x86/mm: Remove CONFIG_DEBUG_NX_TEST

CONFIG_DEBUG_NX_TEST has been broken since CONFIG_DEBUG_SET_MODULE_RONX=y
was added in v2.6.37 via:

  84e1c6b ("x86: Add RO/NX protection for loadable kernel modules")

since the exception table was then made read-only.

Additionally, the manually constructed extables were never fixed when
relative extables were introduced in v3.5 via:

  7062765 ("x86, extable: Switch to relative exception table entries")

However, relative extables won't work for test_nx.c, since test instruction
memory areas may be more than INT_MAX away from an executable fixup
(e.g. stack and heap too far away from executable memory with the fixup).

Since clearly no one has been using this code for a while now, and similar
tests exist in LKDTM, this should just be removed entirely.

Signed-off-by: Kees Cook <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Arjan van de Ven <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Jinbum Park <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/20170131003711.GA74048@beast
Signed-off-by: Ingo Molnar <[email protected]>

arch/x86/Kconfig.debug

@@ -120,14 +120,6 @@ config DEBUG_SET_MODULE_RONX
 	  against certain classes of kernel exploits.
 	  If in doubt, say "N".
 
-config DEBUG_NX_TEST
-	tristate "Testcase for the NX non-executable stack feature"
-	depends on DEBUG_KERNEL && m
-	---help---
-	  This option enables a testcase for the CPU NX capability
-	  and the software setup of this feature.
-	  If in doubt, say "N"
-
 config DOUBLEFAULT
 	default y
 	bool "Enable doublefault exception handler" if EXPERT

arch/x86/kernel/Makefile

@@ -101,7 +101,6 @@ obj-$(CONFIG_APB_TIMER)		+= apb_timer.o
 
 obj-$(CONFIG_AMD_NB)		+= amd_nb.o
 obj-$(CONFIG_DEBUG_RODATA_TEST)	+= test_rodata.o
-obj-$(CONFIG_DEBUG_NX_TEST)	+= test_nx.o
 obj-$(CONFIG_DEBUG_NMI_SELFTEST) += nmi_selftest.o
 
 obj-$(CONFIG_KVM_GUEST)		+= kvm.o kvmclock.o