[PATCH] x86/kconfig/32: Make CONFIG_VM86 default to n and remove EXPERT

[Andy Lutomirski <luto@kernel.org>]

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.

Let's accelerate its slow death.  Remove it from EXPERT and default
it to n.  Distros should not enable it.  In the unlikely event that
some user needs it, they can easily re-enable it.

I've confirmed that 'make oldconfig' will set leave it set to y, so
there should be little or no unexpected breakage from this change.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/Kconfig | 26 ++++++++++++++++++++------
 1 file changed, 20 insertions(+), 6 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index aa94fd014fa2..b54994a28168 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -997,14 +997,28 @@ config X86_THERMAL_VECTOR
 	depends on X86_MCE_INTEL
 
 config VM86
-	bool "Enable VM86 support" if EXPERT
-	default y
+	bool "Enable VM86 support"
+	default n
 	depends on X86_32
 	---help---
-	  This option is required by programs like DOSEMU to run
-	  16-bit real mode legacy code on x86 processors. It also may
-	  be needed by software like XFree86 to initialize some video
-	  cards via BIOS. Disabling this option saves about 6K.
+	  This option allows user programs to put the CPU into V8086
+	  mode, which is an 80286-era approximation of 16-bit real mode.
+
+	  Some very old versions of X and/or vbetool require this option
+	  for user mode setting.  Similarly, DOSEMU will use it if
+	  available to accelerate real mode DOS programs.  However, any
+	  recent version of DOSEMU, X, or vbetool should be fully
+	  functional even without kernel VM86 support, as they will all
+	  fall back to software emulation.
+
+	  Anything that works on a 64-bit kernel is unlikely to need
+	  this option, as 64-bit kernels don't, and can't, support V8086
+	  mode.
+
+	  Unless you use very old userspace or need the last drop of
+	  performance in your real mode DOS games and can't use KVM, say
+	  N here.  It disables a fairly large attack surface in the
+	  kernel.
 
 config X86_16BIT
 	bool "Enable support for 16-bit segments" if EXPERT
-- 
2.4.3