In the Linux kernel, the following vulnerability has been resolved: drm/drm_exec: fix up contended obj when num_objects is 0 drm_exec_prep

In the Linux kernel, the following vulnerability has been resolved:

drm/drm_exec: fix up contended obj when num_objects is 0

drm_exec_prepare_array() silently returns success without calling
drm_exec_lock_contended() when num_objects is zero. This breaks the
invariant upheld by drm_exec_lock_obj(), where every entry point into
the locking sequence must first attempt to lock any previously
contended object before proceeding.

Drivers that chain multiple drm_exec_prepare_array() calls per
drm_exec_until_all_locked() iteration (e.g. amdgpu's userq signal/wait
ioctls, which prepare separate read and write BO arrays) can pass an
empty array for one of the two calls. If contention is hit while
preparing the non-empty array, exec->contended is set and the loop
retries; on retry, the empty-array call preceding it is a no-op that
never clears exec->contended, so drm_exec_retry_on_contention()
immediately jumps back to the top of the loop without ever reaching
the call that would resolve the contention. This spins forever.

Fix it by having drm_exec_prepare_array() call drm_exec_lock_contended()
directly when num_objects is zero, so a pending contended object dont
loop infinitely.