linux-kernel - Re: Compat 32-bit syscall entry from 64-bit task!?

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Date:	Wed, 08 Feb 2012 20:29:00 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Indan Zupancic <indan@....nu>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Jamie Lokier <jamie@...reable.org>,
	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,
	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>,
	"H.J. Lu" <hjl.tools@...il.com>
Subject: Re: Compat 32-bit syscall entry from 64-bit task!?

On 02/08/2012 08:20 PM, Indan Zupancic wrote:
> 
> CS is already available to user space, but any other value than 0x23 or 0x33
> will confuse user space, as that is all they know about. Apparently Xen uses
> different values, but if those are static then user space can check for them
> separately. But if the values change dynamically then some other way may be
> needed.
> 
> But does it make much sense to pass the CPU mode of user space if that mode
> can be changed at any moment? I don't think it really does. Can you give an
> example of how that info can be used by a ptracer?
> 

Uh... you could make THAT argument about ANY register state!

I believe H.J. can fill you in about the usage.

> 
> Only confusion I can think of is someone following the register values
> across a systemcall instruction. Then the swizzling may be unexpected.
> But if they do that they could check how the sycall was entered and
> compensate for that. (I can't think of any requirement why this would
> need to be race-free.)
> 

You'd have to know how you'd entered, which right now you don't have any
way to know.

	-hpa
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