Re: [PATCH] fhandle: fix UAF due to unlocked ->mnt_ns read in may_decode_fh()

[Al Viro]

On Wed, Jun 03, 2026 at 07:38:06PM +0200, Jann Horn wrote:

> Fix it by taking rcu_read_lock() around the mount::mnt_ns access, like
> in __prepend_path().

> +	/*
> +	 * Containing namespace.
> +	 * Normally protected by namespace_sem, but there are also lockless
> +	 * readers (which must use RCU to guard against the namespace being
> +	 * freed).
> +	 */
> +	struct mnt_namespace *mnt_ns;

Umm...  It's somewhat subtle - at the very least you need to explain why
there will be an RCU delay between umount_tree() clearing that and
having the sucker freed.