fanotify: fix fsnotify_prepare_user_wait() failure

If fsnotify_prepare_user_wait() fails, we leave the event on the
notification list.  Which will result in a warning in
fsnotify_destroy_event() and later use-after-free.

Instead of adding a new helper to remove the event from the list in this
case, I opted to move the prepare/finish up into fanotify_handle_event().

This will allow these to be moved further out into the generic code later,
and perhaps let us move to non-sleeping RCU.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: 05f0e38724e8 ("fanotify: Release SRCU lock when waiting for userspace response")
Cc: <stable@vger.kernel.org> # v4.12
Signed-off-by: Jan Kara <jack@suse.cz>

1 files changed, 20 insertions, 13 deletions

diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
index 2fa99aeaa095..df3f484e458a 100644
--- a/fs/notify/fanotify/fanotify.c
+++ b/fs/notify/fanotify/fanotify.c
@@ -64,19 +64,8 @@ static int fanotify_get_response(struct fsnotify_group *group,
 
         pr_debug("%s: group=%p event=%p\n", __func__, group, event);
 
-        /*
-         * fsnotify_prepare_user_wait() fails if we race with mark deletion.
-         * Just let the operation pass in that case.
-         */
-        if (!fsnotify_prepare_user_wait(iter_info)) {
-                event->response = FAN_ALLOW;
-                goto out;
-        }
-
         wait_event(group->fanotify_data.access_waitq, event->response);
 
-        fsnotify_finish_user_wait(iter_info);
-out:
         /* userspace responded, convert to something usable */
         switch (event->response) {
         case FAN_ALLOW:
@@ -211,9 +200,21 @@ static int fanotify_handle_event(struct fsnotify_group *group,
         pr_debug("%s: group=%p inode=%p mask=%x\n", __func__, group, inode,
                  mask);
 
+#ifdef CONFIG_FANOTIFY_ACCESS_PERMISSIONS
+        if (mask & FAN_ALL_PERM_EVENTS) {
+                /*
+                 * fsnotify_prepare_user_wait() fails if we race with mark
+                 * deletion.  Just let the operation pass in that case.
+                 */
+                if (!fsnotify_prepare_user_wait(iter_info))
+                        return 0;
+        }
+#endif
+
         event = fanotify_alloc_event(inode, mask, data);
+        ret = -ENOMEM;
         if (unlikely(!event))
-                return -ENOMEM;
+                goto finish;
 
         fsn_event = &event->fse;
         ret = fsnotify_add_event(group, fsn_event, fanotify_merge);
@@ -223,7 +224,8 @@ static int fanotify_handle_event(struct fsnotify_group *group,
                 /* Our event wasn't used in the end. Free it. */
                 fsnotify_destroy_event(group, fsn_event);
 
-                return 0;
+                ret = 0;
+                goto finish;
         }
 
 #ifdef CONFIG_FANOTIFY_ACCESS_PERMISSIONS
@@ -232,6 +234,11 @@ static int fanotify_handle_event(struct fsnotify_group *group,
                                             iter_info);
                 fsnotify_destroy_event(group, fsn_event);
         }
+finish:
+        if (mask & FAN_ALL_PERM_EVENTS)
+                fsnotify_finish_user_wait(iter_info);
+#else
+finish:
 #endif
         return ret;
 }