[PATCH] drm/amdgpu: don't release the fence reference consumed by the scheduler

[Donggeun Yoo <donggeunyoo.kernel@gmail.com>]

drm_sched_job_add_dependency() consumes the fence reference in both the
success and the error case. The kernel-doc has said so since commit
ebd5f74255b9 ("drm/sched: Add dependency tracking") added the function,
and sched_main.c does the dma_fence_put() itself when its xa_alloc()
fails.

Three amdgpu callers take an extra reference for the callee and then put
it again when the callee returns an error, dropping a reference they no
longer own. The refcount ends up one short of the number of owners, so a
later put consumes someone else's reference and the fence is freed while
that owner still points at it.

In amdgpu_sync_push_to_job() the fence usually comes from
amdgpu_sync_resv(), which walks a BO's dma_resv and hands each fence to
amdgpu_sync_fence(), where the sync entry takes a reference of its own.
The fence stays installed in that dma_resv, so once the entry's reference
is gone amdgpu_sync_free() eats the reservation object's instead and the
fence is freed underneath a dma_resv that still lists it. The
use-after-free lands in an unrelated thread.

amdgpu_vm_sdma_update() is cross-thread by construction. It walks with
dma_resv_for_each_fence_unlocked(), whose cursor holds exactly one
reference, and the dma_resv_iter_end() on the same error path drops it a
second time.

amdgpu_cs_submit() stays local: the extra put takes the gang member's
scheduled fence to zero, handing the whole drm_sched_fence to call_rcu()
while its finished half is still live.

The extra get is correct at all three sites and stays. Only the put on
the error path is wrong.

Every other drm_sched_job_add_dependency() caller in the tree - etnaviv,
imagination, msm, nouveau, panthor, v3d, xe - hands the reference over
and returns the error untouched, as does
drm_sched_job_add_resv_dependencies() in the scheduler itself.

The trigger is xa_alloc() failing during command submission or a VM
update.

Measured on a Radeon 780M (GC 11.0.1) with a forced -ENOMEM return from
drm_sched_job_add_dependency(), printing the fence's reference count in
amdgpu_sync_push_to_job() on entry, after the failed call, and as the
error is returned:

  unpatched  entry=5 after_call=5 on_return=4
             entry=6 after_call=6 on_return=5
  patched    entry=5 after_call=5 on_return=5
             entry=6 after_call=6 on_return=6

after_call equals entry in both, which is the callee consuming the
reference the caller took for it. Unpatched, the dma_fence_put() that
follows takes it one below where it started, and that reference is the
sync entry's own; patched, there is no put left to make and the count
leaves the function as it arrived.

Fixes: 41cc108b2451 ("drm/amdgpu: fix missing dma_fence_put in error path")
Fixes: ed21f6c3fe42 ("drm/amdgpu: fix another missing fence reference in the CS code")
Fixes: c1c4a8b21721 ("drm/amdgpu: grab extra fence reference for drm_sched_job_add_dependency")
Assisted-by: Claude:claude-fable-5
Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@gmail.com>
---
Resent: the original reached lkml but not dri-devel/amd-gfx - I was not
subscribed at the time. Apologies to those seeing it twice.

Tested on a Radeon 780M (GC 11.0.1) running 7.3-rc2 with KASAN, lockdep
and DEBUG_LIST on, driving compute submissions through radeonsi. The
-ENOMEM was forced by a temporary module parameter on gpu-sched that
makes drm_sched_job_add_dependency() fail after N calls, consuming the
fence on the way out exactly as the xa_alloc() failure does; the counts
above come from a probe printed in amdgpu_sync_push_to_job(), with the
unpatched and patched drivers run through the same harness. Neither
produced a KASAN report or a refcount warning: at the point of the extra
put the fence still had four or five other owners, so it is not freed
there. Observing the use-after-free itself needs the stolen reference to
be the last one, which this workload does not arrange.

Only the amdgpu_sync_push_to_job() site is reachable from a plain
compute workload. amdgpu_cs_submit() skips its loop body unless the
submission is a gang of more than one job, and amdgpu_vm_sdma_update()
was not reached; both are fixed here on the same reasoning.

No Cc: stable. I believe this qualifies under
Documentation/process/stable-kernel-rules.rst:19 - it is a use-after-free,
not the theoretical race condition line 27 excludes - but I have not
observed the failure itself, so I am leaving the tag off rather than
claiming more than I measured. No objection if you want to add it.

 drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c      | 4 +---
 drivers/gpu/drm/amd/amdgpu/amdgpu_sync.c    | 4 +---
 drivers/gpu/drm/amd/amdgpu/amdgpu_vm_sdma.c | 1 -
 3 files changed, 2 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
index 03b41f803520..12ec031c0812 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
@@ -1303,10 +1303,8 @@ static int amdgpu_cs_submit(struct amdgpu_cs_parser *p,
 		fence = &p->jobs[i]->base.s_fence->scheduled;
 		dma_fence_get(fence);
 		r = drm_sched_job_add_dependency(&leader->base, fence);
-		if (r) {
-			dma_fence_put(fence);
+		if (r)
 			return r;
-		}
 	}
 
 	if (p->gang_size > 1) {
diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_sync.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_sync.c
index d6ae9974c952..4f31021b2152 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_sync.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_sync.c
@@ -447,10 +447,8 @@ int amdgpu_sync_push_to_job(struct amdgpu_sync *sync, struct amdgpu_job *job)
 
 		dma_fence_get(f);
 		r = drm_sched_job_add_dependency(&job->base, f);
-		if (r) {
-			dma_fence_put(f);
+		if (r)
 			return r;
-		}
 	}
 	return 0;
 }
diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm_sdma.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm_sdma.c
index 50cc0779c340..c070c4a032eb 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm_sdma.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm_sdma.c
@@ -234,7 +234,6 @@ static int amdgpu_vm_sdma_update(struct amdgpu_vm_update_params *p,
 		dma_fence_get(fence);
 		r = drm_sched_job_add_dependency(&p->job->base, fence);
 		if (r) {
-			dma_fence_put(fence);
 			dma_resv_iter_end(&cursor);
 			return r;
 		}
-- 
2.53.0