[syzbot] BUG: unable to handle kernel paging request in truncate_inode_partial_folio

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 941e3e791269 Merge tag 'for_linus' of git://git.kernel.org..
git tree: upstream
console+strace: https://syzkaller.appspot.com/x/log.txt?x=1670ded4080000
kernel config: https://syzkaller.appspot.com/x/.config?x=833001d0819ddbc9
dashboard link: https://syzkaller.appspot.com/bug?extid=9bd2b7adbd34b30b87e4
compiler: gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=140f9ba8080000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=15495188080000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+9bd2b7...@syzkaller.appspotmail.com

BUG: unable to handle page fault for address: ffff888021f7e005
#PF: supervisor write access in kernel mode
#PF: error_code(0x0002) - not-present page
PGD 11401067 P4D 11401067 PUD 11402067 PMD 21f7d063 PTE 800fffffde081060
Oops: 0002 [#1] PREEMPT SMP KASAN
CPU: 0 PID: 3761 Comm: syz-executor281 Not tainted 5.19.0-rc4-syzkaller-00014-g941e3e791269 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:memset_erms+0x9/0x10 arch/x86/lib/memset_64.S:64
Code: c1 e9 03 40 0f b6 f6 48 b8 01 01 01 01 01 01 01 01 48 0f af c6 f3 48 ab 89 d1 f3 aa 4c 89 c8 c3 90 49 89 f9 40 88 f0 48 89 d1 <f3> aa 4c 89 c8 c3 90 49 89 fa 40 0f b6 ce 48 b8 01 01 01 01 01 01
RSP: 0018:ffffc9000329fa90 EFLAGS: 00010202
RAX: 0000000000000000 RBX: 0000000000001000 RCX: 0000000000000ffb
RDX: 0000000000000ffb RSI: 0000000000000000 RDI: ffff888021f7e005
RBP: ffffea000087df80 R08: 0000000000000001 R09: ffff888021f7e005
R10: ffffed10043efdff R11: 0000000000000000 R12: 0000000000000005
R13: 0000000000000000 R14: 0000000000001000 R15: 0000000000000ffb
FS: 00007fb29d8b2700(0000) GS:ffff8880b9a00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffff888021f7e005 CR3: 0000000026e7b000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
zero_user_segments include/linux/highmem.h:272 [inline]
folio_zero_range include/linux/highmem.h:428 [inline]
truncate_inode_partial_folio+0x76a/0xdf0 mm/truncate.c:237
truncate_inode_pages_range+0x83b/0x1530 mm/truncate.c:381
truncate_inode_pages mm/truncate.c:452 [inline]
truncate_pagecache+0x63/0x90 mm/truncate.c:753
simple_setattr+0xed/0x110 fs/libfs.c:535
secretmem_setattr+0xae/0xf0 mm/secretmem.c:170
notify_change+0xb8c/0x12b0 fs/attr.c:424
do_truncate+0x13c/0x200 fs/open.c:65
do_sys_ftruncate+0x536/0x730 fs/open.c:193
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x46/0xb0
RIP: 0033:0x7fb29d900899
Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 11 15 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fb29d8b2318 EFLAGS: 00000246 ORIG_RAX: 000000000000004d
RAX: ffffffffffffffda RBX: 00007fb29d988408 RCX: 00007fb29d900899
RDX: 00007fb29d900899 RSI: 0000000000000005 RDI: 0000000000000003
RBP: 00007fb29d988400 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007fb29d98840c
R13: 00007ffca01a23bf R14: 00007fb29d8b2400 R15: 0000000000022000
</TASK>
Modules linked in:
CR2: ffff888021f7e005
---[ end trace 0000000000000000 ]---
RIP: 0010:memset_erms+0x9/0x10 arch/x86/lib/memset_64.S:64
Code: c1 e9 03 40 0f b6 f6 48 b8 01 01 01 01 01 01 01 01 48 0f af c6 f3 48 ab 89 d1 f3 aa 4c 89 c8 c3 90 49 89 f9 40 88 f0 48 89 d1 <f3> aa 4c 89 c8 c3 90 49 89 fa 40 0f b6 ce 48 b8 01 01 01 01 01 01
RSP: 0018:ffffc9000329fa90 EFLAGS: 00010202
RAX: 0000000000000000 RBX: 0000000000001000 RCX: 0000000000000ffb
RDX: 0000000000000ffb RSI: 0000000000000000 RDI: ffff888021f7e005
RBP: ffffea000087df80 R08: 0000000000000001 R09: ffff888021f7e005
R10: ffffed10043efdff R11: 0000000000000000 R12: 0000000000000005
R13: 0000000000000000 R14: 0000000000001000 R15: 0000000000000ffb
FS: 00007fb29d8b2700(0000) GS:ffff8880b9a00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffff888021f7e005 CR3: 0000000026e7b000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
----------------
Code disassembly (best guess):
0: c1 e9 03 shr $0x3,%ecx
3: 40 0f b6 f6 movzbl %sil,%esi
7: 48 b8 01 01 01 01 01 movabs $0x101010101010101,%rax
e: 01 01 01
11: 48 0f af c6 imul %rsi,%rax
15: f3 48 ab rep stos %rax,%es:(%rdi)
18: 89 d1 mov %edx,%ecx
1a: f3 aa rep stos %al,%es:(%rdi)
1c: 4c 89 c8 mov %r9,%rax
1f: c3 retq
20: 90 nop
21: 49 89 f9 mov %rdi,%r9
24: 40 88 f0 mov %sil,%al
27: 48 89 d1 mov %rdx,%rcx
* 2a: f3 aa rep stos %al,%es:(%rdi) <-- trapping instruction
2c: 4c 89 c8 mov %r9,%rax
2f: c3 retq
30: 90 nop
31: 49 89 fa mov %rdi,%r10
34: 40 0f b6 ce movzbl %sil,%ecx
38: 48 rex.W
39: b8 01 01 01 01 mov $0x1010101,%eax
3e: 01 01 add %eax,(%rcx)


