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

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Date:   Tue, 22 Sep 2020 00:57:58 +0200
From:   Jann Horn <jannh@...gle.com>
To:     YiFei Zhu <zhuyifei1999@...il.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 Tue, Sep 22, 2020 at 12:51 AM YiFei Zhu <zhuyifei1999@...il.com> wrote:
> 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?

I'm talking about the code you're adding in the other patch ("[RFC
PATCH seccomp 1/2] seccomp/cache: Add "emulator" to check if filter is
arg-dependent"). Sorry, that was a bit unclear.