Re: [PATCH 1/2] dma-buf/dma-fence: fix checking signaling bit for timeline and driver name

[Philipp Stanner <phasta@mailbox.org>]

On Wed, 2026-09-09 at 15:14 +0200, Christian König wrote:
> The patch "dma-buf: dma-fence: Fix potential NULL pointer dereference"
> changed the check to test for the ops pointer instead of the signaled
> bit to avoid a potential NULL dereference when the ops pointer has been
> cleared.
> 
> The problem is now that the ops pointer is cleared only when neither the
> release nor the wait callback is implemented and this isn't true for a lot
> of dma_fence implementations yet. So those implementations lost the RCU
> protection after signaling of the returned string resulting in potential
> use after free.
> 
> Add the signaling check additional to the ops pointer check so that we
> have both the protection against NULL dereference as well as the RCU
> protection after signaling for the returned string.
> 
> Signed-off-by: Christian König <christian.koenig@amd.com>
> Fixes: 035219a760ed ("dma-buf: dma-fence: Fix potential NULL pointer dereference")
> CC: stable@vger.kernel.org # 7.2+
> Reported-by: Jonghyuk Kim(MalHyuk) <malhyuk97@gmail.com>
> Tested-by: Jonghyuk Kim(MalHyuk) <malhyuk97@gmail.com>
> ---
>  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 05090fb0fd5a..e92f9df8d63c 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 (ops)
> +	if (!dma_fence_test_signaled_flag(fence) && ops)

It's probably a good idea to comment at an appropriate place (I suggest
here and at dma_fence_ops) why this is necessary, because at all other
places the ops-pointer can serve as decoupling point.

Reason is simply to also support drivers that implement release() and 
wait(), preventing ops from being set to NULL.


P.