linux-kernel - Re: [PATCH v11 06/12] seccomp: add system call filtering using BPF

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Message-ID: <CAGXu5jKyLHLF+HuU=SLHJLg8AaOPJXoqq1=hOBy5oOvffA-xiQ@mail.gmail.com>
Date:	Mon, 27 Feb 2012 12:00:37 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Eric Paris <eparis@...hat.com>
Cc:	Will Drewry <wad@...omium.org>, Kees Cook <kees@...ntu.com>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-doc@...r.kernel.org, kernel-hardening@...ts.openwall.com,
	netdev@...r.kernel.org, x86@...nel.org, arnd@...db.de,
	davem@...emloft.net, hpa@...or.com, mingo@...hat.com,
	oleg@...hat.com, peterz@...radead.org, rdunlap@...otime.net,
	mcgrathr@...omium.org, tglx@...utronix.de, luto@....edu,
	serge.hallyn@...onical.com, djm@...drot.org, scarybeasts@...il.com,
	indan@....nu, pmoore@...hat.com, akpm@...ux-foundation.org,
	corbet@....net, eric.dumazet@...il.com, markus@...omium.org,
	coreyb@...ux.vnet.ibm.com
Subject: Re: [PATCH v11 06/12] seccomp: add system call filtering using BPF

On Mon, Feb 27, 2012 at 11:25 AM, Eric Paris <eparis@...hat.com> wrote:
> On Mon, 2012-02-27 at 10:55 -0800, Kees Cook wrote:
>> On Mon, Feb 27, 2012 at 8:49 AM, Eric Paris <eparis@...hat.com> wrote:
>
>> You mean as used in audit_log_exit() ? It looks like that depends on a
>> lot of state cached in __audit_syscall_entry() and finally triggered
>> in __audit_syscall_exit() (and ..._free()). I don't think this is
>> really want seccomp wants to be involved in.
>>
>> By CONFIG_AUDITSC, you mean CONFIG_AUDITSYSCALL? Without that set,
>> audit_seccomp is a no-op.
>>
>> The reason compat needs to be reported (or rather, arch) is because
>> just reporting syscall is ambiguous. It either needs arch or compat to
>> distinguish it.
>
> Yes, that is what I mean and you are right.  You shouldn't push the
> syscall in this record either.  If !audit_dummy_context() you are
> already going to get arch, syscall, and a0-a4 in the associated audit
> record.  Please do not duplicate that info.

Ah, in that case, please ignore the patch I just sent. Heh.

> It might make sense to have a separate audit_seccomp() path when
> audit_dummy_context() which includes arch, syscall, and a0-a4.

Ah! I think I understand what you mean now. If audit_dummy_context(),
then the syscall, arch, and a0-a4 were not already collected. Gotcha.

How do you envision it looking? I still see it as two distinct events
(the syscall itself, and the rejection). Would you want those details
added to the context structure to be reported at ..._exit() time? It
seems like context->type couldn't be used to see if those fields were
valid.

Something like:

void __audit_seccomp(unsigned long syscall, long signr)
{
        struct audit_buffer *ab;

        if (!audit_dummy_context()) {
                struct audit_context *context = current->audit_context;
                context->syscall_signr = signr;
                context->syscall_ip = KSTK_EIP(current);
                return;
        }

        ab = audit_log_start(NULL, GFP_KERNEL, AUDIT_ANOM_ABEND);
        audit_log_abend(ab, "seccomp", signr);
        audit_log_format(ab, " syscall=%ld", syscall);
        audit_log_format(ab, " ip=0x%lx", KSTK_EIP(current));
        audit_log_end(ab);
}

And then report syscall_ip and syscall_signr if syscall_signr != 0 in
the _exit()? I think everything else from audit_log_abend() will end
up in the _exit() report.

> It is my fault (85e7bac3) that we have syscall at all, but I'm on a new
> crusade to remove audit record duplication.  So I'd happily see a patch
> in this series that removes that instead of adds to it.

Well, I think the abend reporting is nice; I'd hate to see that
totally removed. The seccomp case is a bit different, I agree. I could
see it either way.

-Kees

-- 
Kees Cook
ChromeOS Security
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