[tip:x86/asm] x86/kconfig/32: Mark CONFIG_VM86 as BROKEN

[tip-bot for Andy Lutomirski <tipbot@zytor.com>]

Commit-ID:  0b02e20767a3b4d843d2c58cf031d9e31f60e39d
Gitweb:     http://git.kernel.org/tip/0b02e20767a3b4d843d2c58cf031d9e31f60e39d
Author:     Andy Lutomirski <luto@kernel.org>
AuthorDate: Tue, 7 Jul 2015 18:25:56 -0700
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 8 Jul 2015 11:04:45 +0200

x86/kconfig/32: Mark CONFIG_VM86 as BROKEN

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.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Acked-by: Arjan van de Ven <arjan@linux.intel.com>
Cc: <stable@vger.kernel.org> # Backport it as far back as possible
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/23d4709cee2fe92c32d41b99c7a3c1823725925a.1436312944.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/Kconfig | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index aa94fd0..a7648f9b 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -997,8 +997,8 @@ config X86_THERMAL_VECTOR
 	depends on X86_MCE_INTEL
 
 config VM86
-	bool "Enable VM86 support" if EXPERT
-	default y
+	bool "Enable VM86 support" if BROKEN
+	default n
 	depends on X86_32
 	---help---
 	  This option is required by programs like DOSEMU to run
@@ -1006,6 +1006,12 @@ config VM86
 	  be needed by software like XFree86 to initialize some video
 	  cards via BIOS. Disabling this option saves about 6K.
 
+	  Linux's VM86 support is poorly maintained, essentially never
+	  tested by upstream kernel developers, has quite a few known
+	  bugs, and is probably full of security holes.  The only thing
+	  that appears to use it is DOSEMU, and DOSBOX and KVM are
+	  better options these days.  Don't enable it.
+
 config X86_16BIT
 	bool "Enable support for 16-bit segments" if EXPERT
 	default y