[PATCH 2/2] dma-buf/dma-fence: Order setting of the signaled-bit properly

[Philipp Stanner]

dma_fence_is_signaled() can return true *while* the fence is still
signaling. This is fragile and has been involved in at least two bugs:

	commit c8a5d5ea3ba6 ("nouveau: fix client work fence deletion race")
	commit 7c703a7d3f2b ("drm/amdgpu: Fix one use-after-free of VM")

The signaled-bit is set under lock-protection but read without it.
Adding the lock has been rejected on the grounds of the lock-acquisition
shooting down cache lines of parties that merely want to check whether
the fence is already signaled.

Make sure the bit can only become visible as 1 once the callbacks have
ran. To achieve this, read the bit with test_bit_acquire() and set it
with set_bit_release().

Signed-off-by: Philipp Stanner <phasta@xxxxxxxxxx>
---
 drivers/dma-buf/dma-fence.c | 11 +++++++++--
 include/linux/dma-fence.h   |  6 +++++-
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
index bd58688b81a7..07d499fa35ac 100644
--- a/drivers/dma-buf/dma-fence.c
+++ b/drivers/dma-buf/dma-fence.c
@@ -359,8 +359,7 @@ void dma_fence_signal_timestamp_locked(struct dma_fence *fence,
 
 	dma_fence_assert_held(fence);
 
-	if (unlikely(test_and_set_bit(DMA_FENCE_FLAG_SIGNALED_BIT,
-				      &fence->flags)))
+	if (unlikely(dma_fence_test_signaled_flag(fence)))
 		return;
 
 	trace_dma_fence_signaled(fence);
@@ -384,6 +383,14 @@ void dma_fence_signal_timestamp_locked(struct dma_fence *fence,
 		INIT_LIST_HEAD(&cur->node);
 		cur->func(fence, cur);
 	}
+
+	/*
+	 * This orders the signaled-state with the test_bit_acquire() in
+	 * dma_fence_test_signaled_flag(). This ensures that functions like
+	 * dma_fence_is_signaled() can only ever return true once signaling is
+	 * completed (i.e., after the callbacks above ran).
+	 */
+	set_bit_release(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags);
 }
 EXPORT_SYMBOL(dma_fence_signal_timestamp_locked);
 
diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
index dd07d128adc3..61522027af41 100644
--- a/include/linux/dma-fence.h
+++ b/include/linux/dma-fence.h
@@ -496,7 +496,11 @@ const char __rcu *dma_fence_timeline_name(struct dma_fence *fence);
 static inline bool
 dma_fence_test_signaled_flag(struct dma_fence *fence)
 {
-	return test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags);
+	/*
+	 * The counterpart of this ordered operation can be found in
+	 * dma_fence_signal_timestamp_locked().
+	 */
+	return test_bit_acquire(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags);
 }
 
 /**
-- 
2.56.0