From e21c8c03af20932c15d8b1d3bb9cbad9607a6eab Mon Sep 17 00:00:00 2001
From: Piotr Krysiuk <piotras@gmail.com>
Date: Mon, 27 Apr 2020 11:34:12 +0100
Subject: [PATCH] fs/namespace.c: fix mountpoint reference counter race
References: CVE-2020-12114 bsc#1171098 stable-4.14.y
Git-commit: e21c8c03af20932c15d8b1d3bb9cbad9607a6eab
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git
Patch-mainline: Queued in subsystem maintainer repository

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>
Acked-by: Takashi Iwai <tiwai@suse.de>

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

diff --git a/fs/namespace.c b/fs/namespace.c
index e9c13eedd739..c8acc60c456d 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3216,8 +3216,8 @@ 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) {
-- 
2.16.4