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

[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.

/Thomas





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