execute helpers.

> -fsdevel" in

[Ian Kent]

Is there?

Can you explain how the pre-forking happens please?

AFAICS a workqueue is used to run UMH helpers, I can't see any pre
-forking going on there and it doesn't appear to be possible to do
either.

[Stanislav Kinsburskiy]

13.02.2016 00:39, Ian Kent пишет:

Hi Ian,
I'm not sure, I understand your question.
But there is a generic "khelper" thread, which is responsible for
spawning new kthreads to execute some binary, requested by user.
IOW, when you want to use UMH, you add a request to "khelper" workqueue,
which, in turn, creates another thread. The new one call init callback
and does the actual execve call.

[Ian Kent]

AFAICS kernel/kmod.c used to use create_singlethread_workqueue() and
queue_work() to perform umh calls, now it uses only queue_work() and
the system_unbound_wq workqueue.

Looking at the workqueue sub system there doesn't appear to be a way to
create a workqueue with a thread runner thread, created within the
process context at the time of workqueue creation, that then waits to
run work. So there's no way to create a workqueue to run umh calls
within a specific process context, such as that of a container, by using
the workqueue subsystem as it is now.

The problem being that the process context of the caller requesting umh
isn't necessarily (and shouldn't be used because it could allow the
caller to hijack the environment) the process context that needs to be
used for the request.

It looks like the reply to this thread from Oleg that demonstrates using
child_reaper for the run context could be used though. Capturing the
struct pid of child_reaper and then using that to locate the appropriate
task context later (if it still exists) at request time could be used.

That doesn't take care of working out when this should be captured or
where to put it so it can be obtained at request time (which seems
difficult in itself).

[Eric W. Biederman]

Ccing The containers list because a related discussion is happening there
and somehow this thread has never made it there.

Ian Kent <ra...@themaw.net> writes:

> On Mon, 2013-11-18 at 18:28 +0100, Oleg Nesterov wrote:
>> On 11/15, Eric W. Biederman wrote:
>> >
>> > I don't understand that one. Having a preforked thread with the
>> > proper
>> > environment that can act like kthreadd in terms of spawning user
>> > mode
>> > helpers works and is simple.
>
> Forgive me replying to such an old thread but ...
>
> After realizing workqueues can't be used to pre-create threads to run
> usermode helpers I've returned to look at this.

If someone can wind up with a good implementation I will be happy.

>> Can't we ask ->child_reaper to create the non-daemonized kernel thread
>> with the "right" ->nsproxy, ->fs, etc?
>
> Eric, do you think this approach would be sufficient too?
>
> Probably wouldn't be quite right for user namespaces but should provide
> what's needed for other cases?
>
> It certainly has the advantage of not having to maintain a plague of
> processes waiting around to execute helpers.

That certainly sounds attractive. Especially for the case of everyone
who wants to set a core pattern in a container.

I am fuzzy on all of the details right now, but what I do remember is
that in the kernel the user mode helper concepts when they attempted to
scrub a processes environment were quite error prone until we managed to
get kthreadd(pid 2) on the scene which always had a clean environment.

If we are going to tie this kind of thing to the pid namespace I
recommend simplying denying it if you are in a user namespace without
an approrpriate pid namespace. AKA simply not allowing thigns to be setup
if current->pid_ns->user_ns != current->user_ns.

That still leaves things a little hand-wavy but I hope that helps
conceptually.

Eric

[Eric W. Biederman]

Ian Kent <ra...@themaw.net> writes:

> AFAICS kernel/kmod.c used to use create_singlethread_workqueue() and
> queue_work() to perform umh calls, now it uses only queue_work() and
> the system_unbound_wq workqueue.
>
> Looking at the workqueue sub system there doesn't appear to be a way to
> create a workqueue with a thread runner thread, created within the
> process context at the time of workqueue creation, that then waits to
> run work. So there's no way to create a workqueue to run umh calls
> within a specific process context, such as that of a container, by using
> the workqueue subsystem as it is now.
>
> The problem being that the process context of the caller requesting umh
> isn't necessarily (and shouldn't be used because it could allow the
> caller to hijack the environment) the process context that needs to be
> used for the request.
>
> It looks like the reply to this thread from Oleg that demonstrates using
> child_reaper for the run context could be used though. Capturing the
> struct pid of child_reaper and then using that to locate the appropriate
> task context later (if it still exists) at request time could be used.
>
> That doesn't take care of working out when this should be captured or
> where to put it so it can be obtained at request time (which seems
> difficult in itself).

