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

[Kees Cook <keescook@chromium.org>]

On Mon, Feb 27, 2012 at 12:34 PM, Eric Paris <eparis@redhat.com> wrote:
> On Mon, 2012-02-27 at 12:00 -0800, Kees Cook wrote:
>> On Mon, Feb 27, 2012 at 11:25 AM, Eric Paris <eparis@redhat.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@redhat.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.
>
> Once again I send you down a bad path.  Your original patch was the
> best.  We should consider including a0-aX in a future version.  I was
> mistaken in foolishly believing that audit_syscall_entry() was done
> before secure_computing().  But if you look, that isn't the case.
> Please pretend I never said anything as you had it right the first time.

Heh, okay. But now I know more about audit, so that's good. :)

-Kees

-- 
Kees Cook
ChromeOS Security