Re: [PATCH] x86/kconfig/32: Mark CONFIG_VM86 as BROKEN

[Andy Lutomirski]

On Wed, Jul 8, 2015 at 11:48 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Wed, Jul 8, 2015 at 10:55 AM, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>> On Wed, Jul 8, 2015 at 10:49 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>>
>>> I don't know how to tell whether something is trying to use real mode,
>>> but I can play this just fine in DOSEMU on my 64-bit laptop:
>>
>> So a 64-bit distro obviously will never have used vm86 mode - it
>> doesn't work there. Never has. There's no sane way to get to vm86 mode
>> from long mode, that's just how the 64-bit extensions worked.
>>
>> (64-bit hardware obviously does support vm86 mode, but you have to
>> play games with mixing long mode and CPL0 32-bit protected mode to get
>> there, and we never did that).
>>
>> It's the 32-bit distros I would worry about. The ones that may have
>> well disabled emulation, because they have vm86 mode enabled.
>
> Speaking as the dosemu maintainer in Debian and Ubuntu, I can confirm
> what Andy mentioned: dosemu will kick over to emulation if SYS_vm86
> and SYS_vm86old fail. The other area I remember that used vm86 mode
> was non-KMS Xorg drivers and anything using svgalib that tried to do
> video card BIOS initialization.

Adam Jackson said on the Fedora list that everything uses x86emu these
days.  And haven't modern kernels already dropped most of the UMS
support already?

>
> Also, Andy, I think you weren't looking at i386 builds of Ubuntu.
> Current Ubuntu, and 12.04 ("Precise") LTS (supported until 2017), and
> 14.04 LTS (until 2019) releases all have CONFIG_VM86.

Hmm.  I was going off something someone said an IRC.  Apparently I
should have double-checked.

If you have a test system easily available, can you see what happens
if you try to do:

$ sudo auditctl -e 1
$ sudo auditctl -D # just in case you had a "-a task,never" rule installed
$ dosemu

on a system with CONFIG_VM86=y?  I bet it fails.  Maybe it gets lucky
due to the the bogus vm86 asm code managing to explode with
eax=-ENOSYS, triggering a fallback to emulation.

--Andy
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