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

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] Use-After-Free (Incomplete Fix)
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commit a4cb3c10462917197ec6f8cdfe7a265ad10ef18a
Author: Jonghyuk Kim(MalHyuk) <malhyuk97@gmail.com>

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;
>  }

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Sashiko AI review · https://sashiko.dev/#/patchset/20260902012717.880724-1-malhyuk97@gmail.com?part=1