[PATCH v3 2/2] drm/xe/dma-buf: fix UAF with retry loop

[Matthew Auld <matthew.auld@intel.com>]

Retry doesn't work here, since bo will be freed on error, leading to
UAF. However, now that we do the alloc & init before the attach, we can
now combine this as one unit and have the init do the alloc for us. This
should make the retry safe.

Reported by Sashiko.

v2: Fix up the error unwind (CI)

Closes: https://sashiko.dev/#/patchset/20260506184332.86743-2-matthew.auld%40intel.com
Fixes: eb289a5f6cc6 ("drm/xe: Convert xe_dma_buf.c for exhaustive eviction")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: <stable@vger.kernel.org> # v6.18+
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
---
 drivers/gpu/drm/xe/xe_dma_buf.c | 49 ++++++++-------------------------
 1 file changed, 12 insertions(+), 37 deletions(-)

diff --git a/drivers/gpu/drm/xe/xe_dma_buf.c b/drivers/gpu/drm/xe/xe_dma_buf.c
index 2332db502c8b..1336e048f580 100644
--- a/drivers/gpu/drm/xe/xe_dma_buf.c
+++ b/drivers/gpu/drm/xe/xe_dma_buf.c
@@ -258,16 +258,8 @@ struct dma_buf *xe_gem_prime_export(struct drm_gem_object *obj, int flags)
 	return ERR_PTR(ret);
 }
 
-/*
- * Takes ownership of @storage: on success it is transferred to the returned
- * drm_gem_object; on failure it is freed before returning the error.
- * This matches the contract of xe_bo_init_locked() which frees @storage on
- * its error paths, so callers need not (and must not) free @storage after
- * this call.
- */
 static struct drm_gem_object *
-xe_dma_buf_init_obj(struct drm_device *dev, struct xe_bo *storage,
-		    struct dma_buf *dma_buf)
+xe_dma_buf_create_obj(struct drm_device *dev, struct dma_buf *dma_buf)
 {
 	struct dma_resv *resv = dma_buf->resv;
 	struct xe_device *xe = to_xe_device(dev);
@@ -278,10 +270,8 @@ xe_dma_buf_init_obj(struct drm_device *dev, struct xe_bo *storage,
 	int ret = 0;
 
 	dummy_obj = drm_gpuvm_resv_object_alloc(&xe->drm);
-	if (!dummy_obj) {
-		xe_bo_free(storage);
+	if (!dummy_obj)
 		return ERR_PTR(-ENOMEM);
-	}
 
 	dummy_obj->resv = resv;
 	xe_validation_guard(&ctx, &xe->val, &exec, (struct xe_val_flags) {}, ret) {
@@ -290,8 +280,7 @@ xe_dma_buf_init_obj(struct drm_device *dev, struct xe_bo *storage,
 		if (ret)
 			break;
 
-		/* xe_bo_init_locked() frees storage on error */
-		bo = xe_bo_init_locked(xe, storage, NULL, resv, NULL, dma_buf->size,
+		bo = xe_bo_init_locked(xe, NULL, NULL, resv, NULL, dma_buf->size,
 				       0, /* Will require 1way or 2way for vm_bind */
 				       ttm_bo_type_sg, XE_BO_FLAG_SYSTEM, &exec);
 		drm_exec_retry_on_contention(&exec);
@@ -342,7 +331,6 @@ struct drm_gem_object *xe_gem_prime_import(struct drm_device *dev,
 	const struct dma_buf_attach_ops *attach_ops;
 	struct dma_buf_attachment *attach;
 	struct drm_gem_object *obj;
-	struct xe_bo *bo;
 
 	if (dma_buf->ops == &xe_dmabuf_ops) {
 		obj = dma_buf->priv;
@@ -357,22 +345,14 @@ struct drm_gem_object *xe_gem_prime_import(struct drm_device *dev,
 		}
 	}
 
-	bo = xe_bo_alloc();
-	if (IS_ERR(bo))
-		return ERR_CAST(bo);
-
 	/*
-	 * xe_dma_buf_init_obj() takes ownership of the raw bo, so do not touch
-	 * on fail, since it will already take care of cleanup. On success we
-	 * still need to drop the ref, if something later fails.
-	 *
-	 * In addition this needs to happen before the attach, since
-	 * it will create a new attachment for this, and add it to the list of
-	 * attachments, at which point it is globally visible, and at any point
-	 * the export side can call into on invalidate_mappings callback, which
-	 * require a working object.
+	 * This needs to happen before the attach, since it will create a new
+	 * attachment for this, and add it to the list of attachments, at which
+	 * point it is globally visible, and at any point the export side can
+	 * call into on invalidate_mappings callback, which require a working
+	 * object.
 	 */
-	obj = xe_dma_buf_init_obj(dev, bo, dma_buf);
+	obj = xe_dma_buf_create_obj(dev, dma_buf);
 	if (IS_ERR(obj))
 		return obj;
 
@@ -382,20 +362,15 @@ struct drm_gem_object *xe_gem_prime_import(struct drm_device *dev,
 		attach_ops = test->attach_ops;
 #endif
 
-	attach = dma_buf_dynamic_attach(dma_buf, dev->dev, attach_ops, &bo->ttm.base);
+	attach = dma_buf_dynamic_attach(dma_buf, dev->dev, attach_ops, obj);
 	if (IS_ERR(attach)) {
-		obj = ERR_CAST(attach);
-		goto out_err;
+		xe_bo_put(gem_to_xe_bo(obj));
+		return ERR_CAST(attach);
 	}
 
 	get_dma_buf(dma_buf);
 	obj->import_attach = attach;
 	return obj;
-
-out_err:
-	xe_bo_put(bo);
-
-	return obj;
 }
 
 #if IS_ENABLED(CONFIG_DRM_XE_KUNIT_TEST)
-- 
2.53.0