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

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Message-ID: <CALCETrXr1S=5y734BVzHGTk74+qHAz-D692nd57HwqVN2sOU1g@mail.gmail.com>
Date:	Fri, 10 Jul 2015 07:37:15 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 7:12 AM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
> Andy Lutomirski <luto@...capital.net> writes:
>
>> On the other hand, sys_vm86 fails if the syscall slow path is in use.
>> That means that quite a few Fedora versions (auditing), anything with
>> ptrace, seccomp (before 3.16 IIRC), and anything with context tracking
>> is probably actually *improved* by turning off the vm86 syscalls even
>> for dosemu users.
>
> Is there any chance that vm86 is sufficiently badly broken before this
> that we can conclude vm86 is not in use?  It would really simplify this
> discussion if we could point to code rot and say that it is clear that
> no one has been testing this code path for ages, and that the code can't
> possibly work the way it is now.  That would just let us remove vm86.
>

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 only says "[OK]" because my test case isn't careful enough.  That's
>> a failure.  I suspect it was a much worse failure a couple versions
>> ago before my ENOSYS-reworking patch went in.
>>
>> I'll try to confirm later this week that dosemu can really handle real
>> mode without sys_vm86.
>
> I have not looked in ages but certainly on 64bit dosemu can.
>
> As someone else pointed out dosemu maps the zero page so that may also
> be a point where vm86 support gets broken.

Right.  And someone pointed out that vbetool sometimes needs access to
virtual (or emulated virtual) addresses above 3GB, and vm86 can't do
that.

--Andy
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