Re: [PATCH 4/6] seccomp: Emulate basic filters for constant action results

From: Paul Moore <paul@paul-moore.com>
To: Kees Cook <keescook@chromium.org>,
	Tom Hromatka <tom.hromatka@oracle.com>
Cc: Jann Horn <jannh@google.com>, YiFei Zhu <yifeifz2@illinois.edu>,
	Christian Brauner <christian.brauner@ubuntu.com>,
	Tycho Andersen <tycho@tycho.pizza>,
	Andy Lutomirski <luto@amacapital.net>,
	Will Drewry <wad@chromium.org>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Giuseppe Scrivano <gscrivan@redhat.com>,
	Tobin Feldman-Fitzthum <tobin@ibm.com>,
	Dimitrios Skarlatos <dskarlat@cs.cmu.edu>,
	Valentin Rothberg <vrothber@redhat.com>,
	Hubertus Franke <frankeh@us.ibm.com>,
	Jack Chen <jianyan2@illinois.edu>,
	Josep Torrellas <torrella@illinois.edu>,
	Tianyin Xu <tyxu@illinois.edu>, bpf <bpf@vger.kernel.org>,
	Linux Containers <containers@lists.linux-foundation.org>,
	Linux API <linux-api@vger.kernel.org>,
	kernel list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 4/6] seccomp: Emulate basic filters for constant action results
Date: Thu, 24 Sep 2020 11:28:55 -0400	[thread overview]
Message-ID: <CAHC9VhQpto1KuL7PhjtdjtAjJ2nC+rZNSM7+nSZ_ksqGXbhY+Q@mail.gmail.com> (raw)
In-Reply-To: <202009240038.864365E@keescook>

On Thu, Sep 24, 2020 at 3:46 AM Kees Cook <keescook@chromium.org> wrote:
> On Thu, Sep 24, 2020 at 01:47:47AM +0200, Jann Horn wrote:
> > On Thu, Sep 24, 2020 at 1:29 AM Kees Cook <keescook@chromium.org> wrote:
> > > This emulates absolutely the most basic seccomp filters to figure out
> > > if they will always give the same results for a given arch/nr combo.
> > >
> > > Nearly all seccomp filters are built from the following ops:
> > >
> > > BPF_LD  | BPF_W    | BPF_ABS
> > > BPF_JMP | BPF_JEQ  | BPF_K
> > > BPF_JMP | BPF_JGE  | BPF_K
> > > BPF_JMP | BPF_JGT  | BPF_K
> > > BPF_JMP | BPF_JSET | BPF_K
> > > BPF_JMP | BPF_JA
> > > BPF_RET | BPF_K
> > >
> > > These are now emulated to check for accesses beyond seccomp_data::arch
> > > or unknown instructions.
> > >
> > > Not yet implemented are:
> > >
> > > BPF_ALU | BPF_AND (generated by libseccomp and Chrome)
> >
> > BPF_AND is normally only used on syscall arguments, not on the syscall
> > number or the architecture, right? And when a syscall argument is
> > loaded, we abort execution anyway. So I think there is no need to
> > implement those?
>
> Is that right? I can't actually tell what libseccomp is doing with
> ALU|AND. It looks like it's using it for building jump lists?

There is an ALU|AND op in the jump resolution code, but that is really
just if libseccomp needs to fixup the accumulator because a code block
is expecting a masked value (right now that would only be a syscall
argument, not the syscall number itself).

> Paul, Tom, under what cases does libseccomp emit ALU|AND into filters?

Presently the only place where libseccomp uses ALU|AND is when the
masked equality comparison is used for comparing syscall arguments
(SCMP_CMP_MASKED_EQ).  I can't honestly say I have any good
information about how often that is used by libseccomp callers, but if
I do a quick search on GitHub for "SCMP_CMP_MASKED_EQ" I see 2k worth
of code hits; take that for whatever it is worth.  Tom may have some
more/better information.

Of course no promises on future use :)  As one quick example, I keep
thinking about adding the instruction pointer to the list of things
that can be compared as part of a libseccomp rule, and if we do that I
would expect that we would want to also allow a masked comparison (and
utilize another ALU|AND bpf op there).  However, I'm not sure how
useful that would be in practice.

-- 
paul moore
www.paul-moore.com