linux-kernel - Re: [PATCH] x86: entry_32.S: change ESPFIX test to not touch PT

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Message-ID: <CALCETrWK_qyYHUX3gzCN0EqYN6AAMiJMQVmfMDCsDRRewyra6g@mail.gmail.com>
Date:	Mon, 9 Mar 2015 12:51:56 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: entry_32.S: change ESPFIX test to not touch PT_OLDSS(%esp)

On Mon, Mar 9, 2015 at 12:26 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 03/09/2015 12:13 PM, Andy Lutomirski wrote:
>> On Mon, Mar 9, 2015 at 10:44 AM, H. Peter Anvin <hpa@...or.com> wrote:
>>> On 03/09/2015 09:44 AM, Linus Torvalds wrote:
>>>>
>>>> And remember: those zero-cost out-of-order branches turn quite
>>>> expensive if they *ever* mispredict. Even a 5% mispredict rate is
>>>> likely to mean "it's better to have a data dependency chain".
>>>>
>>>> So it could easily go either way. I'm not convinced the old code is bad at all.
>>>>
>>>
>>> I'm inclined to side with Linus here.  I'm hesitant to change this based
>>> on pure speculation.
>>>
>>> To answer Andy's question: I do believe we need espfix for V86 mode as well.
>>>
>>
>> I think we don't.  Did I screw up my test?
>>
>
> I don't see how your test executes V86 mode code at all, since there
> seems to be nothing mapped at the bottom of memory?

It executes the implicit #PF at the bottom of memory :)

Seriously, though, I think that IRET doesn't check whether the
instruction we're returning to can be fetched, so IRET will complete
successfully and then we'll get #PF.  The resulting SIGSEGV kicks us
out of vm86 mode, and, assuming that the kernel isn't buggy (hah!) the
v86 state will get saved back to the vm86plus_struct and we can see if
esp got corrupted.

--Andy
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