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>
1 file changed
