linux-kernel - Re: For review: seccomp_user

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Date:   Wed, 28 Oct 2020 15:53:04 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Jann Horn <jannh@...gle.com>
Cc:     Tycho Andersen <tycho@...ho.pizza>,
        "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
        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>,
        Alexei Starovoitov <ast@...nel.org>,
        Will Drewry <wad@...omium.org>, bpf <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 Mon, Oct 26, 2020 at 10:51:02AM +0100, Jann Horn wrote:
> The problem is the scenario where a process is interrupted while it's
> waiting for the supervisor to reply.
> 
> Consider the following scenario (with supervisor "S" and target "T"; S
> wants to wait for events on two file descriptors seccomp_fd and
> other_fd):
> 
> S: starts poll() to wait for events on seccomp_fd and other_fd
> T: performs a syscall that's filtered with RET_USER_NOTIF
> S: poll() returns and signals readiness of seccomp_fd
> T: receives signal SIGUSR1
> T: syscall aborts, enters signal handler
> T: signal handler blocks on unfiltered syscall (e.g. write())
> S: starts SECCOMP_IOCTL_NOTIF_RECV
> S: blocks because no syscalls are pending

Oooh, yes, ew. Thanks for the illustration.

Thinking about this from userspace's least-surprise view, I would expect
the "recv" to stay "queued", in the sense we'd see this:

S: starts poll() to wait for events on seccomp_fd and other_fd
T: performs a syscall that's filtered with RET_USER_NOTIF
S: poll() returns and signals readiness of seccomp_fd
T: receives signal SIGUSR1
T: syscall aborts, enters signal handler
T: signal handler blocks on unfiltered syscall (e.g. write())
S: starts SECCOMP_IOCTL_NOTIF_RECV
S: gets (stale) seccomp_notif from seccomp_fd
S: sends seccomp_notif_resp, receives ENOENT (or some better errno?)

This is not at all how things are designed internally right now, but
that behavior would work, yes?

-- 
Kees Cook