[PATCH v3 1/2] drm/xe/dma-buf: handle empty bo and UAF races

[PATCH v3 1/2] drm/xe/dma-buf: handle empty bo and UAF races

[Matthew Auld]

There look to be some nasty races here when triggering the
invalidate_mappings hook:

1) We do xe_bo_alloc() followed by the attach, before the actual full bo
   init step in xe_dma_buf_init_obj(). However the bo is visible on the
   attachments list after the attach.  This is bad since exporter driver,
   say amdgpu, can at any time call back into our invalidate_mappings hook,
   with an empty/bogus bo, leading to potential bugs/crashes.

2) Similar to 1) but here we get a UAF, when the invalidate_mappings
   hook is triggered. For example, we get as far as xe_bo_init_locked()
   but this fails in some way. But here the bo will be freed on error, but
   we still have it attached from dma-buf pov, so if the
   invalidate_mappings is now triggered then the bo we access is gone and
   we trigger UAF and more bugs/crashes.

To fix this, move the attach step until after we actually have a fully
set up buffer object. Note that the bo is not published to userspace
until later, so not sure what the comment "Don't publish the bo
until we have a valid attachment", is referring to.

We have at least two different customers reporting hitting a NULL ptr
deref in evict_flags when importing something from amdgpu, followed by
triggering the evict flow. Hit rate is also pretty low, which would
hint at some kind of race, so something like 1) or 2) might explain
this.

v2:
  - Shuffle the order of the ops slightly (no functional change)
  - Improve the comment to better explain the ordering (Matt B)

Assisted-by: Gemini:gemini-3 #debug
Link: https://gitlab.freedesktop.org/drm/xe/kernel/-/work_items/7903
Link: https://gitlab.freedesktop.org/drm/xe/kernel/-/work_items/4055
Fixes: dd08ebf6c352 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
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> # v6.8+
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Acked-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
---
 drivers/gpu/drm/xe/xe_dma_buf.c | 31 ++++++++++++++++---------------
 1 file changed, 16 insertions(+), 15 deletions(-)

diff --git a/drivers/gpu/drm/xe/xe_dma_buf.c b/drivers/gpu/drm/xe/xe_dma_buf.c
index b9828da15897..2332db502c8b 100644
--- a/drivers/gpu/drm/xe/xe_dma_buf.c
+++ b/drivers/gpu/drm/xe/xe_dma_buf.c
@@ -357,15 +357,25 @@ struct drm_gem_object *xe_gem_prime_import(struct drm_device *dev,
 		}
 	}
 
-	/*
-	 * Don't publish the bo until we have a valid attachment, and a
-	 * valid attachment needs the bo address. So pre-create a bo before
-	 * creating the attachment and publish.
-	 */
 	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.
+	 */
+	obj = xe_dma_buf_init_obj(dev, bo, dma_buf);
+	if (IS_ERR(obj))
+		return obj;
+
 	attach_ops = &xe_dma_buf_attach_ops;
 #if IS_ENABLED(CONFIG_DRM_XE_KUNIT_TEST)
 	if (test)
@@ -378,21 +388,12 @@ struct drm_gem_object *xe_gem_prime_import(struct drm_device *dev,
 		goto out_err;
 	}
 
-	/*
-	 * xe_dma_buf_init_obj() takes ownership of bo on both success
-	 * and failure, so we must not touch bo after this call.
-	 */
-	obj = xe_dma_buf_init_obj(dev, bo, dma_buf);
-	if (IS_ERR(obj)) {
-		dma_buf_detach(dma_buf, attach);
-		return obj;
-	}
 	get_dma_buf(dma_buf);
 	obj->import_attach = attach;
 	return obj;
 
 out_err:
-	xe_bo_free(bo);
+	xe_bo_put(bo);
 
 	return obj;
 }
-- 
2.53.0

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

[Matthew Auld]

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