i915: don't use a vma that didn't match the context VM

In eb_lookup_vma(), the code checks that the context vm matches before
incrementing the i915 vma usage count, but for the non-matching case it
didn't clear the non-matching vma pointer, so it would then mistakenly
be returned, causing potential UaF and refcount issues.

Reported-by: Yassine Mounir <sosohero200@gmail.com>
Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: torvalds

drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c

@@ -898,6 +898,8 @@ static struct i915_vma *eb_lookup_vma(struct i915_execbuffer *eb, u32 handle)
 		vma = radix_tree_lookup(&eb->gem_context->handles_vma, handle);
 		if (likely(vma && vma->vm == vm))
 			vma = i915_vma_tryget(vma);
+		else
+			vma = NULL;
 		rcu_read_unlock();
 		if (likely(vma))
 			return vma;