linux-kernel - Re: [RFC PATCH seccomp 2/2] seccomp/cache: Cache filter results that allow syscalls

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Message-ID: <CABqSeAROcwq0ZGzWaxyPm+LDHu6T_8CD7_1c-hdhaMikr_ECCA@mail.gmail.com>
Date:   Mon, 21 Sep 2020 17:50:55 -0500
From:   YiFei Zhu <zhuyifei1999@...il.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     Linux Containers <containers@...ts.linux-foundation.org>,
        YiFei Zhu <yifeifz2@...inois.edu>, bpf <bpf@...r.kernel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        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>,
        Kees Cook <keescook@...omium.org>,
        Tianyin Xu <tyxu@...inois.edu>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Valentin Rothberg <vrothber@...hat.com>,
        Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>,
        Aleksa Sarai <cyphar@...har.com>,
        kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH seccomp 2/2] seccomp/cache: Cache filter results that
 allow syscalls

On Mon, Sep 21, 2020 at 1:09 PM Jann Horn <jannh@...gle.com> wrote:
>
> On Mon, Sep 21, 2020 at 7:35 AM YiFei Zhu <zhuyifei1999@...il.com> wrote:
> [...]
> > We do this by creating a per-task bitmap of permitted syscalls.
> > If seccomp filter is invoked we check if it is cached and if so
> > directly return allow. Else we call into the cBPF filter, and if
> > the result is an allow then we cache the results.
>
> What? Why? We already have code to statically evaluate the filter for
> all syscall numbers. We should be using the results of that instead of
> re-running the filter and separately caching the results.
>
> > The cache is per-task
>
> Please don't. The static results are per-filter, so the bitmask(s)
> should also be per-filter and immutable.

I do agree that an immutable bitmask is faster and easier to reason
about its correctness. However, I did not find the "code to statically
evaluate the filter for all syscall numbers" while reading seccomp.c.
Would you give me a pointer to that and I will see how to best make
use of it?

YiFei Zhu