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:	Wed, 18 Jan 2012 17:19:53 -0800
From:	Andrew Lutomirski <luto@....edu>
To:	Indan Zupancic <indan@....nu>
Cc:	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, torvalds@...ux-foundation.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,
	ak@...ux.intel.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>,
	Andi Kleen <andi@...stfloor.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 Wed, Jan 18, 2012 at 5:06 PM, Indan Zupancic <indan@....nu> wrote:
> On Wed, January 18, 2012 02:01, Andrew Lutomirski wrote:
>> On Tue, Jan 17, 2012 at 4:56 PM, Indan Zupancic <indan@....nu> wrote:
>> I don't know what your ptrace jailer does.  But a task can switch
>> itself between 32-bit and 64-bit execution at will, and there's
>> nothing the kernel can do about it.  (That isn't quite true -- in
>> theory the kernel could fiddle with the GDT, but that would be
>> expensive and wouldn't work on Xen.)
>
> That's why we don't cache the CS value but check it for every system call.
> But you said elsewhere that checking CS isn't always correct either.
> I grepped arch/x86 for "user_64bit_mode", but couldn't find anything,
> but maybe my kernel sources are too old, I haven't updated this system
> for almost a year. The current code only handles 0x23 and 0x33 and kills
> the jail if it encounters anything else.

I think you're hosed on Xen, then.  Xen regularly runs with a
different Xen-specific cs value.

--Andy
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