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
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syzbot can test patches for this issue, for details see:
https://goo.gl/tpsmEJ#testing-patches

[Eric Biggers]

I think this is a bug in memfd_secret. secretmem_setattr() can race with a page
being faulted in by secretmem_fault(). Specifically, a page can be faulted in
after secretmem_setattr() has set i_size but before it zeroes out the partial
page past i_size. memfd_secret pages aren't mapped in the kernel direct map, so
the crash occurs when the kernel tries to zero out the partial page.

I don't know what the best solution is -- maybe a rw_semaphore protecting
secretmem_fault() and secretmem_setattr()? Or perhaps secretmem_setattr()
should avoid the call to truncate_setsize() by not using simple_setattr(), given
that secretmem_setattr() only supports the size going from zero to nonzero.

The following commit tried to fix a similar bug, but it wasn't enough:

commit f9b141f93659e09a52e28791ccbaf69c273b8e92
Author: Axel Rasmussen <axelra...@google.com>
Date: Thu Apr 14 19:13:31 2022 -0700

mm/secretmem: fix panic when growing a memfd_secret


Here's a simplified reproducer. Note, for memfd_secret to be supported, the
kernel config must contain CONFIG_SECRETMEM=y and the kernel command line must
contain secretmem.enable=1.

#include <pthread.h>
#include <setjmp.h>
#include <signal.h>
#include <sys/mman.h>
#include <sys/syscall.h>
#include <unistd.h>

static volatile int fd;
static jmp_buf jump_buf;

static void *truncate_thread(void *arg)
{
for (;;)
ftruncate(fd, 1000);
}

static void handle_sigbus(int sig)
{
longjmp(jump_buf, 1);
}

int main(void)
{
struct sigaction act = {
.sa_handler = handle_sigbus,
.sa_flags = SA_NODEFER,
};
pthread_t t;
void *addr;

sigaction(SIGBUS, &act, NULL);

pthread_create(&t, NULL, truncate_thread, NULL);
for (;;) {
fd = syscall(__NR_memfd_secret, 0);
addr = mmap(NULL, 8192, PROT_WRITE, MAP_SHARED, fd, 0);
if (setjmp(jump_buf) == 0)
*(unsigned int *)addr = 0;
munmap(addr, 8192);
close(fd);
}
}

[Axel Rasmussen]

From my perspective the rw_semaphore approach sounds reasonable.

simple_setattr() and the functions it calls to do the actual work
isn't a tiny amount of code, it would be a shame to reimplement it in
secretmem.c.

