drm/vmwgfx: Fix stale file descriptors on failed usercopy

A failing usercopy of the fence_rep object will lead to a stale entry in
the file descriptor table as put_unused_fd() won't release it. This
enables userland to refer to a dangling 'file' object through that still
valid file descriptor, leading to all kinds of use-after-free
exploitation scenarios.

Fix this by deferring the call to fd_install() until after the usercopy
has succeeded.

Fixes: c906965dee ("drm/vmwgfx: Add export fence to file descriptor support")
Signed-off-by: Mathias Krause <minipli@grsecurity.net>
Signed-off-by: Zack Rusin <zackr@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/gpu/drm/vmwgfx/vmwgfx_drv.h

@@ -1140,15 +1140,14 @@ extern int vmw_execbuf_fence_commands(struct drm_file *file_priv,
 				      struct vmw_private *dev_priv,
 				      struct vmw_fence_obj **p_fence,
 				      uint32_t *p_handle);
-extern void vmw_execbuf_copy_fence_user(struct vmw_private *dev_priv,
+extern int vmw_execbuf_copy_fence_user(struct vmw_private *dev_priv,
 					struct vmw_fpriv *vmw_fp,
 					int ret,
 					struct drm_vmw_fence_rep __user
 					*user_fence_rep,
 					struct vmw_fence_obj *fence,
 					uint32_t fence_handle,
-					int32_t out_fence_fd,
-					struct sync_file *sync_file);
+					int32_t out_fence_fd);
 bool vmw_cmd_describe(const void *buf, u32 *size, char const **cmd);
 
 /**

drivers/gpu/drm/vmwgfx/vmwgfx_execbuf.c

@@ -3879,17 +3879,17 @@ int vmw_execbuf_fence_commands(struct drm_file *file_priv,
  * Also if copying fails, user-space will be unable to signal the fence object
  * so we wait for it immediately, and then unreference the user-space reference.
  */
-void
+int
 vmw_execbuf_copy_fence_user(struct vmw_private *dev_priv,
 			    struct vmw_fpriv *vmw_fp, int ret,
 			    struct drm_vmw_fence_rep __user *user_fence_rep,
 			    struct vmw_fence_obj *fence, uint32_t fence_handle,
-			    int32_t out_fence_fd, struct sync_file *sync_file)
+			    int32_t out_fence_fd)
 {
 	struct drm_vmw_fence_rep fence_rep;
 
 	if (user_fence_rep == NULL)
-		return;
+		return 0;
 
 	memset(&fence_rep, 0, sizeof(fence_rep));
 
@@ -3917,19 +3917,13 @@ vmw_execbuf_copy_fence_user(struct vmw_private *dev_priv,
 	 * handle.
 	 */
 	if (unlikely(ret != 0) && (fence_rep.error == 0)) {
-		if (sync_file)
-			fput(sync_file->file);
-
-		if (fence_rep.fd != -1) {
-			put_unused_fd(fence_rep.fd);
-			fence_rep.fd = -1;
-		}
-
 		ttm_ref_object_base_unref(vmw_fp->tfile, fence_handle);
 		VMW_DEBUG_USER("Fence copy error. Syncing.\n");
 		(void) vmw_fence_obj_wait(fence, false, false,
 					  VMW_FENCE_WAIT_TIMEOUT);
 	}
+
+	return ret ? -EFAULT : 0;
 }
 
 /**
@@ -4266,16 +4260,23 @@ int vmw_execbuf_process(struct drm_file *file_priv,
 
 			(void) vmw_fence_obj_wait(fence, false, false,
 						  VMW_FENCE_WAIT_TIMEOUT);
+		}
+	}
+
+	ret = vmw_execbuf_copy_fence_user(dev_priv, vmw_fpriv(file_priv), ret,
+				    user_fence_rep, fence, handle, out_fence_fd);
+
+	if (sync_file) {
+		if (ret) {
+			/* usercopy of fence failed, put the file object */
+			fput(sync_file->file);
+			put_unused_fd(out_fence_fd);
 		} else {
 			/* Link the fence with the FD created earlier */
 			fd_install(out_fence_fd, sync_file->file);
 		}
 	}
 
-	vmw_execbuf_copy_fence_user(dev_priv, vmw_fpriv(file_priv), ret,
-				    user_fence_rep, fence, handle, out_fence_fd,
-				    sync_file);
-
 	/* Don't unreference when handing fence out */
 	if (unlikely(out_fence != NULL)) {
 		*out_fence = fence;
@@ -4293,7 +4294,7 @@ int vmw_execbuf_process(struct drm_file *file_priv,
 	 */
 	vmw_validation_unref_lists(&val_ctx);
 
-	return 0;
+	return ret;
 
 out_unlock_binding:
 	mutex_unlock(&dev_priv->binding_mutex);

drivers/gpu/drm/vmwgfx/vmwgfx_fence.c

@@ -1128,7 +1128,7 @@ int vmw_fence_event_ioctl(struct drm_device *dev, void *data,
 	}
 
 	vmw_execbuf_copy_fence_user(dev_priv, vmw_fp, 0, user_fence_rep, fence,
-				    handle, -1, NULL);
+				    handle, -1);
 	vmw_fence_obj_unreference(&fence);
 	return 0;
 out_no_create:

drivers/gpu/drm/vmwgfx/vmwgfx_kms.c

@@ -2501,7 +2501,7 @@ void vmw_kms_helper_validation_finish(struct vmw_private *dev_priv,
 	if (file_priv)
 		vmw_execbuf_copy_fence_user(dev_priv, vmw_fpriv(file_priv),
 					    ret, user_fence_rep, fence,
-					    handle, -1, NULL);
+					    handle, -1);
 	if (out_fence)
 		*out_fence = fence;
 	else