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

[Asahi Lina]

On 14/07/2023 18.57, Christian König wrote:
> Am 14.07.23 um 11:49 schrieb 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.
>>
>> 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.
> 
> Well this is function is not about fences, this function is a callback
> for the entity.

That deals with dependency fences, which could have come from any 
arbitrary source, including another entity and another scheduler.

>>
>> 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.
> 
> I'm certainly trying to help here, but you seem to have unrealistic
> expectations.

I don't think expecting not to be told my changes are "outright 
nonsense" is an unrealistic expectation. If you think it is, maybe 
that's yet another indicator of the culture problems the kernel 
community has...

> I perfectly understand what you are trying to do, but you don't seem to
> understand that this functionality here isn't made for your use case.

I do, that's why I'm trying to change things. Right now, this 
functionality isn't even properly documented, which is why I thought it 
could be used for my use case, and slowly discovered otherwise. Daniel 
suggested documenting it, then fixing the mismatches between 
documentation and reality, which is what I'm doing here.

> We can adjust the functionality to better match your requirements, but
> you can't say it is broken because it doesn't work when you use it not
> in the way it is intended to be used.

I'm saying the idea that a random dma-buf holds onto a chain of 
references that prevents unloading a driver module that wrote into it 
(and keeps a bunch of random unrelated objects alive) is a broken state 
of affairs. It may or may not trickle down to actual problems for users 
(I would bet it does in some cases but I don't know for sure), but it's 
a situation that doesn't make any sense.

I know I'm triggering actual breakage with my new use case due to this, 
which is why I'm trying to improve things. But the current state of 
affairs just doesn't make any sense even if it isn't causing kernel 
oopses today with other drivers.

> You can go ahead and try to re-implement the functionality in Rust, but
> then I would reject that pointing out that this should probably be an
> extension to the existing code.

You keep rejecting my attempts at extending the existing code...

~~ Lina