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

Paul Menzel
2020-07-24T07:45:23Z

Dear Kees,

Am 24.07.20 um 00:32 schrieb Kees Cook:
> 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].
>>
>> [1] https://bugzilla.kernel.org/show_bug.cgi?id=207383
>
> 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.

I understand the Fixes tag mainly a help when backporting commits.

As Linux 5.8-rc7 is going to be released this Sunday, I wonder, if
commit 3202fa62f ("slub: relocate freelist pointer to middle of object")
should be reverted for now to fix the regression for the users according
to Linux’ no regression policy. Once the AMDGPU/DRM driver issue is
fixed, it can be reapplied. I know it’s not optimal, but as some testing
is going to be involved for the fix, I’d argue it’s the best option for
the users.

Kind regards,

Paul