fs/namespace.c: fix mountpoint reference counter race

A race condition between threads updating mountpoint reference counter
affects longterm releases 4.4.220, 4.9.220, 4.14.177 and 4.19.118.

The mountpoint reference counter corruption may occur when:
* one thread increments m_count member of struct mountpoint
  [under namespace_sem, but not holding mount_lock]
    pivot_root()
* another thread simultaneously decrements the same m_count
  [under mount_lock, but not holding namespace_sem]
    put_mountpoint()
      unhash_mnt()
        umount_mnt()
          mntput_no_expire()

To fix this race condition, grab mount_lock before updating m_count in
pivot_root().

Reference: CVE-2020-12114
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Piotr Krysiuk <piotras@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/fs/namespace.c b/fs/namespace.c
index 5acf789efbab224461847df9e63afc04a6941ca8..8c41f38617dbb86bfd5fd3187e32a83616ffc6e6 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2937,8 +2937,8 @@ SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
        /* make certain new is below the root */
        if (!is_path_reachable(new_mnt, new.dentry, &root))
                goto out4;
-       root_mp->m_count++; /* pin it so it won't go away */
        lock_mount_hash();
+       root_mp->m_count++; /* pin it so it won't go away */
        detach_mnt(new_mnt, &parent_path);
        detach_mnt(root_mnt, &root_parent);
        if (root_mnt->mnt.mnt_flags & MNT_LOCKED) {