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

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Date:	Wed, 2 Sep 2015 14:40:50 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Stas Sergeev <stsp@...t.ru>
Cc:	Josh Boyer <jwboyer@...oraproject.org>,
	"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>,
	Kees Cook <keescook@...omium.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 2:12 PM, Stas Sergeev <stsp@...t.ru> wrote:
> 02.09.2015 23:55, Andy Lutomirski пишет:
>
>> On Wed, Sep 2, 2015 at 1:47 PM, Stas Sergeev <stsp@...t.ru> wrote:
>>>
>>> 02.09.2015 23:22, Josh Boyer пишет:
>>>>
>>>> On Wed, Sep 2, 2015 at 1:50 PM, Stas Sergeev <stsp@...t.ru> wrote:
>>>>>
>>>>> 02.09.2015 20:46, Josh Boyer пишет:
>>>>>>
>>>>>> On Wed, Sep 2, 2015 at 10:08 AM, Andy Lutomirski <luto@...capital.net>
>>>>>> wrote:
>>>>>>>
>>>>>>> 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?
>>>>>>
>>>>>> Can you please leave the default as N, and have a sysctl option to
>>>>>> enable it instead?  While dosemu might still be in use, it isn't going
>>>>>> to be the common case at all.  So from a distro perspective, I think
>>>>>> we'd probably rather have the default match the common case.
>>>>>
>>>>> The fact that fedora doesn't package dosemu, doesn't automatically
>>>>> mean all other distros do not too. Since when kernel defaults should
>>>>> match the ones of fedora?
>>>>
>>>> I didn't say that.
>>>
>>> What you said was:
>>> ---
>>>
>>> While dosemu might still be in use, it isn't going
>>> to be the common case at all.  So from a distro perspective
>>>
>>> ---
>>> ... which is likely true only in fedora circe.
>>>
>>>>     The default right now is N.
>>>
>>> In a not yet released kernel, unless I am mistaken.
>>> If fedora already provides that kernel, other distros likely not.
>>>
>>>>     I asked it be left
>>>> that way.  That's all.
>>>
>>> Lets assume its not yet N, unless there was a kernel release already.
>>> Its easy to get back if its not too late.
>>
>> How about CONFIG_SYSCTL_VM86_DEFAULT which defaults to Y?  Fedora
>> could set it to N.
>
> Sorry, I don't understand this sysctl proposal.
> Could you please educate me what is it all about?
> This sysctl will disable or enable the vm86() syscall at run-time,
> right? What does it give us? If you disable something in the
> config, this gives you, say, smaller kernel image. If OTOH you
> add the run-time switch, it gives you a bigger image, regardless
> of its default value.
> I might be missing something, but I don't understand what
> problem will this solve? Have I missed some earlier message
> in this thread?

For the 99%+ of users who don't use dosemu, it prevents exploits that
target vm86 from attacking their kernel.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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