Re: [PATCH] amdgpu_dm: fix nonblocking atomic commit use-after-free

From: Christian KÃnig
Date: Fri Jul 24 2020 - 03:26:46 EST

Am 24.07.20 um 00:58 schrieb Mazin Rezk:
On Thursday, July 23, 2020 6:32 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:

On Thu, Jul 23, 2020 at 09:10:15PM +0000, Mazin Rezk wrote:

When amdgpu_dm_atomic_commit_tail is running in the workqueue,
drm_atomic_state_put will get called while amdgpu_dm_atomic_commit_tail is
running, causing a race condition where state (and then dm_state) is
sometimes freed while amdgpu_dm_atomic_commit_tail is running. This bug has
occurred since 5.7-rc1 and is well documented among polaris11 users [1].
Prior to 5.7, this was not a noticeable issue since the freelist pointer
was stored at the beginning of dm_state (base), which was unused. After
changing the freelist pointer to be stored in the middle of the struct, the
freelist pointer overwrote the context, causing dc_state to become garbage
data and made the call to dm_enable_per_frame_crtc_master_sync dereference
a freelist pointer.
This patch fixes the aforementioned issue by calling drm_atomic_state_get
in amdgpu_dm_atomic_commit before drm_atomic_helper_commit is called and
drm_atomic_state_put after amdgpu_dm_atomic_commit_tail is complete.
According to my testing on 5.8.0-rc6, this should fix bug 207383 on
Bugzilla [1].
Nice work tracking this down!

Fixes: 3202fa62f ("slub: relocate freelist pointer to middle of object")
I do, however, object to this Fixes tag. :) The flaw appears to have
been with amdgpu_dm's reference tracking of "state" in the nonblocking
case. (How this reference counting is supposed to work correctly, though,
I'm not sure.) If I look at where the drm helper was split from being
the default callback, it looks like this was what introduced the bug:

da5c47f682ab ("drm/amd/display: Remove acrtc->stream")

? 3202fa62f certainly exposed it much more quickly, but there was a race
even without 3202fa62f where something could have realloced the memory
and written over it.


Kees Cook

Thanks, I'll be sure to avoid using 3202fa62f as the cause next time.
I just thought to do that because it was what made the use-after-free cause
a noticeable bug.

Also, by the way, I just realised the patch didn't completely solve the bug.
Sorry about that, making an LKML thread on this was hasty on my part. Should
I get further confirmation from the Bugzilla thread before submitting a patch
for this bug in the future?

Submitting stuff as early as possible is mostly a good idea. Just if the code is utterly broken or completely unreadable you should probably expect a harsh response :)

Maybe ask for more testing in the commit message if you are not 100% sure if that really fixes a bug or not.


Mazin Rezk