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

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Message-ID: <CABqD9hbC8v-rUBW+4T_-EwDwe1+VDohQZweQ-cxOg2A-iDH5CQ@mail.gmail.com>
Date:	Wed, 22 Feb 2012 18:05:05 -0600
From:	Will Drewry <wad@...omium.org>
To:	Kees Cook <keescook@...omium.org>
Cc:	Andrew Lutomirski <luto@....edu>, 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 5:53 PM, Kees Cook <keescook@...omium.org> wrote:
> 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.

Hrm, it might be possible to do_exit(SIGSYS) which would be both. It
looks like tsk->exit_code would be SIGSYS then, but I'll look a little
more closely to see what that'll actually do.
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