CVE-2026-98985

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

namespace: check the mounts before reading their parents in pivot root()

path pivot root() reads the parents of new root and of the caller's root and checks whether they are shared before it checks that either mount is in the caller's mount namespace. Only namespace sem is held. That's fine for a mount that is in the caller's mount namespace.

But new root can be a file descriptor to a mount that has been unmounted and that only the file descriptor keeps alive. If that mount stayed attached to its parent when it was unmounted nothing pins the parent for it. The final mntput() of the parent unhooks the children under mount lock alone and frees the parent afterwards:

pivot root() close(fd), last ref on the parent

ex parent = new mnt->mnt parent mntput no expire slowpath() umount mnt(new mnt) cleanup mnt() call rcu() IS MNT SHARED(ex parent)

BUG: KASAN: slab-use-after-free in path pivot root+0xf1a/0x1840 Read of size 4 at addr ffff8881047382f8 by task pivot widen/157 path pivot root+0xf1a/0x1840 x64 sys pivot root+0x165/0x190

The same goes for the caller's root via chroot(). Both outcomes of the check end in EINVAL so nothing but the read itself goes wrong. It's the same thing commit bb4609405752 ("statmount: read the parent of an unmounted mount under mount lock") fixed for statmount().

Check that both mounts are in the caller's namespace before their parents are read. Every path returns EINVAL either way.