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

[Christian König]

Am 27.05.21 um 17:01 schrieb Thomas Hellström:
> 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,

Well then that would be a bug in itself.

>> Why not move that to the check above as well?
> Because the BO may become unpopulated in swap_notify(), i915, like
> vmwgfx, sometimes sets up gpu bindings from system, and when we get a
> notification from user-space that those are purgeable, we don't want to
> purge immediately but wait for a potential swapout.

Uff, good point. But then we need to check that at both locations I think.

Because populating the TT object currently doesn't put the BO back on 
the LRU eventually.

Christian.

>
> /Thomas
>
>
>> Christian.
>>
>>>    out:
>>>    
>>>          /*
>>> @@ -1225,6 +1236,9 @@ int ttm_bo_swapout(struct ttm_buffer_object
>>> *bo, struct ttm_operation_ctx *ctx,
>>>          if (locked)
>>>                  dma_resv_unlock(bo->base.resv);
>>>          ttm_bo_put(bo);
>>> +
>>> +       /* Don't break locking rules. */
>>> +       WARN_ON(ret == -EBUSY);
>>>          return ret;
>>>    }
>>>    
>>> diff --git a/drivers/gpu/drm/ttm/ttm_device.c
>>> b/drivers/gpu/drm/ttm/ttm_device.c
>>> index 460953dcad11..eaa7487ae404 100644
>>> --- a/drivers/gpu/drm/ttm/ttm_device.c
>>> +++ b/drivers/gpu/drm/ttm/ttm_device.c
>>> @@ -143,14 +143,12 @@ int ttm_device_swapout(struct ttm_device
>>> *bdev, struct ttm_operation_ctx *ctx,
>>>    
>>>                  for (j = 0; j < TTM_MAX_BO_PRIORITY; ++j) {
>>>                          list_for_each_entry(bo, &man->lru[j], lru)
>>> {
>>> -                               uint32_t num_pages;
>>> +                               pgoff_t num_pages;
>>>    
>>> -                               if (!bo->ttm ||
>>> -                                   bo->ttm->page_flags &
>>> TTM_PAGE_FLAG_SG ||
>>> -                                   bo->ttm->page_flags &
>>> TTM_PAGE_FLAG_SWAPPED)
>>> +                               if (!READ_ONCE(bo->ttm))
>>>                                          continue;
>>>    
>>> -                               num_pages = bo->ttm->num_pages;
>>> +                               num_pages = bo->base.size >>
>>> PAGE_SHIFT;
>>>                                  ret = ttm_bo_swapout(bo, ctx,
>>> gfp_flags);
>>>                                  /* ttm_bo_swapout has dropped the
>>> lru_lock */
>>>                                  if (!ret)
>