It would be really really nice if the user namespace could be used
for the where do we look at case. As every other namespace already
has a pointer to the user namespace, and fundamentally the user
namespace is the permission boundary (from a namespace perspective).

So for the equivalent of kthreadd in a user namespace we need a thread
that has a full set of namespaces owned by the user namespaces.

On one side this is very easy to obtain if we look at the process that
sets core_pattern or mounts one of the nfs filesystems (such as the
filesystem that when mounted starts nfsd), and just fork a kernel thread
from it.

On another side perhaps what we want is a syscall call it start_umhd
that says repurpose the caller of this thread to handle future user mode
helper calls. That we could tie to a user namespace quite easily.

This definitely does not play particularly nice with queue work and
friends, but that is just infrastructure and we can update user mode
helper to use something else reasonable as long as we have a solid
design.

Perhaps there is a combination of the two ideas that could work.
Instead of a syscall use the invocation of a service that needs a user
mode helper as a trigger to create such a launcher thread.

Eric

[Kamezawa Hiroyuki]

Can't be handled by simple capability like CAP_SYS_USERMODEHELPER ?

User_ns check seems not to allow core-dump-cather in host will not work if user_ns is used.

Thanks,
-Kame

[Ian Kent]

I don't think so but I'm not sure.

The approach I was talking about assumes the init process of the caller
(say within a container, corresponding to ->child_reaper) is an
appropriate template for umh thread execution.

But I don't think that covers the case where unshare has created
different namespaces, like a mount namespace for example.

The current workqueue sub system can't be used to pre-create a thread to
be used for umh execution so, either is needs changes or yet another
mechanism needs to be implemented.

For uses other than core dumping capturing a reference to the struct pid
of the environment init process and using that as an execution template
should be sufficient and takes care of environment existence problems
with some extra checks, not to mention eliminating the need for a
potentially huge number of kernel threads needing to be created to
provide execution templates.

Where to store this and how to access it when needed is another problem.

Not sure a usermode helper capability is the right thing either as I
thought one important use of user namespaces was to allow unprivileged
users to perform operations they otherwise can't.

Maybe a CAP_SYS_USERNSCOREDUMP or similar would be sensible ....

Still an appropriate execution template would be needed and IIUC we
can't trust getting that from within a user created namespace
environment.

>
> Thanks,
> -Kame
>

[Ian Kent]

Perhaps, if a struct cred could be captured at some appropriate time
that could be used to cater for user namespaces.

Eric, do you think that would be possible to do without allowing users
to circumvent security?

>
> >
> > Thanks,
> > -Kame

[Eric W. Biederman]

I wasn't talking about a capability I was talking about how to identify
where the user mode helper lives.

>> > User_ns check seems not to allow core-dump-cather in host will not
>> > work if user_ns is used.

The bottom line is all of this approaches non-sense if user namespaces
are not used. If you just have a pid namespace or a mount namespace (or
perhaps both) and your fire off a new fangled user mode helper you get a
deep problem. The user space process started to handle your core dump or
your nfs callback will have a full set of capabilities (because it is
still in the root user namespace). With a full set of capabilities
and perhaps a little luck there is no containment.

The imperfect solution that currently exists for the core dump helper
is to provide enough information to the user space application that
it can query and find out the context of the core dumping application
and keep everything in that application sandbox if it so desires.
I expect something similar could be done for other user mode helper
style callbacks.

To make starting the user space application other than how we do today
needs a good argument that you are you can allow a lesser privileged
process set things up and that it can be exploited to gain privielge.

The general problem with capturing less than a full process is that
we always mess it up and forget to capture something important.

In a lot of ways this is a very simpilar problem to setting up an at job
or a cron job. You build a script you test it then you tell at to run
it at a certain time and it fails, because your working environment did
not include something important that was in your actuall environment.

Unfortunately in this case the failures we are talking about are
container escapes and privilege escalation, so we do need to tread
carefully.

We might be able to safely define the context as the context of the
currently running init process (Which we can identifiy with a struct
pid). Justifying that looks a little trickier but doable.

After a mechanism is picked it simply becomes a case of making certain
your permission checks for starting something are in sync with your
mechanism.

Personally I am a fan of the don't be clever and capture a kernel thread
approach as it is very easy to see you what if any exploitation
opportunities there are. The justifications for something more clever
is trickier. Of course we do something that from this perspective would
be considered ``clever'' today with kthreadd and user mode helpers.

