namespace: reset the old parent's ->overmount in mnt_change_mountpoint()

Overview

In the Linux kernel, the following vulnerability has been resolved:

namespace: reset the old parent's ->overmount in mnt_change_mountpoint()

__umount_mnt() resets ->overmount of the parent when it unmounts the mount that overmounts it. mnt_change_mountpoint() doesn't. It moves the mount to its new parent and leaves the old one pointing at it.

For a mount that is moved beneath in attach_recursive_mnt() that doesn't matter. The new mount has already reset ->overmount of the old parent by the time the old top mount is moved onto it.

It matters for reparent() though. propagate_umount() moves a surviving overmount onto the first ancestor that isn't going to be unmounted. The mount it was taken from is unmounted immediately afterwards. If a file descriptor keeps that unmounted mount alive ->overmount points at the moved mount. That's a dangling pointer and UAF as the moved mount is unmounted and freed.

do_lock_mount() follows ->overmount from the target path under mount_lock. With a file descriptor on such a mount as the target of a mount beneath topmost_overmount() walks into the freed mount and do_lock_mount() takes references on whatever it finds there.

So reset ->overmount of the old parent in mnt_change_mountpoint() the same way __umount_mnt() does. A mount's ->overmount is then always a mount that is attached to it.

Affected

• Linux >= ffdc52fbbd5835a936ad683c943d6d103a2d4514 < 5e0a1f2c753a79749bc2ead3ae2f2ede3941daa5
• Linux >= ffdc52fbbd5835a936ad683c943d6d103a2d4514 < 08d575ee89393fd812dbdca7b5d7b4574cd5a564
• Linux >= ffdc52fbbd5835a936ad683c943d6d103a2d4514 < 476ef286a7a5db8bda51325dc0c45887b4d99d9a
• Linux 6.17

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.