linux-kernel - Re: [PATCH 5/7] x86: Split syscall_trace

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Message-ID: <CALCETrWuWxZANoKAR=FG55EOHtzvTHmc3euMdtf8yfibtK-QYw@mail.gmail.com>
Date:	Wed, 16 Jul 2014 14:15:44 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Kees Cook <keescook@...omium.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Will Drewry <wad@...omium.org>,
	James Morris <james.l.morris@...cle.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Linux MIPS Mailing List <linux-mips@...ux-mips.org>,
	linux-arch <linux-arch@...r.kernel.org>,
	linux-security-module <linux-security-module@...r.kernel.org>,
	Alexei Starovoitov <ast@...mgrid.com>
Subject: Re: [PATCH 5/7] x86: Split syscall_trace_enter into two phases

On Wed, Jul 16, 2014 at 1:21 PM, Kees Cook <keescook@...omium.org> wrote:
> On Tue, Jul 15, 2014 at 12:32 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>
>> +
>> +               ret = seccomp_phase1(&sd);
>> +               if (ret == SECCOMP_PHASE1_SKIP) {
>> +                       regs->orig_ax = -ENOSYS;
>
> Before, seccomp didn't touch orig_ax on a skip. I don't see any
> problem with this, and it's probably more clear this way, but are you
> sure there aren't unexpected side-effects from this?

It's necessary to cause the syscall to be skipped -- see syscall_trace_enter.

That being said, setting it to -ENOSYS is nonsense and probably
confused you at least as much as it confused me.  It should be
regs->orig_ax = -1.

--Andy
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