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

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Message-Id: <4552118F-BE9B-4905-BF0F-A53DC13D5A82@amacapital.net>
Date:   Wed, 29 May 2019 10:13:54 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Casey Schaufler <casey@...aufler-ca.com>
Cc:     David Howells <dhowells@...hat.com>, Jann Horn <jannh@...gle.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



> On May 29, 2019, at 8:53 AM, Casey Schaufler <casey@...aufler-ca.com> wrote:
> 
>> 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.

Taking a step back, why do we care who triggered the event?  It seems to me that we should care whether the event happened and whether the *receiver* is permitted to know that.

(And receiver means whoever subscribed, presumably, not whoever called read() or mmap().)