Eric

[Kamezawa Hiroyuki]

On 2016/02/19 5:45, Eric W. Biederman wrote:
> Personally I am a fan of the don't be clever and capture a kernel thread
> approach as it is very easy to see you what if any exploitation
> opportunities there are. The justifications for something more clever
> is trickier. Of course we do something that from this perspective would
> be considered ``clever'' today with kthreadd and user mode helpers.
>

I read old discussion....let me allow clarification to create a helper kernel thread
to run usermodehelper with using kthreadd.

0) define a trigger to create an independent usermodehelper environment for a container.
Option A) at creating some namespace (pid, uid, etc...)
Option B) at creating a new nsproxy
Option C).at a new systemcall is called or some sysctl, make_private_usermode_helper() or some,

It's expected this should be triggered by init process of a container with some capability.
And scope of the effect should be defined. pid namespace ? nsporxy ? or new namespace ?

1) create a helper thread.
task = kthread_create(kthread_work_fn, ?, ?, "usermodehelper")
switch task's nsproxy to current.(swtich_task_namespaces())
switch task's cgroups to current (cgroup_attach_task_all())
switch task's cred to current.
copy task's capability from current
(and any other ?)
wake_up_process()

And create a link between kthread_wq and container.

2) modify call_usermodehelper() to use kthread_worker
....

It seems the problem is which object container private user mode helper should be tied to.

Regards,
-Kame

[Ian Kent]

Right, that seems like a fairly straight forward thing to implement
based on Olegs' example patch.

I'll put together a series based on that approach.

Keep in mind that the patches in my previous posts for sub-system usage
are definitely wrong but I can use them (and they will be only an
initial example of how to use the mechanism) to verify that contained
execution happens. They will need to change.

I was thinking that also capturing a struct cred (although I need to
look more at the relationship between the process cred, and the nsproxy
locations) at a particular time combined with a double fork and exec
could allow inclusion of user namespace.

Perhaps at only one level deep, ie. only allowing the first user
namesapec created from init or from container and not user namespaces
created from within a user namespace (if I can work out how to identify
that case).

Again when these are captured and how to get at them when needed is
going to be a challenge.

>
> After a mechanism is picked it simply becomes a case of making certain
> your permission checks for starting something are in sync with your
> mechanism.

Hopefully yourself and others can help with that, ;)

>
> Personally I am a fan of the don't be clever and capture a kernel
> thread
> approach as it is very easy to see you what if any exploitation
> opportunities there are. The justifications for something more clever
> is trickier. Of course we do something that from this perspective
> would
> be considered ``clever'' today with kthreadd and user mode helpers.

Indeed, a good policy, but it seems the choice of the init process
context (of a given container) is fairly straight forward and much of
the tricky stuff and a good measure of checks may already be done in
thread creation and exec code.

As you have pointed out before this is a very difficult problem to deal
with .....

Ian

[Ian Kent]

Not sure I quite understand this but I thought the difficulty with this
approach previously (even though the approach was very much incomplete)
was knowing that all the "moving parts" would not allow vulnerabilities.

And it looks like this would require a kernel thread for each instance.
So for a thousand containers that each mount an NFS mount that means, at
least, 1000 additional kernel threads. Might be able to sell that, if we
were lucky, but from an system administration POV it's horrible.

There's also the question of existence (aka. lifetime) to deal with
since the thread above needs to be created at a time other than the
usermode helper callback.

What happens for SIGKILL on a container?

[Kamezawa Hiroyuki]

Ok, that was discussed.

> And it looks like this would require a kernel thread for each instance.
> So for a thousand containers that each mount an NFS mount that means, at
> least, 1000 additional kernel threads. Might be able to sell that, if we
> were lucky, but from an system administration POV it's horrible.
>

I agree.

> There's also the question of existence (aka. lifetime) to deal with
> since the thread above needs to be created at a time other than the
> usermode helper callback.
>
> What happens for SIGKILL on a container?
>

It depends on how the helper kthread is tied to a container related object.
If kthread is linked with some namespace, we can kill it when a namespace
goes away.

So, with your opinion,
- a helper thread should be spawned on demand
- the lifetime of it should be clear. It will be good to have as same life time as the container.

I wonder there is no solution for "moving part" problem other than calling
do_fork() or copy_process() with container's init process context if we do all in the kernel.
Is that possible ?

Thanks,
-Kame

[Ian Kent]

First understand that the fork and workqueue code is not something I've
needed to look at in the past so it's still quite new to me even now.

> It depends on how the helper kthread is tied to a container related
> object.
> If kthread is linked with some namespace, we can kill it when a
> namespace
> goes away.