For the rwsem, I guess the idea is setattr will take it for write, and
fault will take it for read? Since setattr is a very infrequent
operation - a typical use case is you'd do it exactly once right after
opening the memfd_secret - this seems like it wouldn't make fault
significantly less performant. It's also a pretty small change I
think, just a few lines.

[Mike Rapoport]

Below is my take on adding a semaphore and making ->setattr() and ->fault()
mutually exclusive. It's only lightly tested so I'd appreciate if Eric
could give it a whirl.

With addition of semaphore to secretmem_setattr() it seems we don't need
special care for size changes, just calling simple_setattr() after taking
the semaphore should be fine. Thoughts?

From edfcb2f0d31c2132bda483635dd2a8dd295efb04 Mon Sep 17 00:00:00 2001
From: Mike Rapoport <rp...@linux.ibm.com>
Date: Thu, 30 Jun 2022 11:26:37 +0300
Subject: [PATCH] secretmem: fix unhandled fault in truncate

syzkaller reports the following issue:

BUG: unable to handle page fault for address: ffff888021f7e005

Eric Biggers suggested that this happens when
secretmem_setattr()->simple_setattr() races with secretmem_fault() so
that a page that is faulted in by secretmem_fault() (and thus removed
from the direct map) is zeroed by inode truncation right afterwards.

Use an rw_semaphore to make secretmem_fault() and secretmem_setattr()
mutually exclusive.

Reported-by: syzbot+9bd2b7...@syzkaller.appspotmail.com
Suggested-by: Eric Biggers <ebig...@kernel.org>
Signed-off-by: Mike Rapoport <rp...@linux.ibm.com>
---
mm/secretmem.c | 48 ++++++++++++++++++++++++++++++++++++++++--------
1 file changed, 40 insertions(+), 8 deletions(-)

diff --git a/mm/secretmem.c b/mm/secretmem.c
index 206ed6b40c1d..40573b045c96 100644
--- a/mm/secretmem.c
+++ b/mm/secretmem.c
@@ -47,30 +47,41 @@ bool secretmem_active(void)
return !!atomic_read(&secretmem_users);
}

+struct secretmem_state {
+ struct rw_semaphore rw_sem;
+};
+
static vm_fault_t secretmem_fault(struct vm_fault *vmf)
{
struct address_space *mapping = vmf->vma->vm_file->f_mapping;
struct inode *inode = file_inode(vmf->vma->vm_file);
+ struct secretmem_state *state = inode->i_private;
pgoff_t offset = vmf->pgoff;
gfp_t gfp = vmf->gfp_mask;
unsigned long addr;
struct page *page;
+ vm_fault_t ret;
int err;

if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode))
return vmf_error(-EINVAL);

+ down_read(&state->rw_sem);
+
retry:
page = find_lock_page(mapping, offset);
if (!page) {
page = alloc_page(gfp | __GFP_ZERO);
- if (!page)
- return VM_FAULT_OOM;
+ if (!page) {
+ ret = VM_FAULT_OOM;
+ goto out;
+ }

err = set_direct_map_invalid_noflush(page);
if (err) {
put_page(page);
- return vmf_error(err);
+ ret = vmf_error(err);
+ goto out;
}

__SetPageUptodate(page);
@@ -86,7 +97,8 @@ static vm_fault_t secretmem_fault(struct vm_fault *vmf)
if (err == -EEXIST)
goto retry;

- return vmf_error(err);
+ ret = vmf_error(err);
+ goto out;
}

addr = (unsigned long)page_address(page);
@@ -94,7 +106,11 @@ static vm_fault_t secretmem_fault(struct vm_fault *vmf)
}

vmf->page = page;
- return VM_FAULT_LOCKED;
+ ret = VM_FAULT_LOCKED;
+
+out:
+ up_read(&state->rw_sem);
+ return ret;
}

static const struct vm_operations_struct secretmem_vm_ops = {
@@ -163,11 +179,17 @@ static int secretmem_setattr(struct user_namespace *mnt_userns,
{
struct inode *inode = d_inode(dentry);
unsigned int ia_valid = iattr->ia_valid;
+ struct secretmem_state *state = inode->i_private;
+ int ret;

+ down_write(&state->rw_sem);
if ((ia_valid & ATTR_SIZE) && inode->i_size)
- return -EINVAL;
+ ret = -EINVAL;
+ else
+ ret = simple_setattr(mnt_userns, dentry, iattr);
+ up_write(&state->rw_sem);

