fanotify: disallow mount/sb marks on kernel internal pseudo fs

Hopefully, nobody is trying to abuse mount/sb marks for watching all
anonymous pipes/inodes.

I cannot think of a good reason to allow this - it looks like an
oversight that dated back to the original fanotify API.

Link: https://lore.kernel.org/linux-fsdevel/20230628101132.kvchg544mczxv2pm@quack3/
Fixes: 0ff21db9fcc3 ("fanotify: hooks the fanotify_mark syscall to the vfsmount code")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Message-Id: <20230629042044.25723-1-amir73il@gmail.com>

diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
index 95d7d87..f69c451 100644 (file)
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -1623,6 +1623,20 @@ static int fanotify_events_supported(struct fsnotify_group *group,
                return -EINVAL;
 
        /*
+        * mount and sb marks are not allowed on kernel internal pseudo fs,
+        * like pipe_mnt, because that would subscribe to events on all the
+        * anonynous pipes in the system.
+        *
+        * SB_NOUSER covers all of the internal pseudo fs whose objects are not
+        * exposed to user's mount namespace, but there are other SB_KERNMOUNT
+        * fs, like nsfs, debugfs, for which the value of allowing sb and mount
+        * mark is questionable. For now we leave them alone.
+        */
+       if (mark_type != FAN_MARK_INODE &&
+           path->mnt->mnt_sb->s_flags & SB_NOUSER)
+               return -EINVAL;
+
+       /*
         * We shouldn't have allowed setting dirent events and the directory
         * flags FAN_ONDIR and FAN_EVENT_ON_CHILD in mask of non-dir inode,
         * but because we always allowed it, error only when using new APIs.