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Date: Thu, 19 Jan 2012 02:47:25 +0100
Subject: Re: Compat 32-bit syscall entry from 64-bit task!? [was: Re:
 [RFC,PATCH 1/2] seccomp_filters: system call filtering using BPF]
From: "Indan Zupancic" <indan@nul.nu>
To: "Andrew Lutomirski" <luto@mit.edu>
Cc: "Oleg Nesterov" <oleg@redhat.com>, "Will Drewry" <wad@chromium.org>,
        linux-kernel@vger.kernel.org, keescook@chromium.org,
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On Thu, January 19, 2012 02:19, Andrew Lutomirski wrote:
> On Wed, Jan 18, 2012 at 5:06 PM, Indan Zupancic <indan@nul.nu> wrote:
>> On Wed, January 18, 2012 02:01, Andrew Lutomirski wrote:
>>> On Tue, Jan 17, 2012 at 4:56 PM, Indan Zupancic <indan@nul.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