From: Piotr Krysiuk piotras@gmail.com
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
--- fs/namespace.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/fs/namespace.c +++ b/fs/namespace.c @@ -3161,8 +3161,8 @@ 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) {