linux-kernel - Re: [PATCH v2 seccomp 3/6] seccomp/cache: Add "emulator" to check if filter is arg-dependent

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Message-ID: <202009252134.871EFAB61@keescook>
Date:   Fri, 25 Sep 2020 21:35:01 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     YiFei Zhu <zhuyifei1999@...il.com>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        YiFei Zhu <yifeifz2@...inois.edu>, bpf <bpf@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Aleksa Sarai <cyphar@...har.com>,
        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>,
        Jann Horn <jannh@...gle.com>,
        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 3/6] seccomp/cache: Add "emulator" to check if
 filter is arg-dependent

On Fri, Sep 25, 2020 at 07:47:47PM -0700, Andy Lutomirski wrote:
> 
> > On Sep 25, 2020, at 6:23 PM, YiFei Zhu <zhuyifei1999@...il.com> wrote:
> > 
> > On Fri, Sep 25, 2020 at 4:07 PM Andy Lutomirski <luto@...capital.net> wrote:
> >> We'd need at least three states per syscall: unknown, always-allow,
> >> and need-to-run-filter.
> >> 
> >> The downsides are less determinism and a bit of an uglier
> >> implementation.  The upside is that we don't need to loop over all
> >> syscalls at load -- instead the time that each operation takes is
> >> independent of the total number of syscalls on the system.  And we can
> >> entirely avoid, say, evaluating the x32 case until the task tries an
> >> x32 syscall.
> > 
> > I was really afraid of multiple tasks writing to the bitmaps at once,
> > hence I used bitmap-per-task. Now I think about it, if this stays
> > lockless, the worst thing that can happen is that a write undo a bit
> > set by another task. In this case, if the "known" bit is cleared then
> > the worst would be the emulation is run many times. But if the "always
> > allow" is cleared but not "known" bit then we have an issue: the
> > syscall will always be executed in BPF.
> > 
> 
> If you interleave the bits, then you can read and write them atomically — both bits for any given syscall will be in the same word.

I think we can just hold the spinlock. :)

-- 
Kees Cook