[PATCH] kasan: make depot_fetch_stack more robust

[Dmitry Vyukov]

I've hit a GPF in depot_fetch_stack when it was given
bogus stack handle. I think it was caused by a distant
out-of-bounds that hit a different object, as the result
we treated uninit garbage as stack handle. Maybe there is
something to fix in KASAN logic, but I think it makes
sense to make depot_fetch_stack more robust as well.

Verify that the provided stack handle looks correct.

Signed-off-by: Dmitry Vyukov <dvy...@google.com>
---
For your convenience uploaded to codereview:
https://codereview.appspot.com/295680043

---
include/linux/stackdepot.h | 2 +-
lib/stackdepot.c | 21 +++++++++++++++++----
mm/kasan/report.c | 10 ++++------
mm/page_owner.c | 12 ++++++------
4 files changed, 28 insertions(+), 17 deletions(-)

diff --git a/include/linux/stackdepot.h b/include/linux/stackdepot.h
index 7978b3e..b2dbe02 100644
--- a/include/linux/stackdepot.h
+++ b/include/linux/stackdepot.h
@@ -27,6 +27,6 @@ struct stack_trace;

depot_stack_handle_t depot_save_stack(struct stack_trace *trace, gfp_t flags);

-void depot_fetch_stack(depot_stack_handle_t handle, struct stack_trace *trace);
+bool depot_fetch_stack(depot_stack_handle_t handle, struct stack_trace *trace);

#endif
diff --git a/lib/stackdepot.c b/lib/stackdepot.c
index 53ad6c0..0982331 100644
--- a/lib/stackdepot.c
+++ b/lib/stackdepot.c
@@ -181,16 +181,29 @@ static inline struct stack_record *find_stack(struct stack_record *bucket,
return NULL;
}

-void depot_fetch_stack(depot_stack_handle_t handle, struct stack_trace *trace)
+bool depot_fetch_stack(depot_stack_handle_t handle, struct stack_trace *trace)
{
union handle_parts parts = { .handle = handle };
- void *slab = stack_slabs[parts.slabindex];
- size_t offset = parts.offset << STACK_ALLOC_ALIGN;
- struct stack_record *stack = slab + offset;
+ void *slab;
+ struct stack_record *stack;

+ if (handle == 0)
+ return false;
+ if (parts.valid != 1 || parts.slabindex >= ARRAY_SIZE(stack_slabs))
+ goto bad;
+ slab = stack_slabs[parts.slabindex];
+ if (slab == NULL)
+ goto bad;
+ stack = slab + (parts.offset << STACK_ALLOC_ALIGN);
+ if (stack->handle.handle != handle)
+ goto bad;
trace->nr_entries = trace->max_entries = stack->size;
trace->entries = stack->entries;
trace->skip = 0;
+ return true;
+bad:
+ pr_err("stackdepot: fetching bogus stack %x\n", handle);
+ return false;
}

/**
diff --git a/mm/kasan/report.c b/mm/kasan/report.c
index 861b977..46e4b82 100644
--- a/mm/kasan/report.c
+++ b/mm/kasan/report.c
@@ -118,15 +118,13 @@ static inline bool init_task_stack_addr(const void *addr)

static void print_track(struct kasan_track *track)
{
- pr_err("PID = %u\n", track->pid);
- if (track->stack) {
- struct stack_trace trace;
+ struct stack_trace trace;

- depot_fetch_stack(track->stack, &trace);
+ pr_err("PID = %u\n", track->pid);
+ if (depot_fetch_stack(track->stack, &trace))
print_stack_trace(&trace, 0);
- } else {
+ else
pr_err("(stack is not available)\n");
- }
}

static void kasan_object_err(struct kmem_cache *cache, struct page *page,
diff --git a/mm/page_owner.c b/mm/page_owner.c
index 8fa5083..1862f05 100644
--- a/mm/page_owner.c
+++ b/mm/page_owner.c
@@ -252,10 +252,11 @@ print_page_owner(char __user *buf, size_t count, unsigned long pfn,
if (ret >= count)
goto err;

