Messages in this thread | | | Subject | Re: [PATCH] amdgpu_dm: fix nonblocking atomic commit use-after-free | From | Paul Menzel <> | Date | Fri, 24 Jul 2020 09:45:18 +0200 |
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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
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