dma-buf: dma-fence: Fix potential NULL pointer dereference

The commit mentioned in the fixes tag below introduced a mechanism
through which fence producers can fully decouple from fence consumers.
This, desirable, mechanism is based on the fence's signaled-bit as the
"decoupling point".

A sophisticated interaction between RCU and atomic instructions attempts
to ensure that fence consumers can still interact with fence producers
through the dma_fence_ops (callback pointers into the producer).

This is the desired behavior: to check for decoupling, the signaled-bit
is first checked. If it's not yet signaled, RCU ensures that the ops
pointer cannot yet be NULL.

Hereby, dma_fence_signal_timestamp_locked() first sets the signaled-bit,
and then sets the ops pointer to NULL. Readers first load the ops
pointer, and then check through the signaled-bit whether the pointer can
legally be accessed.

These set and load operations could occur out of order on weakly ordered
platforms. This problem can be solved very elegantly by using the ops
pointer itself as the synchronization point. The pointer is either NULL,
or cannot become NULL while it is being used thanks to RCU.

Replace the signaled-bit check in dma_fence_timeline_name() and
dma_fence_driver_name().

Cc: stable@vger.kernel.org
Fixes: f4cc3ab824d6 ("dma-buf: protected fence ops by RCU v8")
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20260629075636.2513214-2-phasta@kernel.org
Signed-off-by: Christian König <christian.koenig@amd.com>

diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
index 7120610f4850f7467e0af8eff02adbd0df6b4a8e..151c3344459c69dfee298d78d5712994ccf9b1a4 100644 (file)
--- a/drivers/dma-buf/dma-fence.c
+++ b/drivers/dma-buf/dma-fence.c
@@ -1167,7 +1167,7 @@ const char __rcu *dma_fence_driver_name(struct dma_fence *fence)
 
        /* RCU protection is required for safe access to returned string */
        ops = rcu_dereference(fence->ops);
-       if (!dma_fence_test_signaled_flag(fence))
+       if (ops)
                return (const char __rcu *)ops->get_driver_name(fence);
        else
                return (const char __rcu *)"detached-driver";
@@ -1200,7 +1200,7 @@ const char __rcu *dma_fence_timeline_name(struct dma_fence *fence)
 
        /* RCU protection is required for safe access to returned string */
        ops = rcu_dereference(fence->ops);
-       if (!dma_fence_test_signaled_flag(fence))
+       if (ops)
                return (const char __rcu *)ops->get_timeline_name(fence);
        else
                return (const char __rcu *)"signaled-timeline";