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

[Nitin Gote <nitin.r.gote@intel.com>]

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).

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;
 }
-- 
2.50.1