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

[Al Viro <viro@zeniv.linux.org.uk>]

On Wed, Jun 03, 2026 at 07:41:51PM +0100, Al Viro wrote:

> Basically, the store that cleared ->mnt_ns has been done in namespace_sem
> scope and that scope is either no later than the scope in put_mnt_ns()

argh...  s/either//

> that has dropped the active refcount of ns to zero.  At the beginning
> of that scope in put_mnt_ns() we are guaranteed to have the passive
> refcount positive.  Dropping the passive reference happens after an
> rcu delay started in later in the same namespace_sem scope and namespace
> is not freed until the passive refcount reaches zero.

TL;DR: your fix is correct, but needs a better explanation of correctness.
If nothing else, I'd like to have the above findable on lore - I've way
too many pieces of half-baked docs sitting around in local notes as it is ;-/