linux-kernel - Re: [PATCH 3/6] seccomp: Implement constant action bitmaps

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Date:   Thu, 24 Sep 2020 00:38:19 -0700
From:   Kees Cook <keescook@...omium.org>
To:     YiFei Zhu <zhuyifei1999@...il.com>
Cc:     Jann Horn <jannh@...gle.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Tycho Andersen <tycho@...ho.pizza>,
        Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Giuseppe Scrivano <gscrivan@...hat.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Dimitrios Skarlatos <dskarlat@...cmu.edu>,
        Valentin Rothberg <vrothber@...hat.com>,
        Hubertus Franke <frankeh@...ibm.com>,
        bpf <bpf@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        linux-api@...r.kernel.org,
        kernel list <linux-kernel@...r.kernel.org>,
        Jack Chen <jianyan2@...inois.edu>,
        Josep Torrellas <torrella@...inois.edu>,
        Tianyin Xu <tyxu@...inois.edu>,
        YiFei Zhu <yifeifz2@...inois.edu>
Subject: Re: [PATCH 3/6] seccomp: Implement constant action bitmaps

On Wed, Sep 23, 2020 at 07:36:47PM -0500, YiFei Zhu wrote:
> On Wed, Sep 23, 2020 at 6:29 PM Kees Cook <keescook@...omium.org> wrote:
> > In order to optimize these cases from O(n) to O(1), seccomp can
> > use bitmaps to immediately determine the desired action. A critical
> > observation in the prior paragraph bears repeating: the common case for
> > syscall tests do not check arguments. For any given filter, there is a
> > constant mapping from the combination of architecture and syscall to the
> > seccomp action result. (For kernels/architectures without CONFIG_COMPAT,
> > there is a single architecture.). As such, it is possible to construct
> > a mapping of arch/syscall to action, which can be updated as new filters
> > are attached to a process.
> 
> Would you mind educating me how this patch plan one handling MIPS? For
> one kernel they seem to have up to three arch numbers per build,
> AUDIT_ARCH_MIPS{,64,64N32}. Though ARCH_TRACE_IGNORE_COMPAT_SYSCALLS
> does not seem to be defined for MIPS so I'm assuming the syscall
> numbers are the same, but I think it is possible some client uses that
> arch number to pose different constraints for different processes, so
> it would better not accelerate them rather than break them.

I'll take a look, but I'm hoping it won't be too hard to fit into what
I've got designed so for to deal with x86_x32. (Will MIPS want this
optimization at all?)

-- 
Kees Cook