Re: stop breaking dosemu (Re: x86/kconfig/32: Rename CONFIG_VM86 and default it to 'n')

From: Austin S Hemmelgarn
Date: Fri Sep 04 2015 - 08:34:54 EST

On 2015-09-04 06:46, Stas Sergeev wrote:
04.09.2015 13:09, Chuck Ebbert ÐÐÑÐÑ:
On Fri, 4 Sep 2015 00:28:04 +0300
Stas Sergeev <stsp@xxxxxxx> wrote:

03.09.2015 21:51, Austin S Hemmelgarn ÐÐÑÐÑ:
There are servers out there that have this enabled and _never_ use it
at all,
Unless I am mistaken, servers usually use special flavour of the
distro (different from desktop install), where of course this will
be disabled _compile time_.
Many (most?) distros use just one kernel for everything, because it's
just too much work to have a separate flavor for servers.
But for example menuconfig promotes CONFIG_PREEMPT_NONE for server
and CONFIG_PREEMPT for desktop. Also perhaps server would need an
lts version rather than latest.
I wonder if RHEL Server offers the generic desktop-suited kernel
with vm86() enabled?

In any case, if there is some generic mechanism to selectively
disable syscalls at run-time for server, then vm86() is of course
a good candidate. I wonder how many other syscalls are currently
run-time controlled? (those that are not marked as an "attack surface"
and defaulted to Y; I suppose the "attack surface" is currently only vm86())

OK, I think I need to clarify something here.

The attack surface of a given system refers to the number of different ways that someone could potentially attack that system. An individual syscall is not in itself an attack surface, but is part of the attack surface for the whole system. One of the core concepts of proactive security is to minimize the attack surface, because the fewer ways someone could possibly attack you, the less likely it is that they will succeed.

I however, referred to vm86 as a potential attack vector, which refers one way in which someone could attempt to attack the system (be it through arbitrary code execution , privilege escalation, or some other type of exploit), note that something does not need to have a known exploit to be classified as a potential attack vector (most black hat's out there will keep quiet about discovered exploits until they can actually make use of them themselves). By their very definition, every single site that userspace can call into the kernel is a _potential_ attack vector, including vm86(). vm86() is one of the more attractive syscalls to attempt to use as an attack vector on 32-bit x86 systems because it's relatively unaudited, significantly modifies the execution state of the processor, and is available on a majority of 32-bit x85 systems in the wild. This does not mean that it is exploitable directly, just that it's a possible target for an exploit.

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