[PATCH][next] dma-buf: Fix NULL pointer dereference in dma_fence_enable_sw_signaling()

["Gustavo A. R. Silva" <gustavoars@xxxxxxxxxx>]

Currently, a NULL pointer dereference will happen in function
`dma_fence_enable_sw_signaling()` (at line 615), in case `chain`
is not allocated in `mock_chain()` and this function returns
`NULL` (at line 86). See below:

drivers/dma-buf/st-dma-fence-chain.c:
 86         chain = mock_chain(NULL, f, 1);
 87         if (!chain)
 88                 err = -ENOMEM;
 89
 90         dma_fence_enable_sw_signaling(chain);

drivers/dma-buf/dma-fence.c:
 611 void dma_fence_enable_sw_signaling(struct dma_fence *fence)
 612 {
 613         unsigned long flags;
 614
 615         spin_lock_irqsave(fence->lock, flags);
			       ^^^^^^^^^^^
				    |
			  NULL pointer reference
			  if fence == NULL

 616         __dma_fence_enable_signaling(fence);
 617         spin_unlock_irqrestore(fence->lock, flags);
 618 }

Fix this by adding a NULL check before dereferencing `fence` in
`dma_fence_enable_sw_signaling()`. This will prevent any other NULL
pointer dereference when the `fence` passed as an argument is `NULL`.

Addresses-Coverity: ("Dereference after null check")
Fixes: d62c43a953ce ("dma-buf: Enable signaling on fence for selftests")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
---
 drivers/dma-buf/dma-fence.c | 9 ++++++++-
 include/linux/dma-fence.h   | 2 +-
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
index 8aa8f8cb7071..4d2f13560d0f 100644
--- a/drivers/dma-buf/dma-fence.c
+++ b/drivers/dma-buf/dma-fence.c
@@ -607,14 +607,21 @@ static bool __dma_fence_enable_signaling(struct dma_fence *fence)
  * This will request for sw signaling to be enabled, to make the fence
  * complete as soon as possible. This calls &dma_fence_ops.enable_signaling
  * internally.
+ *
+ * Returns 0 on success and a negative error value when @fence is NULL.
  */
-void dma_fence_enable_sw_signaling(struct dma_fence *fence)
+int dma_fence_enable_sw_signaling(struct dma_fence *fence)
 {
 	unsigned long flags;
 
+	if (!fence)
+		return -EINVAL;
+
 	spin_lock_irqsave(fence->lock, flags);
 	__dma_fence_enable_signaling(fence);
 	spin_unlock_irqrestore(fence->lock, flags);
+
+	return 0;
 }
 EXPORT_SYMBOL(dma_fence_enable_sw_signaling);
 
diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
index ebe78bd3d121..1e4025e925e6 100644
--- a/include/linux/dma-fence.h
+++ b/include/linux/dma-fence.h
@@ -399,7 +399,7 @@ int dma_fence_add_callback(struct dma_fence *fence,
 			   dma_fence_func_t func);
 bool dma_fence_remove_callback(struct dma_fence *fence,
 			       struct dma_fence_cb *cb);
-void dma_fence_enable_sw_signaling(struct dma_fence *fence);
+int dma_fence_enable_sw_signaling(struct dma_fence *fence);
 
 /**
  * dma_fence_is_signaled_locked - Return an indication if the fence
-- 
2.34.1