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

[Christian König]

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.

Christian.

>
> /Thomas
>