linux-kernel - Re: [RFC/PATCH 5/7] x86/vm86: Teach handle_vm86

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Date:	Thu, 9 Jul 2015 15:41:54 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Rik van Riel <riel@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Borislav Petkov <bp@...en8.de>,
	Kees Cook <keescook@...omium.org>,
	Brian Gerst <brgerst@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC/PATCH 5/7] x86/vm86: Teach handle_vm86_trap to return to
 32bit mode directly

On Wed, Jul 8, 2015 at 12:24 PM, Andy Lutomirski <luto@...nel.org> wrote:
> The TIF_NOTIFY_RESUME hack it was using was buggy and unsupportable.
> vm86 mode was completely broken under ptrace, for example, because
> we'd never make it to v8086 mode.
>
> This code is still a huge, scary mess, but at least it's no longer
> tangled with the exit-to-userspace loop.

This patch is incorrect.  Brian, what's the ETA for your vm86 cleanup?
 If it's very soon, then I'll see if I can rely on it.  If not, I'll
have to come up with a way to fix this patch.

Grr.  The kernel state when handle_vm86_trap is called is absurd right
now.  Somehow we're supposed to survive do_trap, send a signal
corresponding to the outside-vm86 state, and exit vm86 cleanly (with
ax = 0), all before returning to user mode.  I doubt these semantics
are even intentional.

This code sucks.

--Andy
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