I don't know how to do that so without knowing any better I assume it
could be difficult and complicated but, of course, I don't know.

>
> So, with your opinion,
> - a helper thread should be spawned on demand
> - the lifetime of it should be clear. It will be good to have as
> same life time as the container.

This was always what I believed to be the best way to do it but ...

Not sure you've seen the other threads on this by me so let me provide
some history.

I started out posting a series (totally untested, an RFC only) in the
hope of finding a way to do this.

After a few iterations that lead to the conclusion that a kernel thread
would need to be created to provide context for subsequent helper
execution (for every distinct context), much the same as we have here,
and that the init process of the required context would probably be
sufficient for this, required as the environment of the thread
requesting helper execution itself could be used subvert execution.

I ended up accepting that even if I could work out what needed to be
captured and work out what needed to be done to switch to the
namspace(s) and other bits that would be high maintenance as it would be
fairly complicated and subsystems may be added or changed over time.

Also I had assumed a singlethread workqueue would create a single thread
for helper execution which was wrong.

After realizing what I had was far from what's needed I went back and
started reviewing the previous threads.

That lead me to following a link Oleg had posted to this thread where I
finally saw his suggestion about using ->child_reaper as the execution
template.

That really got my attention because of its simplicity and that's why I
want to give that a try now and see where it leads. However user
namespaces do sound like a problem even with this.

Having finally got a simple test scenario I see now that the palaces I
use to capture the information used to run the helper is also wrong but
that's less important than getting an execution method that works, is
safe, and is as simple as it can be.

>
> I wonder there is no solution for "moving part" problem other than
> calling
> do_fork() or copy_process() with container's init process context if
> we do all in the kernel.

Not sure I understand this but I believe that ultimately there will be
the equivalent of a fork (perhaps two) and exec (we need to exec the
helper anyway) no matter how this is done.

For example, IIUC, a fork must be done to change pid namespace but a
template like the container init process would already have that pid
namespace in cases other than possibly user namespaces.

I hope I understood what you were asking and haven't needlessly rambled
on, ;)

Ian

[Ian Kent]

You know, wrt. the mechanism Oleg suggested, I've been wondering if it's
even necessary to capture process template information for execution.

Isn't the main issue the execution of unknown arbitrary objects getting
access to a privileged context?

Then perhaps it is sufficient to require registration of an SHA hash (of
some sort) for these objects by a suitably privileged process and only
allow helper execution of valid objects.

If that is sufficient then helper execution from within a container or
user namespace could just use the callers environment itself.

What else do we need to be wary of, any thoughts Eric?

Ian

[J. Bruce Fields]

On Tue, Feb 23, 2016 at 10:55:30AM +0800, Ian Kent wrote:
> You know, wrt. the mechanism Oleg suggested, I've been wondering if it's
> even necessary to capture process template information for execution.
>
> Isn't the main issue the execution of unknown arbitrary objects getting
> access to a privileged context?
>
> Then perhaps it is sufficient to require registration of an SHA hash (of
> some sort) for these objects by a suitably privileged process and only
> allow helper execution of valid objects.

That executable probably also depends on libraries, services, and tons
of other miscellaneous stuff in its environment. The NFSv4 client
idmapper, for example, may be doing ldap calls. Unless the helper is
created with incredible care, I don't think that it's enough just to
verify that you're executing the correct helper.

--b.

[Ian Kent]

On Tue, 2016-02-23 at 09:36 -0500, J. Bruce Fields wrote:
> On Tue, Feb 23, 2016 at 10:55:30AM +0800, Ian Kent wrote:
> > You know, wrt. the mechanism Oleg suggested, I've been wondering if
> > it's
> > even necessary to capture process template information for
> > execution.
> >
> > Isn't the main issue the execution of unknown arbitrary objects
> > getting
> > access to a privileged context?
> >
> > Then perhaps it is sufficient to require registration of an SHA hash
> > (of
> > some sort) for these objects by a suitably privileged process and
> > only
> > allow helper execution of valid objects.
>
> That executable probably also depends on libraries, services, and tons
> of other miscellaneous stuff in its environment. The NFSv4 client
> idmapper, for example, may be doing ldap calls. Unless the helper is
> created with incredible care, I don't think that it's enough just to
> verify that you're executing the correct helper.

Yeah, I was thinking the logistics of keeping something like this up to
date would be hard but calculating this for every call would be too much
overhead I think.

