linux-kernel - Re: [PATCH 2/3] drm/scheduler: Fix UAF in drm_sched_fence_get_timeline

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Date:   Fri, 14 Jul 2023 18:49:44 +0900
From:   Asahi Lina <lina@...hilina.net>
To:     Christian König <christian.koenig@....com>,
        Luben Tuikov <luben.tuikov@....com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Sumit Semwal <sumit.semwal@...aro.org>
Cc:     Faith Ekstrand <faith.ekstrand@...labora.com>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        linux-media@...r.kernel.org, asahi@...ts.linux.dev
Subject: Re: [PATCH 2/3] drm/scheduler: Fix UAF in
 drm_sched_fence_get_timeline_name

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@...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 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.

As I already explained, the fences can outlive their scheduler. That 
means *this* entity certainly exists for *this* scheduler, but the 
*dependency* fence might have come from a past scheduler that was 
already destroyed along with all of its entities, and its address reused.

Christian, I'm really getting tired of your tone. I don't appreciate 
being told my comments are "outright nonsense" when you don't even take 
the time to understand what the issue is and what I'm trying to 
do/document. If you aren't interested in working with me, I'm just going 
to give up on drm_sched, wait until Rust gets workqueue support, and 
reimplement it in Rust. You can keep your broken fence lifetime 
semantics and I'll do my own thing.

~~ Lina