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 02:04:53 +0000
From:	Jamie Lokier <jamie@...reable.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Indan Zupancic <indan@....nu>, Andrew Lutomirski <luto@....edu>,
	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>
Subject: Re: Compat 32-bit syscall entry from 64-bit task!? [was: Re:
 [RFC,PATCH 1/2] seccomp_filters: system call filtering using BPF]

Andi Kleen wrote:
> > 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.
..
> Look up far jumps in any x86 manual.

I'm pretty sure this isn't about changing cs or far jumps

I think Indan means code is running with 64-bit cs, but the kernel
treats int $0x80 as a 32-bit syscall and sysenter as a 64-bit syscall,
and there's no way for the ptracer to know which syscall the kernel
will perform, even by looking at all registers.  It looks like a hole
in ptrace which could be fixed.

-- Jamie
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