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

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Message-ID: <CAK1hOcNbG1caVkaP062w62CG6fdcbXDnDe5=-GOhvRmO-CKy3A@mail.gmail.com>
Date:	Mon, 9 Mar 2015 20:31:55 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>, 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>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: entry_32.S: change ESPFIX test to not touch PT_OLDSS(%esp)

>> What we definitely should do here is at least frame this check with
>>> That being said, what ends up in the high bits of esp when we iret to
>>> vm86 mode?
>>
>> I don't know. I guess it's time to write an actual vm86 testcase :)
>
> Ick.  I can try...

I found an example which runs small bit of 16-bit code using vm86 machinery.
Tried in 32-bit kernel under qemu, it worked: printed "Hello".

View attachment "vm86.c" of type "text/x-csrc" (4700 bytes)