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@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;
