Messages in this thread | | | Date | Mon, 21 Mar 2022 15:44:13 +0000 | Subject | Re: [PATCH] drm/ttm: fix potential null ptr deref in when mem space alloc fails | From | Robert Beckett <> |
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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://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c?h=v5.17#n384
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.
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.
> > 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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