[RFC PATCH] drm/ttm: Fix swapping dereferences of freed memory

[Christian König]

Am 27.05.21 um 17:51 schrieb Thomas Hellström:
> On Thu, 2021-05-27 at 17:32 +0200, Christian König wrote:
>> Am 27.05.21 um 17:05 schrieb Thomas Hellström:
>>> On Thu, 2021-05-27 at 17:01 +0200, Thomas Hellström wrote:
>>>> On Thu, 2021-05-27 at 16:54 +0200, Christian König wrote:
>>>>> Am 27.05.21 um 16:19 schrieb Thomas Hellström:
>>>>>> The swapping code was dereference bo->ttm pointers without
>>>>>> having
>>>>>> the
>>>>>> dma-resv lock held. Also it might try to swap out unpopulated
>>>>>> bos.
>>>>>>
>>>>>> Fix this by moving the bo->ttm dereference until we have the
>>>>>> reservation
>>>>>> lock. Check that the ttm_tt is populated after the
>>>>>> swap_notify
>>>>>> callback.
>>>>>>
>>>>>> Signed-off-by: Thomas Hellström
>>>>>> <thomas.hellstrom at linux.intel.com>
>>>>>> ---
>>>>>>     drivers/gpu/drm/ttm/ttm_bo.c     | 16 +++++++++++++++-
>>>>>>     drivers/gpu/drm/ttm/ttm_device.c |  8 +++-----
>>>>>>     2 files changed, 18 insertions(+), 6 deletions(-)
>>>>>>
>>>>>> diff --git a/drivers/gpu/drm/ttm/ttm_bo.c
>>>>>> b/drivers/gpu/drm/ttm/ttm_bo.c
>>>>>> index 9f53506a82fc..86213d37657b 100644
>>>>>> --- a/drivers/gpu/drm/ttm/ttm_bo.c
>>>>>> +++ b/drivers/gpu/drm/ttm/ttm_bo.c
>>>>>> @@ -1163,6 +1163,16 @@ int ttm_bo_swapout(struct
>>>>>> ttm_buffer_object
>>>>>> *bo, struct ttm_operation_ctx *ctx,
>>>>>>           if (!ttm_bo_evict_swapout_allowable(bo, ctx, &place,
>>>>>> &locked, NULL))
>>>>>>                   return -EBUSY;
>>>>>>     
>>>>>> +       dma_resv_assert_held(bo->base.resv);
>>>>>> +
>>>>>> +       if (!bo->ttm ||
>>>>>> +           bo->ttm->page_flags & TTM_PAGE_FLAG_SG ||
>>>>>> +           bo->ttm->page_flags & TTM_PAGE_FLAG_SWAPPED) {
>>>>>> +               if (locked)
>>>>>> +                       dma_resv_unlock(bo->base.resv);
>>>>>> +               return -EBUSY;
>>>>>> +       }
>>>>>> +
>>>>>>           if (!ttm_bo_get_unless_zero(bo)) {
>>>>>>                   if (locked)
>>>>>>                           dma_resv_unlock(bo->base.resv);
>>>>>> @@ -1215,7 +1225,8 @@ int ttm_bo_swapout(struct
>>>>>> ttm_buffer_object
>>>>>> *bo, struct ttm_operation_ctx *ctx,
>>>>>>           if (bo->bdev->funcs->swap_notify)
>>>>>>                   bo->bdev->funcs->swap_notify(bo);
>>>>>>     
>>>>>> -       ret = ttm_tt_swapout(bo->bdev, bo->ttm, gfp_flags);
>>>>>> +       if (ttm_tt_is_populated(bo->ttm))
>>>>>> +               ret = ttm_tt_swapout(bo->bdev, bo->ttm,
>>>>>> gfp_flags);
>>>>> Exactly that is what I won't recommend. We would try to swap
>>>>> out
>>>>> the
>>>>> same BO over and over again with that.
>>>> But we wouldn't since the BO is taken off the LRU and never re-
>>>> added,
>>>>
>>>>
>>> In fact, we'd probably might want to take the !bo->ttm bos off the
>>> LRU
>>> as well..
>> No, we don't want to take any BOs of the LRU unless they are pinned.
>>
>> Adding a TT object or populating it doesn't necessarily put the BO
>> back
>> to the LRU.
> OK, but swapped bos are also taken off the LRU list so these
> unpopulated bos are just taking the same path. Only difference to
> swapped is that they don't get read back on re-populate, but typically
> cleared.
>
> But what would be the point of keeping swapped-out bos on the LRU
> list?, particularly when we're iterating under a spinlock?
> Shouldn't we try to re-add to LRU (if not already on an LRU) just
> before populating? There aren't really that many calls in core TTM.

I want to avoid removing BOs from the LRU as much as possible since we 
forgot on multiple places that we want to re-add them.

Conceptual I think the swapped BOs should have a separate memory domain, 
this way we can ignore them cleanly when swapping things out.

Going to pick this patch up, modifying it a bit more and then pushing it 
to drm-misc-fixes for upstreaming.

Thanks,
Christian.

>
> /Thomas
>
>
>
>
>
>> Christian.
>>
>>> /Thomas
>>>
>