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

[Thomas Hellström]

On 5/28/21 4:10 PM, Christian König wrote:
> Am 28.05.21 um 09:33 schrieb Thomas Hellström:
>> On Fri, 2021-05-28 at 09:16 +0200, Christian König wrote:
>>> 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.
>> Yes, that would of course work as well. Keeping them on the system LRU
>> is IMO highly undesirable.
>>
>>> Going to pick this patch up, modifying it a bit more and then pushing
>>> it
>>> to drm-misc-fixes for upstreaming.
>> OK, I dropped the TTM fix for the purge-in-swap-notify from the i915
>> series, hoping that the reworked variant of this patch lands first.
>
> You will still need to add the second ttm_tt_populated() check since I 
> dropped that for the back which I want to push to -fixes.
>
> Regards,
> Christian.
>
OK, great. then you have my S-O-B on this patch.

BTW that original patch that added the ttm_tt_is_populated() was 
considered "LGTM" by you, except for this ttm_tt_is_populated(). So do I 
have an Acked-by: on that now?

That is

https://patchwork.freedesktop.org/patch/435833/?series=90681&rev=2

plus the check added?

Thanks,

Thomas




>>
>> Thanks,
>> Thomas
>>
>>> Thanks,
>>> Christian.
>>>
>>>> /Thomas
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>> Christian.
>>>>>
>>>>>> /Thomas
>>>>>>
>>
>