fsnotify: fix inode reference leak in fsnotify_recalc_mask()

fsnotify_recalc_mask() fails to handle the return value of
__fsnotify_recalc_mask(), which may return an inode pointer that needs
to be released via fsnotify_drop_object() when the connector's HAS_IREF
flag transitions from set to cleared.

This manifests as a hung task with the following call trace:

  INFO: task umount:1234 blocked for more than 120 seconds.
  Call Trace:
   __schedule
   schedule
   fsnotify_sb_delete
   generic_shutdown_super
   kill_anon_super
   cleanup_mnt
   task_work_run
   do_exit
   do_group_exit

The race window that triggers the iref leak:

  Thread A (adding mark)              Thread B (removing mark)
  ──────────────────────              ────────────────────────
  fsnotify_add_mark_locked():
    fsnotify_add_mark_list():
      spin_lock(conn->lock)
      add mark_B(evictable) to list
      spin_unlock(conn->lock)
    return

    /* ---- gap: no lock held ---- */

                                      fsnotify_detach_mark(mark_A):
                                        spin_lock(mark_A->lock)
                                        clear ATTACHED flag on mark_A
                                        spin_unlock(mark_A->lock)
                                        fsnotify_put_mark(mark_A)

    fsnotify_recalc_mask():
      spin_lock(conn->lock)
      __fsnotify_recalc_mask():
        /* mark_A skipped: ATTACHED cleared */
        /* only mark_B(evictable) remains */
        want_iref = false
        has_iref = true  /* not yet cleared */
        -> HAS_IREF transitions true -> false
        -> returns inode pointer
      spin_unlock(conn->lock)
      /* BUG: return value discarded!
       * iput() and fsnotify_put_sb_watched_objects()
       * are never called */

Fix this by deferring the transition true -> false of HAS_IREF flag from
fsnotify_recalc_mask() (Thread A) to fsnotify_put_mark() (thread B).

Fixes: c3638b5b13 ("fsnotify: allow adding an inode mark without pinning inode")
Signed-off-by: Xin Yin <yinxin.x@bytedance.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Link: https://patch.msgid.link/CAOQ4uxiPsbHb0o5voUKyPFMvBsDkG914FYDcs4C5UpBMNm0Vcg@mail.gmail.com
Signed-off-by: Jan Kara <jack@suse.cz>

fs/notify/mark.c

@@ -238,7 +238,12 @@ static struct inode *fsnotify_update_iref(struct fsnotify_mark_connector *conn,
 	return inode;
 }
 
-static void *__fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
+/*
+ * Calculate mask of events for a list of marks.
+ *
+ * Return true if any of the attached marks want to hold an inode reference.
+ */
+static bool __fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
 {
 	u32 new_mask = 0;
 	bool want_iref = false;
@@ -262,6 +267,34 @@ static void *__fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
 	 */
 	WRITE_ONCE(*fsnotify_conn_mask_p(conn), new_mask);
 
+	return want_iref;
+}
+
+/*
+ * Calculate mask of events for a list of marks after attach/modify mark
+ * and get an inode reference for the connector if needed.
+ *
+ * A concurrent add of evictable mark and detach of non-evictable mark can
+ * lead to __fsnotify_recalc_mask() returning false want_iref, but in this
+ * case we defer clearing iref to fsnotify_recalc_mask_clear_iref() called
+ * from fsnotify_put_mark().
+ */
+static void fsnotify_recalc_mask_set_iref(struct fsnotify_mark_connector *conn)
+{
+	bool has_iref = conn->flags & FSNOTIFY_CONN_FLAG_HAS_IREF;
+	bool want_iref = __fsnotify_recalc_mask(conn) || has_iref;
+
+	(void) fsnotify_update_iref(conn, want_iref);
+}
+
+/*
+ * Calculate mask of events for a list of marks after detach mark
+ * and return the inode object if its reference is no longer needed.
+ */
+static void *fsnotify_recalc_mask_clear_iref(struct fsnotify_mark_connector *conn)
+{
+	bool want_iref = __fsnotify_recalc_mask(conn);
+
 	return fsnotify_update_iref(conn, want_iref);
 }
 
@@ -298,7 +331,7 @@ void fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
 
 	spin_lock(&conn->lock);
 	update_children = !fsnotify_conn_watches_children(conn);
-	__fsnotify_recalc_mask(conn);
+	fsnotify_recalc_mask_set_iref(conn);
 	update_children &= fsnotify_conn_watches_children(conn);
 	spin_unlock(&conn->lock);
 	/*
@@ -419,7 +452,7 @@ void fsnotify_put_mark(struct fsnotify_mark *mark)
 		/* Update watched objects after detaching mark */
 		if (sb)
 			fsnotify_update_sb_watchers(sb, conn);
-		objp = __fsnotify_recalc_mask(conn);
+		objp = fsnotify_recalc_mask_clear_iref(conn);
 		type = conn->type;
 	}
 	WRITE_ONCE(mark->connector, NULL);