linux-kernel - Re: Compat 32-bit syscall entry from 64-bit task!? [was: Re: [RFC,PATCH 1/2] seccomp_filters: system call filtering using BPF]

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Date:	Tue, 17 Jan 2012 18:31:14 -0800
From:	Andi Kleen <ak@...ux.intel.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Jamie Lokier <jamie@...reable.org>,
	Andi Kleen <andi@...stfloor.org>,
	Indan Zupancic <indan@....nu>,
	Andrew Lutomirski <luto@....edu>,
	Oleg Nesterov <oleg@...hat.com>,
	Will Drewry <wad@...omium.org>, linux-kernel@...r.kernel.org,
	keescook@...omium.org, john.johansen@...onical.com,
	serge.hallyn@...onical.com, coreyb@...ux.vnet.ibm.com,
	pmoore@...hat.com, eparis@...hat.com, djm@...drot.org,
	segoon@...nwall.com, rostedt@...dmis.org, jmorris@...ei.org,
	scarybeasts@...il.com, avi@...hat.com, penberg@...helsinki.fi,
	viro@...iv.linux.org.uk, mingo@...e.hu, akpm@...ux-foundation.org,
	khilman@...com, borislav.petkov@....com, amwang@...hat.com,
	eric.dumazet@...il.com, gregkh@...e.de, dhowells@...hat.com,
	daniel.lezcano@...e.fr, linux-fsdevel@...r.kernel.org,
	linux-security-module@...r.kernel.org, olofj@...omium.org,
	mhalcrow@...gle.com, dlaor@...hat.com,
	Roland McGrath <mcgrathr@...omium.org>
Subject: Re: Compat 32-bit syscall entry from 64-bit task!? [was: Re:
 [RFC,PATCH 1/2] seccomp_filters: system call filtering using BPF]

On Tue, Jan 17, 2012 at 06:27:19PM -0800, Linus Torvalds wrote:
> On Tue, Jan 17, 2012 at 6:04 PM, Jamie Lokier <jamie@...reable.org> wrote:
> >
> > I think Indan means code is running with 64-bit cs, but the kernel
> > treats int $0x80 as a 32-bit syscall and sysenter as a 64-bit syscall,
> > and there's no way for the ptracer to know which syscall the kernel
> > will perform, even by looking at all registers.  It looks like a hole
> > in ptrace which could be fixed.
> 
> We could possibly munge the "orig_ax" field to be different for the
> int80 vs syscall cases. That's really the only field that isn't direct
> x86 state. And it's 64 bits wide, but we really only care about the
> low 32 bits in the kernel. So a bit in the high bits that says "this
> was a int80 entry" would be possible.

That would be incompatible. However you could just add another virtual
register with such information (in fact I thought about that
when I did the compat code originally). However I don't think it'll salvage
the original broken by design ptrace jailer. And everyone else
so far has done fine without it.

-Andi
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