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
@@ -3216,8 +3216,8 @@ 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) {
