[RFC 0/4] Some (drm_sched_|dma_)fence lifetime issues

[Tvrtko Ursulin]

Hi all,

Recently I mentioned to Danilo about some fence lifetime issues so here is a
rough series, more than anything intended to start the discussion.

Most of the problem statement can be found in the first patch but to briefly
summarise - because sched fence can outlive the scheduler, we can trivially
engineer an use after free with xe and possibly other drivers. All that is
needed is to convert a syncobj into a sync file behind drivers back, and I don't
see what the driver can do about it.

IGT that exploits the problem:
https://patchwork.freedesktop.org/patch/642709/?series=146211&rev=2

Different flavour of the problem space is if we had a close(drm_fd) in that test
before the sleep. In that case we can even unload xe.ko and gpu-sched.ko for
even more fun. Last two patches in the series close that gap.

But first two patches are just shrinking the race window. They are not proper
fixes. This is what I want to discuss since I understand reference counting all
the involved objects has been rejected in the past. And since the problem
probably expands to all dma fences it certainly isn't easy.

To be clear once more - lets not focus on how this does not fix it fully - I am
primarily trying to start the conversation.

Cc: Christian König <christian.koenig at amd.com>
Cc: Danilo Krummrich <dakr at kernel.org>
Cc: Lucas De Marchi <lucas.demarchi at intel.com>
Cc: Matthew Brost <matthew.brost at intel.com>
Cc: Philipp Stanner <phasta at kernel.org>
Cc: Rodrigo Vivi <rodrigo.vivi at intel.com>

Tvrtko Ursulin (4):
  sync_file: Weakly paper over one use-after-free resulting race
  dma-fence: Slightly safer dma_fence_set_deadline
  drm/sched: Keep module reference while there are active fences
  drm/xe: Keep module reference while there are active fences

 drivers/dma-buf/dma-fence.c             |  2 +-
 drivers/dma-buf/sync_file.c             | 29 ++++++++++++++++++++-----
 drivers/gpu/drm/scheduler/sched_fence.c | 12 ++++++++--
 drivers/gpu/drm/xe/xe_hw_fence.c        | 13 ++++++++++-
 4 files changed, 47 insertions(+), 9 deletions(-)

-- 
2.48.0