linux-kernel - Re: seccomp and ptrace. what is the correct order?

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Date:	Tue, 22 May 2012 22:07:05 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Will Drewry <wad@...omium.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Indan Zupancic <indan@....nu>,
	Roland McGrath <mcgrathr@...gle.com>,
	Eric Paris <netdev@...isplace.org>,
	linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org,
	kernel-hardening@...ts.openwall.com, mingo@...hat.com,
	oleg@...hat.com, peterz@...radead.org, rdunlap@...otime.net,
	tglx@...utronix.de, luto@....edu, eparis@...hat.com,
	serge.hallyn@...onical.com, pmoore@...hat.com,
	akpm@...ux-foundation.org, corbet@....net, eric.dumazet@...il.com,
	markus@...omium.org, coreyb@...ux.vnet.ibm.com,
	keescook@...omium.org
Subject: Re: seccomp and ptrace. what is the correct order?

On Tue, May 22, 2012 at 03:48:40PM -0500, Will Drewry wrote:
> On Tue, May 22, 2012 at 3:34 PM, H. Peter Anvin <hpa@...or.com> wrote:
> > The proposed patch seems to duplicate the functionality in
> > <asm/syscall.h>. ?Those macros also try to do the right thing in the
> > presence of compat.
> 
> That was my first thought too, so I ran a few simple tests.  gcc isn't
> smart enough to not add ~344 bytes of code to get the number and
> arguments for the x86/kernel/ptrace.c case I included (in the
> naive-est of integrations).  But I don't know that it justifies the
> extra patchwork or enforcing shared code across arches.
> 
> Regardless, the syscall entry + trace code can use some attention
> across the architectures. I don't know that
> one-more-layer-of-abstraction is the right answer (rather than just
> fixing the code). The biggest benefit would be having one-true
> syscall_trace_entry slow path. That said, the fast paths will be
> forever divergent so the opportunity for bugs like the ones pointed
> out will still be there.

FWIW, I'd prefer to have all that done inside __audit_syscall_entry(),
with
        context->arch       = syscall_get_arch(current, regs);
        context->major      = syscall_get_nr(current, regs);
	syscall_get_arguments(current, regs, 0, 4, context->argv);
done instead of initializations from arguments we are doing there now.
I seriously doubt that it would lead to worse code than what we currently
have.  If nothing else, we won't be passing that pile of arguments around.

And yes, asm/syscall.h stuff is probably the right approach here.  For
biarch ones syscall_get_arguments() is saner than doing them one by one...
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