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

["Christian König" <christian.koenig@amd.com>]

On 7/1/26 14:59, Thomas Hellström wrote:
> 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.

IIRC at least for AMDGPU that shouldn't be possible.

We intentionally create the imported BO as empty shell without ttm_resource object, so it is not on any LRU list.

But to be honest I haven't looked into that in years, so it is perfectly possible that this is messed up again.

Regards,
Christian.

> 
> /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;
>>  }