linux-kernel - Re: [PATCH 3/7] vfs: Add a mount-notification facility

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Date:   Wed, 29 May 2019 12:00:57 +0100
From:   David Howells <dhowells@...hat.com>
To:     Jann Horn <jannh@...gle.com>, casey@...aufler-ca.com
Cc:     dhowells@...hat.com, Al Viro <viro@...iv.linux.org.uk>,
        raven@...maw.net, linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        linux-block@...r.kernel.org, keyrings@...r.kernel.org,
        linux-security-module <linux-security-module@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/7] vfs: Add a mount-notification facility

Jann Horn <jannh@...gle.com> wrote:

> > +void post_mount_notification(struct mount *changed,
> > +                            struct mount_notification *notify)
> > +{
> > +       const struct cred *cred = current_cred();
> 
> This current_cred() looks bogus to me. Can't mount topology changes
> come from all sorts of places? For example, umount_mnt() from
> umount_tree() from dissolve_on_fput() from __fput(), which could
> happen pretty much anywhere depending on where the last reference gets
> dropped?

IIRC, that's what Casey argued is the right thing to do from a security PoV.
Casey?

Maybe I should pass in NULL creds in the case that an event is being generated
because an object is being destroyed due to the last usage[*] being removed.

 [*] Usage, not ref - Superblocks are a bit weird in their accounting.

David