Re: [PATCH v2] dma-buf: dma-fence: Fix potential NULL pointer dereference

[Boris Brezillon <boris.brezillon@collabora.com>]

On Mon, 29 Jun 2026 09:56:37 +0200
Philipp Stanner <phasta@kernel.org> wrote:

> 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>

> ---
> Changes since v1:
>   - Use ops pointer instead of memory barriers. (Christian)
>   - Rephrase commit message.
> ---
>  drivers/dma-buf/dma-fence.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> index c7ea1e75d38a..0a025dfdf131 100644
> --- a/drivers/dma-buf/dma-fence.c
> +++ b/drivers/dma-buf/dma-fence.c
> @@ -1170,7 +1170,7 @@ 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";
> @@ -1203,7 +1203,7 @@ 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_driver_name(fence);
>  	else
>  		return (const char __rcu *)"signaled-timeline";
> 
> base-commit: cdeb2ccd993ed8647adbbda2c3b103aa717fd6f7