Re: [PATCH] drm/nouveau: RCU-free the scheduler-containing nouveau_sched

2026-09-17 Thread lyude 
Reviewed-by: Lyude Paul 

Will push to drm-misc-fixes in just a moment

On Wed, 2026-09-02 at 10:27 +0900, Jonghyuk Kim(MalHyuk) wrote:
> struct nouveau_sched embeds a struct drm_gpu_scheduler (base).
> nouveau_sched_destroy() calls nouveau_sched_fini() (which does
> drm_sched_fini(&sched->base)) and then frees the object with plain
> kfree(sched).
> 
> drm_sched_fence_get_timeline_name() returns fence->sched->name, and
> the
> scheduler fence keeps a .release callback so it is not ops-detached
> on
> signalling.  A finished fence exported to userspace via drm_syncobj /
> sync_file therefore keeps pointing at &sched->base after
> nouveau_sched_destroy(),
> and a later get_timeline_name() -- reachable unprivileged through
> SYNC_IOC_FILE_INFO -- dereferences freed memory (KASAN slab-use-
> after-free
> read).
> 
> Per the dma-fence lifetime contract the exporter must keep the data
> backing a
> signalled fence alive for an RCU grace period.  Free the scheduler-
> containing
> object with kfree_rcu() instead of kfree().
> 
> Fixes: 5f03a507b29e ("drm/nouveau: implement 1:1 scheduler - entity
> relationship")
> Cc: [email protected]
> Signed-off-by: Jonghyuk Kim(MalHyuk) 
> ---
>  drivers/gpu/drm/nouveau/nouveau_sched.c | 2 +-
>  drivers/gpu/drm/nouveau/nouveau_sched.h | 1 +
>  2 files changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/nouveau/nouveau_sched.c
> b/drivers/gpu/drm/nouveau/nouveau_sched.c
> index 2cbae003d6de..48dbb851df2f 100644
> --- a/drivers/gpu/drm/nouveau/nouveau_sched.c
> +++ b/drivers/gpu/drm/nouveau/nouveau_sched.c
> @@ -516,7 +516,7 @@ nouveau_sched_destroy(struct nouveau_sched
> **psched)
>   struct nouveau_sched *sched = *psched;
>  
>   nouveau_sched_fini(sched);
> - kfree(sched);
> + kfree_rcu(sched, rcu);
>  
>   *psched = NULL;
>  }
> diff --git a/drivers/gpu/drm/nouveau/nouveau_sched.h
> b/drivers/gpu/drm/nouveau/nouveau_sched.h
> index 20cd1da8db73..51ce8dcf6285 100644
> --- a/drivers/gpu/drm/nouveau/nouveau_sched.h
> +++ b/drivers/gpu/drm/nouveau/nouveau_sched.h
> @@ -98,6 +98,7 @@ void nouveau_job_free(struct nouveau_job *job);
>  
>  struct nouveau_sched {
>   struct drm_gpu_scheduler base;
> + struct rcu_head rcu;
>   struct drm_sched_entity entity;
>   struct workqueue_struct *wq;
>   struct mutex mutex;

Re: [PATCH] drm/nouveau: RCU-free the scheduler-containing nouveau_sched

2026-09-01 Thread sashiko-bot 
Thank you for your contribution! Sashiko AI review found 1 potential issue(s) 
to consider:

Pre-existing issues:
- [High] Use-After-Free (Incomplete Fix)
--

commit a4cb3c10462917197ec6f8cdfe7a265ad10ef18a
Author: Jonghyuk Kim(MalHyuk) 

drm/nouveau: RCU-free the scheduler-containing nouveau_sched

This commit attempts to fix a use-after-free issue by deferring the freeing
of the scheduler object to an RCU callback.

> diff --git a/drivers/gpu/drm/nouveau/nouveau_sched.c 
> b/drivers/gpu/drm/nouveau/nouveau_sched.c
> index 8b9f935afe094..b3f02c490ecb6 100644
> --- a/drivers/gpu/drm/nouveau/nouveau_sched.c
> +++ b/drivers/gpu/drm/nouveau/nouveau_sched.c
> @@ -517,7 +517,7 @@ nouveau_sched_destroy(struct nouveau_sched **psched)
>   struct nouveau_sched *sched = *psched;
>  
>   nouveau_sched_fini(sched);
> - kfree(sched);
> + kfree_rcu(sched, rcu);

[Severity: High]
This isn't a bug introduced by this patch, but does this actually fix the
underlying use-after-free issue?

Using kfree_rcu() only defers freeing the scheduler by a single RCU grace
period. However, dma_fence objects exported to userspace (e.g., via
sync_file) can outlive this grace period since userspace controls the file
descriptor lifetime.

When userspace eventually queries the fence info, sync_fill_fence_info()
acquires a new rcu_read_lock() and calls dma_fence_timeline_name(), which
invokes drm_sched_fence_get_timeline_name():

drivers/gpu/drm/scheduler/sched_fence.c:drm_sched_fence_get_timeline_name() {
...
return (const char *)fence->sched->name;
}

Because the nouveau_sched memory is physically freed after the initial RCU
grace period elapses, this new RCU read-side critical section does not
protect it. Won't this unconditionally dereference fence->sched->name on
the already-freed memory, resulting in the exact same use-after-free read
this patch intends to fix?

>  
>   *psched = NULL;
>  }

-- 
Sashiko AI review · 
https://sashiko.dev/#/patchset/[email protected]?part=1