linux-kernel - Re: [PATCH] drm/scheduler: Fix UAF in drm_sched_fence_get_timeline

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Date:   Thu, 6 Apr 2023 10:29:57 +0200
From:   Christian König <christian.koenig@....com>
To:     Asahi Lina <lina@...hilina.net>,
        Luben Tuikov <luben.tuikov@....com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Sumit Semwal <sumit.semwal@...aro.org>
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        linux-media@...r.kernel.org, asahi@...ts.linux.dev
Subject: Re: [PATCH] drm/scheduler: Fix UAF in
 drm_sched_fence_get_timeline_name

Am 05.04.23 um 18:34 schrieb Asahi Lina:
> A signaled scheduler fence can outlive its scheduler, since fences are
> independently reference counted.

Well that is actually not correct. Schedulers are supposed to stay 
around until the hw they have been driving is no longer present.

E.g. the reference was scheduler_fence->hw_fence->driver->scheduler.

Your use case is now completely different to that and this won't work 
any more.

This here might just be the first case where that breaks.

Regards,
Christian.

>   Therefore, we can't reference the
> scheduler in the get_timeline_name() implementation.
>
> Fixes oopses on `cat /sys/kernel/debug/dma_buf/bufinfo` when shared
> dma-bufs reference fences from GPU schedulers that no longer exist.
>
> Signed-off-by: Asahi Lina <lina@...hilina.net>
> ---
>   drivers/gpu/drm/scheduler/sched_entity.c | 7 ++++++-
>   drivers/gpu/drm/scheduler/sched_fence.c  | 4 +++-
>   include/drm/gpu_scheduler.h              | 5 +++++
>   3 files changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/scheduler/sched_entity.c b/drivers/gpu/drm/scheduler/sched_entity.c
> index 15d04a0ec623..8b3b949b2ce8 100644
> --- a/drivers/gpu/drm/scheduler/sched_entity.c
> +++ b/drivers/gpu/drm/scheduler/sched_entity.c
> @@ -368,7 +368,12 @@ static bool drm_sched_entity_add_dependency_cb(struct drm_sched_entity *entity)
>   
>   		/*
>   		 * Fence is from the same scheduler, only need to wait for
> -		 * it to be scheduled
> +		 * it to be scheduled.
> +		 *
> +		 * Note: s_fence->sched could have been freed and reallocated
> +		 * as another scheduler. This false positive case is okay, as if
> +		 * the old scheduler was freed all of its jobs must have
> +		 * signaled their completion fences.
>   		 */
>   		fence = dma_fence_get(&s_fence->scheduled);
>   		dma_fence_put(entity->dependency);
> diff --git a/drivers/gpu/drm/scheduler/sched_fence.c b/drivers/gpu/drm/scheduler/sched_fence.c
> index 7fd869520ef2..33b145dfa38c 100644
> --- a/drivers/gpu/drm/scheduler/sched_fence.c
> +++ b/drivers/gpu/drm/scheduler/sched_fence.c
> @@ -66,7 +66,7 @@ static const char *drm_sched_fence_get_driver_name(struct dma_fence *fence)
>   static const char *drm_sched_fence_get_timeline_name(struct dma_fence *f)
>   {
>   	struct drm_sched_fence *fence = to_drm_sched_fence(f);
> -	return (const char *)fence->sched->name;
> +	return (const char *)fence->sched_name;
>   }
>   
>   static void drm_sched_fence_free_rcu(struct rcu_head *rcu)
> @@ -168,6 +168,8 @@ void drm_sched_fence_init(struct drm_sched_fence *fence,
>   	unsigned seq;
>   
>   	fence->sched = entity->rq->sched;
> +	strlcpy(fence->sched_name, entity->rq->sched->name,
> +		sizeof(fence->sched_name));
>   	seq = atomic_inc_return(&entity->fence_seq);
>   	dma_fence_init(&fence->scheduled, &drm_sched_fence_ops_scheduled,
>   		       &fence->lock, entity->fence_context, seq);
> diff --git a/include/drm/gpu_scheduler.h b/include/drm/gpu_scheduler.h
> index 9db9e5e504ee..49f019731891 100644
> --- a/include/drm/gpu_scheduler.h
> +++ b/include/drm/gpu_scheduler.h
> @@ -295,6 +295,11 @@ struct drm_sched_fence {
>            * @lock: the lock used by the scheduled and the finished fences.
>            */
>   	spinlock_t			lock;
> +        /**
> +         * @sched_name: the name of the scheduler that owns this fence. We
> +         * keep a copy here since fences can outlive their scheduler.
> +         */
> +	char sched_name[16];
>           /**
>            * @owner: job owner for debugging
>            */
>
> ---
> base-commit: fe15c26ee26efa11741a7b632e9f23b01aca4cc6
> change-id: 20230406-scheduler-uaf-1-994ec34cac93
>
> Thank you,
> ~~ Lina
>