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

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Date:   Wed, 29 May 2019 08:53:34 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     David Howells <dhowells@...hat.com>, Jann Horn <jannh@...gle.com>
Cc:     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

On 5/29/2019 4:00 AM, David Howells wrote:
> 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?

You need to identify the credential of the subject that triggered
the event. If it isn't current_cred(), the cred needs to be passed
in to post_mount_notification(), or derived by some other means.

> 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.

You should pass the cred of the process that removed the
last usage. If the last usage was removed by something like
the power being turned off on a disk drive a system cred
should be used. Someone or something caused the event. It can
be important who it was.

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



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