[PATCH 2/3] drm/scheduler: Fix UAF in drm_sched_fence_get_timeline_name

[Asahi Lina]

On 14/07/2023 17.43, Christian König wrote:
> Am 14.07.23 um 10:21 schrieb Asahi Lina:
>> A signaled scheduler fence can outlive its scheduler, since fences are
>> independencly reference counted. 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 at asahilina.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 b2bbc8a68b30..17f35b0b005a 100644
>> --- a/drivers/gpu/drm/scheduler/sched_entity.c
>> +++ b/drivers/gpu/drm/scheduler/sched_entity.c
>> @@ -389,7 +389,12 @@ 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.
> 
> This is outright nonsense. As long as an entity for a scheduler exists
> it is not allowed to free up this scheduler.
> 
> So this function can't be called like this.
> 
>>    		 */
>>    		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 ef120475e7c6..06a0eebcca10 100644
>> --- a/drivers/gpu/drm/scheduler/sched_fence.c
>> +++ b/drivers/gpu/drm/scheduler/sched_fence.c
>> @@ -68,7 +68,7 @@ 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)
>> @@ -216,6 +216,8 @@ 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 e95b4837e5a3..4fa9523bd47d 100644
>> --- a/include/drm/gpu_scheduler.h
>> +++ b/include/drm/gpu_scheduler.h
>> @@ -305,6 +305,11 @@ 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];
> 
> This just mitigates the problem, but doesn't fix it.

Could you point out any remaining issues so we can fix them? Right now 
this absolutely *is* broken and this fixes the breakage I observed. If 
there are other bugs remaining, I'd like to know what they are so I can 
fix them.

> The real issue is that the hw fence is kept around much longer than that.

As far as I know, the whole point of scheduler fences is to decouple the 
hw fences from the consumers. I already talked with Daniel about this. 
The current behavior is broken. These fences can live forever. It is 
broken to require that they outlive the driver that produced them.

> Additional to that I'm not willing to increase the scheduler fence size
> like that just to decouple them from the scheduler.

Did you read my explanation on the cover letter as to how this is just 
broken right now? We need to fix this. If you have a better suggestion 
I'll take it. Doing nothing is not an option.

~~ Lina