[Ian Kent]

On Mon, 2013-11-18 at 18:28 +0100, Oleg Nesterov wrote:

> On 11/15, Eric W. Biederman wrote:
> >
> > I don't understand that one. Having a preforked thread with the
> > proper
> > environment that can act like kthreadd in terms of spawning user
> > mode
> > helpers works and is simple.
>
> Can't we ask ->child_reaper to create the non-daemonized kernel thread
> with the "right" ->nsproxy, ->fs, etc?
>
> IOW. Please the the "patch" below. It is obviously incomplete and
> wrong,
> and it can be more clear/clean. And probably we need another API. Just
> to explain what I mean.
>
> With this patch call_usermodehelper(..., UMH_IN_MY_NS) should do exec
> from the caller's namespace.

Umm ... I don't think this can work.

I don't think it can be assumed that the init process of a container
will behave like an init process.

If you try and do this with a Docker container that has /bin/bash as the
init process signals never arrive and work doesn't start until some
other signal arrives at which time it fails to create the kernel thread
returning an error ERESTARTNOINTER (IIRC).

In fact a number of other things relating to signalling processes to
cleanly shutdown in a container suffer the same problem.

I probably don't understand what's actually going on, this is just my
impression of what I'm seeing.

> -fsdevel" in

[Oleg Nesterov]

Hi Ian,

I can't really recall this old discussion, so I can be easily wrong...

On 03/24, Ian Kent wrote:
>
> On Mon, 2013-11-18 at 18:28 +0100, Oleg Nesterov wrote:
> >
> > IOW. Please the the "patch" below. It is obviously incomplete and
> > wrong,
> > and it can be more clear/clean. And probably we need another API. Just
> > to explain what I mean.

I hope you didn't miss this part ;)

In particular, we want to turn task_work_add(..., bool notify) into
task_work_add(..., how_to_notify mask) and this "mask" should allow
to force TIF_SIGPENDING.

> > With this patch call_usermodehelper(..., UMH_IN_MY_NS) should do exec
> > from the caller's namespace.
>
> Umm ... I don't think this can work.
>
> I don't think it can be assumed that the init process of a container
> will behave like an init process.
>
> If you try and do this with a Docker container that has /bin/bash as the
> init process signals never arrive and work doesn't start until some
> other signal arrives

only if it blocks/ignores SIGCHLD? But this doesn't matter, see above and
note the "until we have task_work_add_interruptibel()" in the pseudo-code
I showed.

> I probably don't understand what's actually going on, this is just my
> impression of what I'm seeing.

Or perhaps it is me who misunderstands your concerns.

Oleg.

[Ian Kent]

On Fri, 2016-03-25 at 02:28 +0100, Oleg Nesterov wrote:
> Hi Ian,
>
> I can't really recall this old discussion, so I can be easily wrong...
>
> On 03/24, Ian Kent wrote:
> >
> > On Mon, 2013-11-18 at 18:28 +0100, Oleg Nesterov wrote:
> > >
> > > IOW. Please the the "patch" below. It is obviously incomplete and
> > > wrong,
> > > and it can be more clear/clean. And probably we need another API.
> > > Just
> > > to explain what I mean.
>
> I hope you didn't miss this part ;)

Not at all.

>
> In particular, we want to turn task_work_add(..., bool notify) into
> task_work_add(..., how_to_notify mask) and this "mask" should allow
> to force TIF_SIGPENDING.

The point of posting the reply was to try and get some advice as my
understanding of the signalling subsystem is fairly poor.

LOL, I'll have another look at the task_work_add() code and see if I can
understand what your trying to tell me.

>
> > > With this patch call_usermodehelper(..., UMH_IN_MY_NS) should do
> > > exec
> > > from the caller's namespace.
> >
> > Umm ... I don't think this can work.
> >
> > I don't think it can be assumed that the init process of a container
> > will behave like an init process.
> >
> > If you try and do this with a Docker container that has /bin/bash as
> > the
> > init process signals never arrive and work doesn't start until some
> > other signal arrives
>
> only if it blocks/ignores SIGCHLD? But this doesn't matter, see above
> and
> note the "until we have task_work_add_interruptibel()" in the pseudo
> -code
> I showed.

It seems, and this is not the only case I've encountered, that the init
process in docker containers can be a problem when you want to capture
and handle signals.

I've seen this with /bin/bash and supervisord so far.
I don't know if it is the docker container creation doing this or
something else .... certainly I can catch signals within subordinate
processes.

The other thing that occurs to me is that just about anything in a
container could be subverted so the definition of a privileged process
which can be used as a template form execution is essentially undefined.

Mmm ... maybe I've got that wrong too, ;)

Ian