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

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Message-ID: <CAObL_7FsNQaPQg-3=AO9RuMNZZsRgDSJYPhPevDKERws9vQJWg@mail.gmail.com>
Date:	Wed, 18 Jan 2012 11:38:16 -0800
From:	Andrew Lutomirski <luto@....edu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Indan Zupancic <indan@....nu>, Andi Kleen <andi@...stfloor.org>,
	Jamie Lokier <jamie@...reable.org>,
	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>
Subject: Re: Compat 32-bit syscall entry from 64-bit task!?

On Wed, Jan 18, 2012 at 11:31 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> The actually used code just does
>
>                /* Check CS register value. On x86-64 linux it is:
>                 *      0x33    for long mode (64 bit)
>                 *      0x23    for compatibility mode (32 bit)
>                 * It takes only one ptrace and thus doesn't need
>                 * to be cached.
>                 */
>                if (upeek(tcp, 8*CS, &val) < 0)
>                        return -1;
>                switch (val) {
>                        case 0x23: currpers = 1; break;
>                        case 0x33: currpers = 0; break;
>
> which is the reasonable and obvious approach.

*sigh*

It's reasonable, obvious, and even more wrong than it appears.  On
Xen, there's an extra 64-bit GDT entry, and it gets used by default.
(I got bitten by this in some iteration of the vsyscall emulation
patches -- see user_64bit_mode for the correct and
unusable-from-user-mode way to do this.)

--Andy
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