linux-kernel - Re: [PATCH] x86_64,xen,espfix: Initialize espfix on secondary CPUs

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Date:	Tue, 15 Jul 2014 09:00:19 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	stable <stable@...r.kernel.org>
Subject: Re: [PATCH] x86_64,xen,espfix: Initialize espfix on secondary CPUs

On Tue, Jul 15, 2014 at 8:50 AM, H. Peter Anvin <hpa@...or.com> wrote:
> On 07/15/2014 08:38 AM, Konrad Rzeszutek Wilk wrote:
>>
>> I think just disallowing would be preferrable.
>>
>
> So I have yet to hear anyone talk about what the Xen PV IRET does for
> 16-bit stack segments.  Can we get the answer for that, please?
>

If I'm reading the right thing, it does:

 59 .Lforce_iret:
60         /* Mimic SYSRET behavior. */
61         movq  8(%rsp),%rcx            # RIP
62         movq  24(%rsp),%r11           # RFLAGS
63         ALIGN
64 /* No special register assumptions. */
65 iret_exit_to_guest:
66         addq  $8,%rsp
67 .Lft0:  iretq

while running on Xen's stack.  IOW, it leaks bits 31:16 of the Xen
stack, and there's nothing the guest can do about it short of using a
user-mode trampoline or disabling 16-bit stack segments entirely.

See:

http://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=xen/arch/x86/x86_64/entry.S;h=a3ed216b390c2e87a21ff377850ee34ee7f2bc74;hb=HEAD

and (search for do_iret):

http://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=xen/arch/x86/traps.c;h=677074b4e628ed99d407b1045d859355e590d604;hb=HEAD

>         -hpa
>
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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