[PATCH 4.9 33/80] fs/namespace.c: fix mountpoint reference counter race

[Greg Kroah-Hartman]

From: Piotr Krysiuk <piotras@xxxxxxxxx>

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@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Piotr Krysiuk <piotras@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 fs/namespace.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3184,8 +3184,8 @@ SYSCALL_DEFINE2(pivot_root, const char _
 	/* 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) {