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From:   Jann Horn <jannh@google.com>
Date:   Thu, 24 Sep 2020 02:45:45 +0200
Message-ID: <CAG48ez1BXWdWA5zPzOD21bQ4RsHQ6bSDWR8soTkkNphJ=zdHWw@mail.gmail.com>
Subject: Re: [PATCH 2/6] x86: Enable seccomp architecture tracking
To:     Kees Cook <keescook@chromium.org>
Cc:     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>
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On Thu, Sep 24, 2020 at 1:29 AM Kees Cook <keescook@chromium.org> wrote:
> Provide seccomp internals with the details to calculate which syscall
> table the running kernel is expecting to deal with. This allows for
> efficient architecture pinning and paves the way for constant-action
> bitmaps.
[...]
> diff --git a/arch/x86/include/asm/seccomp.h b/arch/x86/include/asm/seccomp.h
[...]
> +#ifdef CONFIG_X86_64
[...]
> +#else /* !CONFIG_X86_64 */
> +# define SECCOMP_ARCH                                  AUDIT_ARCH_I386
> +#endif

If we are on a 32-bit kernel, performing architecture number checks in
the kernel is completely pointless, because we know that there is only
a single architecture identifier under which syscalls can happen.

While this patch is useful for enabling the bitmap logic in the
following patches, I think it adds unnecessary overhead in the context
of the previous patch.