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

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Date:   Mon, 21 Mar 2022 10:51:38 +0100
From:   Christian König <christian.koenig@....com>
To:     Robert Beckett <bob.beckett@...labora.com>,
        dri-devel@...ts.freedesktop.org, Huang Rui <ray.huang@....com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Matthew Auld <matthew.auld@...el.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/ttm: fix potential null ptr deref in when mem space
 alloc fails

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?

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@...labora.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;