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

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Message-ID: <CABqD9ha8qWFZFp1RCSgseH-AbX+00dKpxoN4zguctDQTWuXbuQ@mail.gmail.com>
Date:	Thu, 23 Feb 2012 16:34:45 -0600
From:	Will Drewry <wad@...omium.org>
To:	kernel-hardening@...ts.openwall.com
Cc:	Roland McGrath <mcgrathr@...gle.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Kees Cook <keescook@...omium.org>,
	Andrew Lutomirski <luto@....edu>, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, linux-doc@...r.kernel.org,
	netdev@...r.kernel.org, x86@...nel.org, arnd@...db.de,
	davem@...emloft.net, mingo@...hat.com, oleg@...hat.com,
	peterz@...radead.org, rdunlap@...otime.net, 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: [kernel-hardening] Re: [PATCH v10 07/11] signal, x86: add SIGSYS
 info and make it synchronous.

On Thu, Feb 23, 2012 at 4:15 PM, Indan Zupancic <indan@....nu> wrote:
> On Thu, February 23, 2012 20:26, Will Drewry wrote:
>> Seems like there's an argument for another return code,
>> SECCOMP_RET_CORE, that resets/unblocks the SIGSYS handler since the
>> existing TRAP and KILL options seem to cover the other paths (signal
>> handler and do_exit).
>
> What about making SECCOMP_RET_TRAP dump core/send SIGSYS if there is
> no tracer with PTRACE_O_SECCOMP set? And perhaps go for a blockable
> SIGSYS? That way you only have KILL, ERRNO and TRAP, with the last
> one meaning deny, but giving someone else a chance to do something.
> Or is that just confusing?

I don't think it makes sense to mix up signal delivery for in-process
handling and ptrace. In particular, TRACE calls must assume t the
ptracer actually enacted a policy, but with TRAP as is, it always
rejects it.

> I don't think there should be too many return values, or else you
> put too much runtime policy into the filters.

I'd rather make it explicit than not.  This will be a quagmire if any
behavior is implicit.

> Sending SIGSYS is useful, but it's quite a bit less useful if user
> space can't handle it in a signal handler, so I don't think it's
> worth it to make a unblockable version.

I believe the point here would be that you'd get a useful coredump
without needing to enforce that the process can't handle normal SIGSYS
or other syscalls by blocking signal masking.

cheers!
will
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