linux-kernel - Re: stop breaking dosemu (Re: x86/kconfig/32: Rename CONFIG

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Date:	Wed, 2 Sep 2015 08:31:15 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Stas Sergeev <stsp@...t.ru>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Andrew Bird (Sphere Systems)" <ajb@...eresystems.co.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>, Brian Gerst <brgerst@...il.com>
Subject: Re: stop breaking dosemu (Re: x86/kconfig/32: Rename CONFIG_VM86 and
 default it to 'n')

On Wed, Sep 2, 2015 at 7:08 AM, Andy Lutomirski <luto@...capital.net> wrote:
> On Sep 2, 2015 2:51 AM, "Stas Sergeev" <stsp@...t.ru> wrote:
>>
>> https://lkml.org/lkml/2015/7/21/208
>>
>> Guys, you gonna be kidding.
>> Is this a new trend of breaking dosemu, or what?
>>
>>> VM86 is entirely broken if ptrace, syscall auditing, or
>>> NOHZ_FULL is in use.  The code is a big undocumented mess, it's
>>> a real PITA to test, and it looks like a big chunk of vm86_32.c
>>
>>
>> It is a CPU feature that kernel should support, and always
>> did without any problems. If it started to have problems because
>> of your actions, then you can as well fix your code.
>>
>> > No one should be using it anyway. Use DOSBOX or KVM instead.
>>
>> Have you done the benchmarks between dosbox and dosemu
>> before saying that? Please do, thanks. (don't forget to include
>> dosemu2 in your benchmarks too, as it outperforms both)
>
> I wasn't aware of your dosemu variant at the time, and I incorrectly
> thought that dosemu was unmaintained.
>
> Does real mode performance matter any more?
>
>>
>>> Let's accelerate its slow death.
>>
>>
>
> Dosemu is much less dead than I thought it was when I wrote that
> patch.  Sorry :(
>
>>
>> > + Enabling this option adds considerable attack surface to the
>> > + kernel and slows down system calls and exception handling.
>>
>> Yes, I realize that threatening people with the "considerable attack surface"
>> is a good way to "accelerate its slow death", but please care to explain
>> that attack surface, thankyou.
>
> The user_mode vs user_mode_vm thing was scary and contained at least
> one real bug.  That particular issue is gone now, though.
>
> Before Brian's cleanups, vm86 did horrible things to the entry asm,
> the stack layout, and signal handling, and that scared me.  That's
> hopefully in much better shape now, though.
>
> The mark_screen_rdonly thing is still kind of scary.  It changes PTEs
> on arbitrary mappings behind the vm's back.
>
> I'd be amenable to switching the default back to y and perhaps adding
> a sysctl to make the distros more comfortable.  Ingo, Kees, Brian,
> what do you think?

Yeah, I think putting this under distro control (and user control) is
the best option here.

-Kees


-- 
Kees Cook
Chrome OS Security
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