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From: Andrew Lutomirski <luto@mit.edu>
Date: Wed, 18 Jan 2012 17:19:53 -0800
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Subject: Re: Compat 32-bit syscall entry from 64-bit task!? [was: Re:
 [RFC,PATCH 1/2] seccomp_filters: system call filtering using BPF]
To: Indan Zupancic <indan@nul.nu>
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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.

--Andy