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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Message-ID: <2b59b7f90f89952223d4b4c063c62b75.squirrel@webmail.greenhost.nl>
Date:	Thu, 19 Jan 2012 02:47:25 +0100
From:	"Indan Zupancic" <indan@....nu>
To:	"Andrew Lutomirski" <luto@....edu>
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 Thu, January 19, 2012 02:19, Andrew Lutomirski wrote:
> 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.

That's fine as long as a cs value of 0x23 or 0x33 gives reliable information.
Not running is highly prefered above running insecurely. Security first,
functionality second.

Greetings,

Indan


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