- depot_fetch_stack(handle, &trace);
- ret += snprint_stack_trace(kbuf + ret, count - ret, &trace, 0);
- if (ret >= count)
- goto err;
+ if (depot_fetch_stack(handle, &trace)) {
+ ret += snprint_stack_trace(kbuf + ret, count - ret, &trace, 0);
+ if (ret >= count)
+ goto err;
+ }

if (page_ext->last_migrate_reason != -1) {
ret += snprintf(kbuf + ret, count - ret,
@@ -307,12 +308,11 @@ void __dump_page_owner(struct page *page)
}

handle = READ_ONCE(page_ext->handle);
- if (!handle) {
+ if (!depot_fetch_stack(handle, &trace)) {
pr_alert("page_owner info is not active (free page?)\n");
return;
}

- depot_fetch_stack(handle, &trace);
pr_alert("page allocated via order %u, migratetype %s, gfp_mask %#x(%pGg)\n",
page_ext->order, migratetype_names[mt], gfp_mask, &gfp_mask);
print_stack_trace(&trace, 0);
--
2.8.0.rc3.226.g39d4020

[Joonsoo Kim]

Please do 'goto err' if depot_fetch_stack() return false here.

Others looks fine to me.

Thanks.

[Andrey Ryabinin]

I don't think that adding the kernel code to work around bugs in the
kernel code makes a lot of sense.
depot_fetch_stack() fails if invalid handler is passed, and that is a
bug. You can just add WARN_ON() in
depot_fetch_stack() if you want to detect such cases..
Note that KASAN detects corruption of object's metadata, so such check
may help only in case of
corruption page owner's data.

[Kuthonuzo Luruo]

On Mon, Jul 4, 2016 at 8:11 PM, Andrey Ryabinin <ryabin...@gmail.com> wrote:
> 2016-07-01 20:38 GMT+03:00 Dmitry Vyukov <dvy...@google.com>:
>> I've hit a GPF in depot_fetch_stack when it was given
>> bogus stack handle. I think it was caused by a distant
>> out-of-bounds that hit a different object, as the result
>> we treated uninit garbage as stack handle. Maybe there is
>> something to fix in KASAN logic, but I think it makes
>> sense to make depot_fetch_stack more robust as well.
>>
>> Verify that the provided stack handle looks correct.
>>

> I don't think that adding the kernel code to work around bugs in the
> kernel code makes a lot of sense.
> depot_fetch_stack() fails if invalid handler is passed, and that is a
> bug. You can just add WARN_ON() in
> depot_fetch_stack() if you want to detect such cases..

In this case, the code happens to be a debugging tool that actively anticipates
bad memory accesses. If the tool can reliably detect bad input that could
potentially cause a crash inside the debugger itself, and take actions
to prevent it,
I believe that's a good thing.

> Note that KASAN detects corruption of object's metadata, so such check
> may help only in case of
> corruption page owner's data.

It will also help in case of bad access by non-instrumented code.

>
>> if (page_ext->last_migrate_reason != -1) {
>> ret += snprintf(kbuf + ret, count - ret,
>> @@ -307,12 +308,11 @@ void __dump_page_owner(struct page *page)
>> }
>>
>> handle = READ_ONCE(page_ext->handle);
>> - if (!handle) {
>> + if (!depot_fetch_stack(handle, &trace)) {
>> pr_alert("page_owner info is not active (free page?)\n");
>> return;
>> }
>>
>> - depot_fetch_stack(handle, &trace);
>> pr_alert("page allocated via order %u, migratetype %s, gfp_mask %#x(%pGg)\n",
>> page_ext->order, migratetype_names[mt], gfp_mask, &gfp_mask);
>> print_stack_trace(&trace, 0);
>> --
>> 2.8.0.rc3.226.g39d4020
>>
>

> --
> You received this message because you are subscribed to the Google Groups "kasan-dev" group.
> To unsubscribe from this group and stop receiving emails from it, send an email to kasan-dev+...@googlegroups.com.
> To post to this group, send email to kasa...@googlegroups.com.
> To view this discussion on the web visit https://groups.google.com/d/msgid/kasan-dev/CAPAsAGxj61%3DtrcAAPqODX1Z7vV%3D7-faG1oJBL5WCn%3DrBXAsvNA%40mail.gmail.com.
> For more options, visit https://groups.google.com/d/optout.