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

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Message-ID: <54FDDB90.9010706@zytor.com>
Date:	Mon, 09 Mar 2015 10:42:40 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Denys Vlasenko <dvlasenk@...hat.com>
CC:	Andy Lutomirski <luto@...capital.net>,
	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 03/09/2015 09:08 AM, Linus Torvalds wrote:
> 
> Sure, the btl is easier to explain in the source code, but instead of this:
> 
>> +       btl     $X86_EFLAGS_VM_BIT,PT_EFLAGS(%esp)
> 
> you'd have to add a comment, like
> 
>     testb $2, PT_EFLAGS+2(%esp)  # X86_EFLAGS_VM_BIT
> 
> or something.
> 

Maybe:

	testb $(X86_EFLAGS_VM-16), PT_EFLAGS+2(%esp)

> Or just at least *partially* do what we used to do, and make it all be
> 
>     movb  PT_EFLAGS+2(%esp),%al
>     andb $2,%al
>     orb PT_CS(%esp),%al
>     testb $3,%al
>     je restore_nocheck
>     testb $SEGMENT_TI_MASK,PT_OLDSS(%esp)
>     jne ldt_ss
> 
> which still avoids looking at SS unless needed, and is smaller and
> faster than the btl, afaik.

The question is if avoiding looking at a field on the stack matters at all.

	-hpa


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