linux-kernel - Re: SECCOMP_RET_USER_NOTIF: listener improvements

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Message-ID: <20190424152001.GL3758@cisco>
Date:   Wed, 24 Apr 2019 09:20:01 -0600
From:   Tycho Andersen <tycho@...ho.ws>
To:     Christian Brauner <christian@...uner.io>
Cc:     keescook@...omium.org, luto@...capital.net, jannh@...gle.com,
        linux-kernel@...r.kernel.org, stgraber@...ntu.com
Subject: Re: SECCOMP_RET_USER_NOTIF: listener improvements

On Wed, Apr 24, 2019 at 05:04:26PM +0200, Christian Brauner wrote:
> Hey everyone,
> 
> So I was working on making use of the seccomp listener stuff and I
> stumbled upon a problem. Imagine a scenario where:
> 
> 1. Task T1 installs Filter F1 and gets and listener fd for that filter FD1
> 2. T1 sends FD1 via SCM_RIGHTS to task T2
>    T2 now holds a reference to the same underlying struct file as FD1 via FD2
> 3. T2 registers FD2 in an event loop and starts listening for events
> 4. T1 exits and wipes FD1
> 
> Now, T2 still holds a reference to the filter via FD2 which references
> the same underlying file as FD1 which has the seccomp filter stashed in
> private_data.
> So T2 will never get notified that the filter is essentially unused and
> doesn't know when to exit, i.e. it has no way of telling when T1 and all
> of its children using the same filter are gone.
> 
> I think we should have a way to do this

Since the only way we ever allow creating a struct file * that points
to a struct seccomp_filter *, if there is a notifier attached, the
number of tasks still being monitored by a particular filter should be
filter->usage - 1 (assuming there is a notifier attached). So we could
augment __put_seccomp_filter() to check for this and send out a
message with a SECCOMP_NOTIF_FLAG_DEAD flag or something.

> *or* alternatively have a way to attach a process to an existing
> filter.

I also think this wouldn't be too hard, since the struct file * has a
reference to the filter. So I guess the question is: which of these
makes more sense?

Tycho