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

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Date:	Thu, 19 Jan 2012 08:13:27 -0800
From:	Andrew Lutomirski <luto@....edu>
To:	Jamie Lokier <jamie@...reable.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Indan Zupancic <indan@....nu>,
	Andi Kleen <andi@...stfloor.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 Thu, Jan 19, 2012 at 8:01 AM, Jamie Lokier <jamie@...reable.org> wrote:
> Andrew Lutomirski wrote:
>> 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.)
>
> Here it is:
>
>        static inline bool user_64bit_mode(struct pt_regs *regs)
>        {
>        #ifndef CONFIG_PARAVIRT
>                /*
>                 * On non-paravirt systems, this is the only long mode CPL 3
>                 * selector.  We do not allow long mode selectors in the LDT.
>                 */
>                return regs->cs == __USER_CS;
>        #else
>                /* Headers are too twisted for this to go in paravirt.h. */
>                return regs->cs == __USER_CS || regs->cs == pv_info.extra_user_64bit_cs;
>        #endif
>        }
>
> Perhaps userspace can do that.
> Would it be right for a ptracer to say:
>
>   CS == 0x23 -> 32-bit
>   (CS & 4)   -> 32-bit (LDT, "we do not allow long mode selectors in the LDT")
>   else       -> 64-bit (__USER_CS or some other GDT entry which must be pv_info's)
>
> I.e. assume that no other *GDT* CS values are available to userspace?
> There are other 32-bit GDT entries, but are they not all for data or kernel use only?

I suspect not.  asm/xen/interface_64.h has:

#define FLAT_RING3_CS32 0xe023  /* GDT index 260 */
#define FLAT_RING3_CS64 0xe033  /* GDT index 261 */
#define FLAT_RING3_DS32 0xe02b  /* GDT index 262 */
#define FLAT_RING3_DS64 0x0000  /* NULL selector */
#define FLAT_RING3_SS32 0xe02b  /* GDT index 262 */
#define FLAT_RING3_SS64 0xe02b  /* GDT index 262 */

which sounds like there's an extra 32-bit selector as well.  I haven't
checked, though.

--Andy
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