linux-kernel - Re: [PATCH v10 07/11] signal, x86: add SIGSYS info and make it synchronous.

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Message-ID: <CAGXu5jJQrnkCZtcLqK8NEihRgB25C12b0nKp+XWXV_=4QfcVoA@mail.gmail.com>
Date:	Wed, 22 Feb 2012 15:53:08 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Andrew Lutomirski <luto@....edu>
Cc:	Will Drewry <wad@...omium.org>, Indan Zupancic <indan@....nu>,
	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, eparis@...hat.com,
	serge.hallyn@...onical.com, djm@...drot.org, scarybeasts@...il.com,
	pmoore@...hat.com, akpm@...ux-foundation.org, corbet@....net,
	eric.dumazet@...il.com, markus@...omium.org
Subject: Re: [PATCH v10 07/11] signal, x86: add SIGSYS info and make it synchronous.

On Wed, Feb 22, 2012 at 3:38 PM, Andrew Lutomirski <luto@....edu> wrote:
> On Wed, Feb 22, 2012 at 11:48 AM, Will Drewry <wad@...omium.org> wrote:
>> On Wed, Feb 22, 2012 at 2:34 AM, Indan Zupancic <indan@....nu> wrote:
>>> On Tue, February 21, 2012 18:30, Will Drewry wrote:
>>>> This change enables SIGSYS, defines _sigfields._sigsys, and adds
>>>> x86 (compat) arch support.  _sigsys defines fields which allow
>>>> a signal handler to receive the triggering system call number,
>>>> the relevant AUDIT_ARCH_* value for that number, and the address
>>>> of the callsite.
>>>>
>>>> To ensure that SIGSYS delivery occurs on return from the triggering
>>>> system call, SIGSYS is added to the SYNCHRONOUS_MASK macro.  I'm
>>>> this is enough to ensure it will be synchronous or if it is explicitly
>>>> required to ensure an immediate delivery of the signal upon return from
>>>> the blocked system call.
>>>>
>>>> The first consumer of SIGSYS would be seccomp filter.  In particular,
>>>> a filter program could specify a new return value, SECCOMP_RET_TRAP,
>>>> which would result in the system call being denied and the calling
>>>> thread signaled.  This also means that implementing arch-specific
>>>> support can be dependent upon HAVE_ARCH_SECCOMP_FILTER.
>>>
>>> I think others said this is useful, but I don't see how. Easier
>>> debugging compared to checking return values?
>>>
>>> I suppose SIGSYS can be blocked, so there is no guarantee the process
>>> will be killed.
>>
>> Yeah, this allows for in-process system call emulation, if desired, or
>> for the process to dump core/etc.  With RET_ERRNO or RET_KILL, there
>> isn't any feedback to the system about the state of the process.  Kill
>> populates audit_seccomp and dmesg, but if the application
>> user/developer isn't the system admin, installing audit bits or
>> checking system logs seems onerous.
>
> [Warning: this suggestion may be bad for any number of reasons]
>
> I wonder if it would be helpful to change the semantics of RET_KILL
> slightly.  Rather than killing via do_exit, what if it killed via a
> forcibly-fatal SIGSYS?  That way, the parent's waitid() / SIGCHLD
> would indicate CLD_KILLED with si_status == SIGSYS.  The parent could
> check that and report that the child was probably compromised.
>
> --Andy

I'd prefer sticking with do_exit. This provides much less chance of
things going wrong. A parent seeing a child killed with SIGKILL is
already pretty distinct, IMO.

-Kees

-- 
Kees Cook
ChromeOS Security
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