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

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Date:   Fri, 25 Sep 2020 19:47:47 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     YiFei Zhu <zhuyifei1999@...il.com>
Cc:     Kees Cook <keescook@...omium.org>,
        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 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.

> Is it worth holding a spinlock here?
> 
> Though I'll try to get the benchmark numbers for the emulator later tonight.
> 
> YiFei Zhu