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

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

[Philipp Stanner]

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>
---
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
-- 
2.54.0

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

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Replacing the signaled-bit check with an `ops` null-check causes driver callbacks to be executed on signaled fences, violating the dma-fence contract and leading to a Use-After-Free.
--

commit f3771b2534b5990287b2b373aaf5a26b9dea2923
Author: Philipp Stanner <phasta@kernel.org>

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

The patch removes the signaled-bit check in dma_fence_timeline_name() and
dma_fence_driver_name(), replacing it with a check for a non-NULL ops pointer
to prevent potential out-of-order execution issues.

> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> index c7ea1e75d38a2..0a025dfdf131e 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)

[Severity: High]
Does this expose a use-after-free for signaled fences?

If a fence has a .release or .wait callback,
dma_fence_signal_timestamp_locked() does not set fence->ops to NULL. For
those fences, ops will remain non-NULL indefinitely even after they are
signaled.

By removing the signaled-bit check, won't ops->get_driver_name(fence) be
called on signaled fences? The dma-fence contract states that backing data
can be freed after an RCU grace period once the fence is signaled, so calling
this callback could access freed memory.

>  		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)

[Severity: High]
Could this lead to the same use-after-free as above?

Since ops can remain non-NULL for signaled fences that provide a .release
or .wait callback, this allows the ops callback to be executed after the
fence is signaled and its backing data is potentially freed.

>  		return (const char __rcu *)ops->get_driver_name(fence);
>  	else
>  		return (const char __rcu *)"signaled-timeline";

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260629075636.2513214-2-phasta@kernel.org?part=1

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

[Boris Brezillon]

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

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

[Christian König]

On 6/29/26 09:56, Philipp Stanner 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: Christian König <christian.koenig@amd.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

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

[Danilo Krummrich]

On 6/29/26 9:56 AM, Philipp Stanner 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: Danilo Krummrich <dakr@kernel.org>

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

[Philipp Stanner]

On Mon, 2026-06-29 at 10:49 +0200, Christian König wrote:
> On 6/29/26 09:56, Philipp Stanner wrote:
> > 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: Christian König <christian.koenig@amd.com>

As the maintainer you push that one, don't you Christian?

Regards
P.

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

[Christian König]

On 6/29/26 12:43, Philipp Stanner wrote:
> On Mon, 2026-06-29 at 10:49 +0200, Christian König wrote:
>> On 6/29/26 09:56, Philipp Stanner wrote:
>>> 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: Christian König <christian.koenig@amd.com>
> 
> As the maintainer you push that one, don't you Christian?

Already done so, but you based the patch on the wrong branch and I had to manually rebase it to drm-misc-fixes.

Christian.

> 
> Regards
> P.