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From: Andrew Lutomirski <luto@mit.edu>
Date: Wed, 22 Feb 2012 15:38:57 -0800
Message-ID: <CAObL_7Gc0U2JMBQr_kbcVUJwyJq7LRfgdFr+84LR=ec4kSCwEA@mail.gmail.com>
Subject: Re: [PATCH v10 07/11] signal, x86: add SIGSYS info and make it synchronous.
To: Will Drewry <wad@chromium.org>
Cc: Indan Zupancic <indan@nul.nu>, linux-kernel@vger.kernel.org,
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On Wed, Feb 22, 2012 at 11:48 AM, Will Drewry <wad@chromium.org> wrote:
> On Wed, Feb 22, 2012 at 2:34 AM, Indan Zupancic <indan@nul.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
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