fs: let backing files inherit FMODE_NONOTIFY from the user file

When fanotify reports a permission event, copy_event_to_user() creates
the event fd with dentry_open_nonotify(), so that the listener's use of
that fd does not generate further events. If the event's path is on a
stacked filesystem, the stacked filesystem's ->open() opens the real
file with backing_file_open(). Since commit ebe559609d ("fs: get rid
of __FMODE_NONOTIFY kludge"), FMODE_NONOTIFY lives in f_mode only and
the backing file does not inherit it: it is allocated with the default
fsnotify mode, and fsnotify_open_perm_and_set_mode() raises FS_OPEN_PERM
on the real path if the backing filesystem has permission event
watchers.

Before that commit __FMODE_NONOTIFY travelled in f_flags, and overlayfs
copies the user file's f_flags into the backing open
(ovl_open_realfile()), so the backing file was exempt implicitly. Commit
bc2473c90f ("ovl: enable fsnotify events on underlying real files")
relied on that when it dropped FMODE_NONOTIFY from OVL_OPEN_FLAGS.

The nested event is queued to the same group, from inside the
listener's read(2), and can only be answered by another reader thread
of that group. A listener holding filesystem marks on both the stacked
and the backing filesystem therefore deadlocks as soon as all of its
reader threads are inside such a read(2) at the same time, and every
other task that touches a marked filesystem queues up behind it.

Observed with a commercial anti-malware agent (16 reader threads) on an
ostree/composefs host: / is an overlayfs whose data layer is the ostree
object store on the same btrfs that holds /etc and /home, and a
systemd-sysext overlay sits on /usr. Every on-demand scan of /usr wedged
the whole machine until the group was killed. Stack of a blocked reader
thread on 7.2.7:

  fanotify_get_response
  fanotify_handle_event
  send_to_group
  fsnotify
  __fsnotify_parent
  do_dentry_open
  vfs_open
  backing_file_open
  ovl_open_realfile [overlay]
  ovl_open [overlay]
  do_dentry_open
  vfs_open
  backing_file_open
  ovl_open_realfile [overlay]
  ovl_open [overlay]
  do_dentry_open
  vfs_open
  dentry_open_nonotify
  copy_event_to_user
  fanotify_read
  vfs_read
  ksys_read

Userspace cannot avoid this: the backing open happens while the kernel
builds the fd, before the event is reported, and fid reporting is not
available for permission event classes.

Make a backing file inherit FMODE_NONOTIFY when the user file it is
opened for must not generate events. This restores the pre-6.14
behaviour for the fds that fanotify creates (and for any other
FMODE_NONOTIFY file), while ordinary opens through a stacked filesystem
keep generating events on the real path, as intended by bc2473c90f.

Verified with a self-contained reproducer (private tmpfs lower layers, an
overlayfs over them, FAN_OPEN_PERM filesystem marks on the overlay and on
one lower layer, a child opening a file through the overlay) in a qemu
VM on this tree built with defconfig plus OVERLAY_FS, FANOTIFY and
FANOTIFY_ACCESS_PERMISSIONS: without this patch the listener's read(2)
never returns and the child's open is released only by the listener's
death; with it read(2) returns, the event is answered and the open
completes.

backing_tmpfile_open() is left alone: its only caller is
ovl_create_tmpfile(), whose user file is a regular overlayfs file.

Fixes: ebe559609d ("fs: get rid of __FMODE_NONOTIFY kludge")
Cc: stable@vger.kernel.org
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Edvard Conrad <edvard@tuta.io>
Link: https://patch.msgid.link/P58Wrer--F-9@tuta.io
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

fs/backing-file.c

@@ -55,6 +55,17 @@ struct file *backing_file_open(const struct file *user_file, int flags,
 	if (IS_ERR(f))
 		return f;
 
+	/*
+	 * A user file that must not generate fsnotify events (e.g. the fd
+	 * that fanotify creates for a permission event with
+	 * dentry_open_nonotify()) must not generate them through its backing
+	 * file either. Otherwise a listener watching the backing filesystem
+	 * gets a nested permission event from its own read(2), which only
+	 * another reader of the same group can answer.
+	 */
+	if (FMODE_FSNOTIFY_NONE(user_file->f_mode))
+		file_set_fsnotify_mode(f, FMODE_NONOTIFY);
+
 	path_get(user_path);
 	backing_file_set_user_path(f, user_path);
 	error = vfs_open(real_path, f);