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Date:	Wed, 22 Feb 2012 15:53:08 -0800
From:	Kees Cook <>
To:	Andrew Lutomirski <>
Cc:	Will Drewry <>, Indan Zupancic <>,,,,,,,,,,,,,,,,,,,,,,,,
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 <> wrote:
> On Wed, Feb 22, 2012 at 11:48 AM, Will Drewry <> wrote:
>> On Wed, Feb 22, 2012 at 2:34 AM, Indan Zupancic <> 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 Cook
ChromeOS Security
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