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Date: Wed, 18 Jan 2012 02:50:13 +0100
From: Andi Kleen <andi@firstfloor.org>
To: Indan Zupancic <indan@nul.nu>
Cc: Andrew Lutomirski <luto@mit.edu>, Oleg Nesterov <oleg@redhat.com>,
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        Andi Kleen <andi@firstfloor.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]
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> Our ptrace jailer is checking cs to figure out if a task is a compat task
> or not, if the kernel can change that behind our back it means our jailer

Every user program change it behind your back.

Your ptrace jailer isn't.

> I think this behaviour is so unexpected that it can only cause security
> problems in the long run. Is anyone counting on this? Where is this
> behaviour documented?

Look up far jumps in any x86 manual.

-Andi
-- 
ak@linux.intel.com -- Speaking for myself only.