CVE-2026-98977

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

namespace: handle mount locking for automounts correctly

If mounts are propagated across user namespaces, attach_recursive_mnt() locks every copy of the source mount to protect overmounts from vanishing and revealing the underlying files or directories.

The user namespace is taken from the callers mount namespaces since this is where the mounts end up. Except, thats not always true. Automounts may legitimately get popped in by tasks located in a different mount and user namespace during path lookup.

Then check doesnt make sense at that point. The copy in the namespace of the parent - which may be the hosts - is now locked and the host cannot change the flags of its own mounts anymore.

Heres the reproducer:

• host has debugfs mounted nosuid,nodev,noexec and shared

• tracefs automount below it is not yet active

• hand a child process a directory descriptor on that mount

• child process enters auser and mount namespace

• child process namespace now holds a locked copy of the hosts debugfs mount that receives propagation from it

• child process stats tracing/. through the descriptor

• lookup runs on the hosts mount so the automount lands below the hosts mount

• propagation puts a copy below the childs copy

• both try to clear the flags on the mount they got, with a bind remount:

  host, on its own automount: MS_REMOUNT|MS_BIND = EPERM child, on the copy in its namespace: MS_REMOUNT|MS_BIND = 0

So the lock landed on the hosts mount instead of the childs copy. Congrats. So we need to compare with the owner of the namespace the mount actually gets mounted on. For all regular cases that is the callers mount namespace and so nothing changes.

Detached trees in anonymous mount namespaces by be handed over via SCM_RIGHTS or inherited in other ways on purpose so the attaching tasks mount namespace is authoritative, not the creator of the detached tree.

Affected Software

References

• https://git.kernel.org/stable/c/33aadc7c72055fca93cb5a3691d836ebf8b31c9b
• https://git.kernel.org/stable/c/587d25c4b13d49c8fd05df2f71240f73bb2a058c
• https://git.kernel.org/stable/c/5e8f3ac8fa3d411b422ad32b9168c46a4824cc79
• https://git.kernel.org/stable/c/6ab37fae14196ba2262b152a7d9222c50356d278
• https://git.kernel.org/stable/c/91676705756af6a59a515087b8b854c8a28d12eb
• https://git.kernel.org/stable/c/b3ccde78f1608836e62320428abc0e8aa9cf2329
• https://git.kernel.org/stable/c/e01c79761c7b3373e80094c5eadb21cb864fa8bc
• https://git.kernel.org/stable/c/f9b488641cfa63f9b41321530c543688b2e34f03