Re: [PATCH] drm/ttm: fix potential null ptr deref in when mem space alloc fails

[Christian König]

Am 21.03.22 um 16:44 schrieb Robert Beckett:
> On 21/03/2022 09:51, Christian König wrote:
> > Am 18.03.22 um 20:50 schrieb Robert Beckett:
> > > when allocating a resource in place it is common to free the buffer's
> > > resource, then allocate a new resource in a different placement.
> > >
> > > e.g. amdgpu_bo_create_kernel_at calls ttm_resource_free, then calls
> > > ttm_bo_mem_space.
> >
> > Well yes I'm working the drivers towards this, but NAK at the moment. 
> > Currently bo->resource is never expected to be NULL.
> >
> > And yes I'm searching for this bug in amdgpu for quite a while. Where 
> > exactly does that happen?
>
> in my case, I am writing new code for i915 that does this. I will 
> switch it to allocate the new resource first, then free the old one if 
> successful.
>
> For the existing amd case, see 
> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgit.kernel.org%2Fpub%2Fscm%2Flinux%2Fkernel%2Fgit%2Ftorvalds%2Flinux.git%2Ftree%2Fdrivers%2Fgpu%2Fdrm%2Famd%2Famdgpu%2Famdgpu_object.c%3Fh%3Dv5.17%23n384&data=04%7C01%7Cchristian.koenig%40amd.com%7C81f91d39683e4991181008da0b51a8d0%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637834742606744919%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=mjHPvBStFM7tsYf%2BL9fYCWqddZuIaeza6BsRF3fAmao%3D&reserved=0
>
>
> amdgpu_bo_create_kernel_at calls ttm_resource_free, then calls 
> ttm_bo_mem_space. If the ttm_bo_mem_space call fails (e.g. due to 
> memory pressure), then the error path will try to deref bo->resource, 
> which will be null at that point.

Yeah, but that's a special handling only used during driver startup. We 
somehow have this on systems with DMA-buf sharing as well.

> to fix this, I honestly don't see a reason to not also have the safety 
> check for null there. It could check early and return an error if it 
> is null. I think that defensive programming here makes sense, better 
> than a null deref if someone programs it wrong.

Having it here is fine, the problem is you need to have that at tons of 
other places as well.

Maybe I should send you my WIP patch set for this? If you handle all the 
other cases as well I'm perfectly fine with this.

Regards,
Christian.

> > Amdgpu is supposed to allocate a new resource first, then do a swap 
> > and the free the old one.
> >
> > Thanks,
> > Christian.
> >
> > > In this situation, bo->resource will be null as it is cleared during
> > > the initial freeing of the previous resource.
> > > This leads to a null deref.
> > >
> > > Fixes: d3116756a710 (drm/ttm: rename bo->mem and make it a pointer)
> > >
> > > Signed-off-by: Robert Beckett <bob.beckett@xxxxxxxxxxxxx>
> > > ---
> > >   drivers/gpu/drm/ttm/ttm_bo.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/gpu/drm/ttm/ttm_bo.c 
> > > b/drivers/gpu/drm/ttm/ttm_bo.c
> > > index db3dc7ef5382..62b29ee7d040 100644
> > > --- a/drivers/gpu/drm/ttm/ttm_bo.c
> > > +++ b/drivers/gpu/drm/ttm/ttm_bo.c
> > > @@ -875,7 +875,7 @@ int ttm_bo_mem_space(struct ttm_buffer_object *bo,
> > >       }
> > >   error:
> > > -    if (bo->resource->mem_type == TTM_PL_SYSTEM && !bo->pin_count)
> > > +    if (bo->resource && bo->resource->mem_type == TTM_PL_SYSTEM && 
> > > !bo->pin_count)
> > >           ttm_bo_move_to_lru_tail_unlocked(bo);
> > >       return ret;