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

[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..

/Thomas