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

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Date:	Thu, 9 Jul 2015 11:03:25 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Pavel Machek <pavel@....cz>, Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...en8.de>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Oleg Nesterov <oleg@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Matthew Garrett <mjg59@...f.ucam.org>
Subject: Re: [PATCH] x86/kconfig/32: Mark CONFIG_VM86 as BROKEN

On Thu, Jul 9, 2015 at 10:57 AM, Andy Lutomirski <luto@...capital.net> wrote:
> On Jul 9, 2015 2:03 AM, "Pavel Machek" <pavel@....cz> wrote:
>>
>> On Wed 2015-07-08 16:00:48, Thomas Gleixner wrote:
>> > On Tue, 7 Jul 2015, Arjan van de Ven wrote:
>> >
>> > > On 7/7/2015 6:25 PM, Andy Lutomirski wrote:
>> > > > 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 is dead code.  It
>> > > > also plays awful games with the entry asm.
>> > > >
>> > > > No one should be using it anyway.  Use DOSBOX or KVM instead.
>> > > >
>> > > > Mark it BROKEN.  I want to remove some (obviously incorrect) exit
>> > > > asm that it depends on, and I don't want to figure out how to run
>> > > > severely obsolete programs just to test something that no one uses
>> > > > for anything other than exploits anyway.
>> > > >
>> > >
>> > > while it is never great to deprecate features, in this case I am not sure
>> > > there is another choice unless someone steps up to seriously revamp this code.
>> > > (and look at it from a PREEMPT, NO_HZ etc etc angle)
>> >
>> > Aside of being broken in so many aspects it's even more obsolete than
>> > 386 support, we should just remove it right away.
>>
>> Bad news for you:
>>
>> vbetool-0.5/lrmi.c:#include <asm/vm86.h>
>> vbetool-0.5/lrmi.c:#include <sys/vm86.h>
>> vbetool-0.5/lrmi.c:#include <machine/vm86.h>
>> vbetool-0.5/lrmi.c:         struct vm86_struct vm;
>> vbetool-0.5/lrmi.c:                struct vm86_init_args init;
>> ...
>> vbetool-0.5/lrmi.c:lrmi_vm86(struct vm86_struct *vm)
>> vbetool-0.5/lrmi.c:#define lrmi_vm86 vm86
>> vbetool-0.5/lrmi.c:        fputs("vm86() failed\n", stderr);
>> vbetool-0.5/lrmi.c:run_vm86(void)
>> vbetool-0.5/lrmi.c:             vret = lrmi_vm86(&context.vm);
>> vbetool-0.5/lrmi.c:vm86_callback(int sig, int code, struct sigcontext
>> *sc)
>> vbetool-0.5/lrmi.c:vm86_callback(int sig, int code, struct sigcontext
>> *sc)
>> vbetool-0.5/lrmi.c:run_vm86(void)
>> vbetool-0.5/lrmi.c:             fprintf(stderr, "run_vm86: callback
>> already installed\n");
>>
>> vbetool depends on it, and s2ram depends on vbetool. When we get
>> proper kernel drivers, this one will be solved, but it is not "more
>> obsolete than 386".
>>
>
> vmetool has an x86 emulator.  As far as I know, it's been there for a
> long time, and I'd be surprised if it doesn't work on CONFIG_VM86=n
> kernels.  That being said, the code is kind of tangled and it's not
> quite clear to me what's going on.
>
> See: http://www.codon.org.uk/~mjg59/libx86/
>
> Perhaps we should instead move CONFIG_VM86 out of EXPERT, default it
> to n, and suggest that everyone running a reasonably modern distro
> (2006 and up?) turn it off.

That seems like a good idea to me.

-Kees

-- 
Kees Cook
Chrome OS Security
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