Re: [PATCH] drm/ttm: Fix UAF on dma-buf attach failure for sg BOs

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

Hi, Nitin

On Wed, 2026-07-01 at 11:56 +0530, Nitin Gote wrote:
> When a dma-buf importer creates a ttm_bo_type_sg BO with bo-
> >base.resv
> pointing at the exporter's dma_buf->resv and dma_buf_dynamic_attach()
> fails, no dma_buf reference is held. The exporter can be freed before
> the delayed_delete worker calls dma_resv_lock(bo->base.resv), causing
> a
> use-after-free:
> 
>   Oops: general protection fault, probably for non-canonical address
>         0x6b6b6b6b6b6b6b9c
>   Workqueue: ttm ttm_bo_delayed_delete [ttm]
>   RIP: 0010:mutex_can_spin_on_owner+0x3f/0xc0
> 
> ttm_bo_individualize_resv() skips the resv swap for all sg BOs to
> keep
> the shared resv available for delayed_delete to release the dma-buf
> mapping. A BO whose attach never succeeded has no mapping to release,
> yet it keeps bo->base.resv pointing at the exporter resv that
> delayed_delete later locks once the exporter is gone.
> 
> Fix this by checking bo->base.import_attach, which is set only after
> a
> successful attach. The check is placed after dma_resv_copy_fences()
> so
> successful imports still copy fences to _resv before returning,
> keeping
> the shared resv for delayed_delete. Failed imports fall through to
> swap
> resv to _resv, so delayed_delete never locks the stale exporter resv.
> 
> Closes:
> https://gitlab.freedesktop.org/drm/xe/kernel/-/work_items/8023
> Fixes: d99fbd9aab62 ("drm/ttm: Always take the bo delayed cleanup
> path for imported bos")
> Cc: stable@vger.kernel.org # v6.8+
> Cc: Thomas Hellstrom <thomas.hellstrom@linux.intel.com>
> Cc: Christian Konig <christian.koenig@amd.com>
> Cc: Matthew Auld <matthew.auld@intel.com>
> Assisted-by: GitHub_Copilot:claude-sonnet-4.6
> Signed-off-by: Nitin Gote <nitin.r.gote@intel.com>
> ---
> Hi Thomas/Christian,
> Thank you for the review. Addressed the v3 review comments in this 
> v4 version.
> 
> v4:
> - Moved import_attach check to after dma_resv_copy_fences() so fences
>   are copied before returning for successful imports (Thomas).
> - Removed exporter-alive claim from commit message (Thomas).

That's not sufficient. What I meant was that this invalidates the
approach in its current form:

A			B
prime_import()		
exported_get();
exported_lock();
bo_create();		lru_walk():
attach_fail();		bo_get();
bo_put();		
exported_unlock();	bo_lock() // exporter_lock
exporter_put();		
exporter_free();	
			bo_unlock(); //UAF
			
There is no guarantee that the exporter stays alive until
resv individualization happens.

/Thomas


> 
> v3:
> - Dropped the xe-side reordering approach since importer_priv must be
>   valid when dma_buf_dynamic_attach() publishes the attachment.
> - Per Christian's suggestion on the v1 thread, keyed the check on
>   import_attach rather than removing the sg guard entirely.
> - Fixes both xe and amdgpu in a single TTM patch.
> 
>  drivers/gpu/drm/ttm/ttm_bo.c | 24 +++++++++++++++---------
>  1 file changed, 15 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/gpu/drm/ttm/ttm_bo.c
> b/drivers/gpu/drm/ttm/ttm_bo.c
> index bcd76f6bb7f0..9b6341f69805 100644
> --- a/drivers/gpu/drm/ttm/ttm_bo.c
> +++ b/drivers/gpu/drm/ttm/ttm_bo.c
> @@ -203,15 +203,21 @@ static int ttm_bo_individualize_resv(struct
> ttm_buffer_object *bo)
>  	if (r)
>  		return r;
>  
> -	if (bo->type != ttm_bo_type_sg) {
> -		/* This works because the BO is about to be
> destroyed and nobody
> -		 * reference it any more. The only tricky case is
> the trylock on
> -		 * the resv object while holding the lru_lock.
> -		 */
> -		spin_lock(&bo->bdev->lru_lock);
> -		bo->base.resv = &bo->base._resv;
> -		spin_unlock(&bo->bdev->lru_lock);
> -	}
> +	/*
> +	 * Successfully imported sg BOs need the shared resv for
> dma-buf
> +	 * cleanup. Failed imports have no attachment or mapping and
> can
> +	 * use the private _resv.
> +	 */
> +	if (bo->type == ttm_bo_type_sg && bo->base.import_attach)
> +		return 0;
> +
> +	/* This works because the BO is about to be destroyed and
> nobody
> +	 * references it any more. The only tricky case is the
> trylock on
> +	 * the resv object while holding the lru_lock.
> +	 */
> +	spin_lock(&bo->bdev->lru_lock);
> +	bo->base.resv = &bo->base._resv;
> +	spin_unlock(&bo->bdev->lru_lock);
>  
>  	return r;
>  }