linux-kernel - Re: [PATCH] x86/kconfig/32: Mark CONFIG

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Message-ID: <CA+55aFxWfNb4Bj1tsxkCgpbaSseioEV=AenGX7=sgz2mzJUWYA@mail.gmail.com>
Date:	Fri, 10 Jul 2015 09:35:34 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Andy Lutomirski <luto@...nel.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Kees Cook <keescook@...omium.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH] x86/kconfig/32: Mark CONFIG_VM86 as BROKEN

On Fri, Jul 10, 2015 at 7:37 AM, Andy Lutomirski <luto@...capital.net> wrote:
>
> Having just written a pile of tests for it, I don't think so, as long as none
> of the syscall slow path stuff is in use :(

It seems that you are thinking that people actually use vm86 mode as a
real Linux mode, and do system calls from it etc.

I'm sure that has happened in some crazy situation (people doing some
random pseudo-BIOS etc), but it's not the common situation at all.

The common situation is that you enter vm86 mode with vm86(), and that
you exit it due to one of the (many) unhandled situations or a signal
or whatever. Yeah,we handle a few sad instructions directly, but most
vm86 exits just return to user mode.

The system call paths just aren't an issue in reality, because they
just aren't used.

And I'm personally violently against Ingo's idea of emulating this
with an instruction emulator. Hell no. That's what user mode does, and
it's fine there. In the kernel, we either support the hardware vm86
mode, or we phase it out because we can show that nobody uses it any
more. None of that "let's emulate it in software" crud.

                      Linus
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