linux-kernel - Re: For review: seccomp_user

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Date:   Wed, 30 Sep 2020 16:39:55 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Cc:     Tycho Andersen <tycho@...ho.pizza>,
        Sargun Dhillon <sargun@...gun.me>,
        Christian Brauner <christian@...uner.io>,
        linux-man <linux-man@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Aleksa Sarai <cyphar@...har.com>, Jann Horn <jannh@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>, wad@...omium.org,
        bpf@...r.kernel.org, Song Liu <songliubraving@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andy Lutomirski <luto@...capital.net>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Giuseppe Scrivano <gscrivan@...hat.com>,
        Robert Sesek <rsesek@...gle.com>
Subject: Re: For review: seccomp_user_notif(2) manual page

On Wed, Sep 30, 2020 at 01:07:38PM +0200, Michael Kerrisk (man-pages) wrote:
> [...] I did :-)

Yay! Thank you!

> [...]
>    Overview
>        In conventional usage of a seccomp filter, the decision about how
>        to  treat  a particular system call is made by the filter itself.
>        The user-space notification mechanism allows the handling of  the
>        system  call  to  instead  be handed off to a user-space process.
>        The advantages of doing this are that, by contrast with the  sec‐
>        comp  filter,  which  is  running on a virtual machine inside the
>        kernel, the user-space process has access to information that  is
>        unavailable to the seccomp filter and it can perform actions that
>        can't be performed from the seccomp filter.

I might clarify a bit with something like (though maybe the
target/supervisor paragraph needs to be moved to the start):

	This is used for performing syscalls on behalf of the target,
	rather than having the supervisor make security policy decisions
	about the syscall, which would be inherently race-prone. The
	target's syscall should either be handled by the supervisor or
	allowed to continue normally in the kernel (where standard security
	policies will be applied).

I'll comment more later, but I've run out of time today and I didn't see
anyone mention this detail yet in the existing threads... :)

-- 
Kees Cook