Messages in this thread | | | Date | Tue, 22 Mar 2022 08:17:32 +0100 | Subject | Re: [PATCH] drm/ttm: fix potential null ptr deref in when mem space alloc fails | From | Christian König <> |
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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@collabora.com> >>> --- >>> 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; >>
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