[PATCH 15901/15901] drm/vmwgfx: fix NULL pointer dereference in vmw_validation_bo_fence()

[popov . nkv]

From: Vladimir Popov <popov.nkv@xxxxxxxxx>

If vmw_execbuf_fence_commands() call fails in
vmw_kms_helper_validation_finish(), it sets *p_fence = NULL. If 
ctx->bo_list is not empty, the caller, vmw_kms_helper_validation_finish(),
passes the fence through a chain of functions to dma_fence_is_array(),
which causes a NULL pointer dereference in dma_fence_is_array():

vmw_kms_helper_validation_finish() // pass NULL fence
  vmw_validation_done()
    vmw_validation_bo_fence()
      ttm_eu_fence_buffer_objects() // pass NULL fence
        dma_resv_add_fence()
          dma_fence_is_container()
            dma_fence_is_array() // NULL deref

Fix this by adding a NULL check in vmw_validation_bo_fence(): if the fence
is NULL, fall back to ttm_eu_backoff_reservation()to safely release
the buffer object reservations without attempting to add a NULL fence to
dma_resv. This is safe because when fence is NULL, vmw_fallback_wait()
has already been called inside vmw_execbuf_fence_commands() to synchronize
the GPU.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 038ecc503236 ("drm/vmwgfx: Add a validation module v2")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Vladimir Popov <popov.nkv@xxxxxxxxx>
---
 drivers/gpu/drm/vmwgfx/vmwgfx_validation.h | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_validation.h b/drivers/gpu/drm/vmwgfx/vmwgfx_validation.h
index 353d837907d8..fc04555ca505 100644
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_validation.h
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_validation.h
@@ -127,16 +127,23 @@ vmw_validation_bo_reserve(struct vmw_validation_context *ctx,
  * vmw_validation_bo_fence - Unreserve and fence buffer objects registered
  * with a validation context
  * @ctx: The validation context
+ * @fence: Fence with which to fence all buffer objects taking part in the
+ * command submission.
  *
  * This function unreserves the buffer objects previously reserved using
- * vmw_validation_bo_reserve, and fences them with a fence object.
+ * vmw_validation_bo_reserve, and fences them with a fence object if the
+ * given fence object is not NULL.
  */
 static inline void
 vmw_validation_bo_fence(struct vmw_validation_context *ctx,
 			struct vmw_fence_obj *fence)
 {
-	ttm_eu_fence_buffer_objects(&ctx->ticket, &ctx->bo_list,
-				    (void *) fence);
+	/* fence is able to be NULL if vmw_execbuf_fence_commands() fails */
+	if (fence)
+		ttm_eu_fence_buffer_objects(&ctx->ticket, &ctx->bo_list,
+					    (void *)fence);
+	else
+		ttm_eu_backoff_reservation(&ctx->ticket, &ctx->bo_list);
 }
 
 /**
-- 
2.43.0