- return simple_setattr(mnt_userns, dentry, iattr);
+ return ret;
}

static const struct inode_operations secretmem_iops = {
@@ -179,22 +201,30 @@ static struct vfsmount *secretmem_mnt;
static struct file *secretmem_file_create(unsigned long flags)
{
struct file *file = ERR_PTR(-ENOMEM);
+ struct secretmem_state *state;
struct inode *inode;

inode = alloc_anon_inode(secretmem_mnt->mnt_sb);
if (IS_ERR(inode))
return ERR_CAST(inode);

+ state = kzalloc(sizeof(*state), GFP_KERNEL);
+ if (!state)
+ goto err_free_inode;
+
file = alloc_file_pseudo(inode, secretmem_mnt, "secretmem",
O_RDWR, &secretmem_fops);
if (IS_ERR(file))
- goto err_free_inode;
+ goto err_free_state;

mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER);
mapping_set_unevictable(inode->i_mapping);

+ init_rwsem(&state->rw_sem);
+
inode->i_op = &secretmem_iops;
inode->i_mapping->a_ops = &secretmem_aops;
+ inode->i_private = state;

/* pretend we are a normal file with zero size */
inode->i_mode |= S_IFREG;
@@ -202,6 +232,8 @@ static struct file *secretmem_file_create(unsigned long flags)

return file;

+err_free_state:
+ kfree(state);
err_free_inode:
iput(inode);
return file;

base-commit: 03c765b0e3b4cb5063276b086c76f7a612856a9a
--
2.34.1

--
Sincerely yours,
Mike.

[Axel Rasmussen]

The patch below looks correct to me. I do think we still need the
check which prevents truncating a memfd_secret with an existing
nonzero size, though, because I think simple_setattr's way of doing
that still BUGs in a non-racy way (rwsem doesn't help with this). The
patch below keeps this, so maybe I'm just misinterpreting "we don't
need special care for size changes".

I haven't booted+tested it, I'll leave that to Eric since he already
has a reproducer setup for this. But, for what it's worth, feel free
to take:

Reviewed-by: Axel Rasmussen <axelra...@google.com>

[Mike Rapoport]

On Thu, Jun 30, 2022 at 09:14:07AM -0700, Axel Rasmussen wrote:
> On Thu, Jun 30, 2022 at 1:47 AM Mike Rapoport <rp...@kernel.org> wrote:
> > On Wed, Jun 29, 2022 at 09:30:12AM -0700, Axel Rasmussen wrote:
> > > On Tue, Jun 28, 2022 at 9:41 PM Eric Biggers <ebig...@kernel.org> wrote:
> > > > On Tue, Jun 28, 2022 at 03:59:26PM -0700, syzbot wrote:
> > > > > Hello,
> > > > >
> > > > > syzbot found the following issue on:
> > > > >
> > > > > HEAD commit: 941e3e791269 Merge tag 'for_linus' of git://git.kernel.org..
> > > > > git tree: upstream
> > > > > console+strace: https://syzkaller.appspot.com/x/log.txt?x=1670ded4080000
> > > > > kernel config: https://syzkaller.appspot.com/x/.config?x=833001d0819ddbc9
> > > > > dashboard link: https://syzkaller.appspot.com/bug?extid=9bd2b7adbd34b30b87e4
> > > > > compiler: gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> > > > > syz repro: https://syzkaller.appspot.com/x/repro.syz?x=140f9ba8080000
> > > > > C reproducer: https://syzkaller.appspot.com/x/repro.c?x=15495188080000
> > > > >
> > > > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > > > Reported-by: syzbot+9bd2b7...@syzkaller.appspotmail.com
> > > > >
> > > >

It really was a question, because I was too lazy to dig into
simple_setattr() and I know you investigated it :)

> I haven't booted+tested it, I'll leave that to Eric since he already
> has a reproducer setup for this. But, for what it's worth, feel free
> to take:
>
> Reviewed-by: Axel Rasmussen <axelra...@google.com>

Thanks!

--
Sincerely yours,
Mike.

[Hillf Danton]

Given inode_lock() in do_truncate(), another simpler option is to lock
inode in the fault path if the suggested race is the root cause.

Hillf

[Mike Rapoport]

Yeah, it makes sense. It does not look like a race would happen anywhere
but do_truncate().

> Hillf
>

--
Sincerely yours,
Mike.