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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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