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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [PATCH v2 seccomp 3/6] seccomp/cache: Add "emulator" to check if filter is arg-dependent
Date:   Fri, 25 Sep 2020 12:51:20 -0700
Message-Id: <2FA23A2E-16B0-4E08-96D5-6D6FE45BBCF6@amacapital.net>
References: <202009251223.8E46C831E2@keescook>
Cc:     YiFei Zhu <zhuyifei1999@gmail.com>,
        Linux Containers <containers@lists.linux-foundation.org>,
        YiFei Zhu <yifeifz2@illinois.edu>, bpf <bpf@vger.kernel.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        Aleksa Sarai <cyphar@cyphar.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Dimitrios Skarlatos <dskarlat@cs.cmu.edu>,
        Giuseppe Scrivano <gscrivan@redhat.com>,
        Hubertus Franke <frankeh@us.ibm.com>,
        Jack Chen <jianyan2@illinois.edu>,
        Jann Horn <jannh@google.com>,
        Josep Torrellas <torrella@illinois.edu>,
        Tianyin Xu <tyxu@illinois.edu>,
        Tobin Feldman-Fitzthum <tobin@ibm.com>,
        Tycho Andersen <tycho@tycho.pizza>,
        Valentin Rothberg <vrothber@redhat.com>,
        Will Drewry <wad@chromium.org>
In-Reply-To: <202009251223.8E46C831E2@keescook>
To:     Kees Cook <keescook@chromium.org>
Precedence: bulk


> On Sep 25, 2020, at 12:42 PM, Kees Cook <keescook@chromium.org> wrote:
>=20
> =EF=BB=BFOn Fri, Sep 25, 2020 at 11:45:05AM -0500, YiFei Zhu wrote:
>> On Thu, Sep 24, 2020 at 10:04 PM YiFei Zhu <zhuyifei1999@gmail.com> wrote=
:
>>>> Why do the prepare here instead of during attach? (And note that it
>>>> should not be written to fail.)
>>>=20
>>> Right.
>>=20
>> During attach a spinlock (current->sighand->siglock) is held. Do we
>> really want to put the emulator in the "atomic section"?
>=20
> It's a good point, but I had some other ideas around it that lead to me
> a different conclusion. Here's what I've got in my head:
>=20
> I don't view filter attach (nor the siglock) as fastpath: the lock is
> rarely contested and the "long time" will only be during filter attach.
>=20
> When performing filter emulation, all the syscalls that are already
> marked as "must run filter" on the previous filter can be skipped for
> the new filter, since it cannot change the outcome, which makes the
> emulation step faster.
>=20
> The previous filter's bitmap isn't "stable" until siglock is held.
>=20
> If we do the emulation step before siglock, we have to always do full
> evaluation of all syscalls, and then merge the bitmap during attach.
> That means all filters ever attached will take maximal time to perform
> emulation.
>=20
> I prefer the idea of the emulation step taking advantage of the bitmap
> optimization, since the kernel spends less time doing work over the life
> of the process tree. It's certainly marginal, but it also lets all the
> bitmap manipulation stay in one place (as opposed to being split between
> "prepare" and "attach").
>=20
> What do you think?
>=20
>=20

I=E2=80=99m wondering if we should be much much lazier. We could potentially=
 wait until someone actually tries to do a given syscall before we try to ev=
aluate whether the result is fixed.=