Re: [Intel-gfx] [PATCH v4 01/14] drm/ttm: stop calling tt_swapin in vm_access

Thomas Hellström
2021-09-21T11:37:45Z

HI, Christian,
On Tue, 2021-09-21 at 13:28 +0200, Christian König wrote:
> Am 21.09.21 um 13:01 schrieb Matthew Auld:
> > In commit:
> >
> > commit 09ac4fcb3f255e9225967c75f5893325c116cdbe
> > Author: Felix Kuehling <Felix.Kuehling@amd.com>
> > Date:   Thu Jul 13 17:01:16 2017 -0400
> >
> >      drm/ttm: Implement vm_operations_struct.access v2
> >
> > we added the vm_access hook, where we also directly call tt_swapin
> > for
> > some reason. If something is swapped-out then the ttm_tt must also
> > be
> > unpopulated, and since access_kmap should also call tt_populate, if
> > needed, then swapping-in will already be handled there.
>
> Sounds like you completely misunderstand what that is good for.
>
> This is for debugger attaching to a process and peek/poke into the
> VMA
> and completely unrelated to kmap.

I think what Matthew is saying is that there is a fallthrough to
TTM_PL_TT which calls

ttm_bo_vm_access_kmap

which calls

ttm_tt_populate().

So from my pow, unless there are other concerns, this is

Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>

>
> >
> > If anything, calling tt_swapin directly here would likely always
> > fail
> > since the tt->pages won't yet be populated, or worse since the tt-
> > >pages
> > array is never actually cleared in unpopulate this might lead to a
> > nasty
> > uaf.
>
> That's indeed true, but we just need to unconditionally call
> ttm_tt_populate() here instead.
>
> Regards,
> Christian.
>
> >
> > Fixes: 09ac4fcb3f25 ("drm/ttm: Implement
> > vm_operations_struct.access v2")
> > Signed-off-by: Matthew Auld <matthew.auld@intel.com>
> > Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> > Cc: Christian König <christian.koenig@amd.com>
> > ---
> >   drivers/gpu/drm/ttm/ttm_bo_vm.c | 5 -----
> >   1 file changed, 5 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c
> > b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> > index f56be5bc0861..5b9b7fd01a69 100644
> > --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
> > +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> > @@ -519,11 +519,6 @@ int ttm_bo_vm_access(struct vm_area_struct
> > *vma, unsigned long addr,
> >
> >         switch (bo->resource->mem_type) {
> >         case TTM_PL_SYSTEM:
> > -               if (unlikely(bo->ttm->page_flags &
> > TTM_PAGE_FLAG_SWAPPED)) {
> > -                       ret = ttm_tt_swapin(bo->ttm);
> > -                       if (unlikely(ret != 0))
> > -                               return ret;
> > -               }
> >                 fallthrough;
> >         case TTM_PL_TT:
> >                 ret = ttm_bo_vm_access_kmap(bo, offset, buf, len,
> > write);
>