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

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Message-ID: <20120119160113.GN7180@jl-vm1.vm.bytemark.co.uk>
Date:	Thu, 19 Jan 2012 16:01:13 +0000
From:	Jamie Lokier <jamie@...reable.org>
To:	Andrew Lutomirski <luto@....edu>
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!?

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?

-- Jamie
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