linux-kernel - Re: [PATCH v2 seccomp 2/6] asm/syscall.h: Add syscall

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Date:   Fri, 25 Sep 2020 02:24:43 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Kees Cook <keescook@...omium.org>,
        YiFei Zhu <yifeifz2@...inois.edu>,
        YiFei Zhu <zhuyifei1999@...il.com>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        bpf <bpf@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dimitrios Skarlatos <dskarlat@...cmu.edu>,
        Giuseppe Scrivano <gscrivan@...hat.com>,
        Hubertus Franke <frankeh@...ibm.com>,
        Jack Chen <jianyan2@...inois.edu>,
        Josep Torrellas <torrella@...inois.edu>,
        Tianyin Xu <tyxu@...inois.edu>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Tycho Andersen <tycho@...ho.pizza>,
        Valentin Rothberg <vrothber@...hat.com>,
        Will Drewry <wad@...omium.org>
Subject: Re: [PATCH v2 seccomp 2/6] asm/syscall.h: Add syscall_arches[] array

On Fri, Sep 25, 2020 at 2:18 AM Al Viro <viro@...iv.linux.org.uk> wrote:
> On Fri, Sep 25, 2020 at 02:15:50AM +0200, Jann Horn wrote:
> > On Fri, Sep 25, 2020 at 2:01 AM Kees Cook <keescook@...omium.org> wrote:
> > > 2) seccomp needs to handle "multiplexed" tables like x86_x32 (distros
> > >    haven't removed CONFIG_X86_X32 widely yet, so it is a reality that
> > >    it must be dealt with), which means seccomp's idea of the arch
> > >    "number" can't be the same as the AUDIT_ARCH.
> >
> > Sure, distros ship it; but basically nobody uses it, it doesn't have
> > to be fast. As long as we don't *break* it, everything's fine. And if
> > we ignore the existence of X32 in the fastpath, that'll just mean that
> > syscalls with the X32 marker bit always hit the seccomp slowpath
> > (because it'll look like the syscall number is out-of-bounds ) - no
> > problem.
>
> You do realize that X32 is amd64 counterpart of mips n32, right?  And that's
> not "basically nobody uses it"...

What makes X32 weird for seccomp is that it has the syscall tables for
X86-64 and X32 mushed together, using the single architecture
identifier AUDIT_ARCH_X86_64. I believe that's what Kees referred to
by "multiplexed tables".

As far as I can tell, MIPS is more well-behaved there and uses the
separate architecture identifiers
AUDIT_ARCH_MIPS|__AUDIT_ARCH_64BIT
and
AUDIT_ARCH_MIPS|__AUDIT_ARCH_64BIT|__AUDIT_ARCH_CONVENTION_MIPS64_N32.

(But no, I did not actually realize that that's what N32 is. Thanks
for the explanation, I was wondering why MIPS was the only
architecture with three architecture identifiers...)