x86-32: Use symbolic constants, safer CPUID when enabling EFER.NX

Use symbolic constants rather than hard-coded values when setting
EFER.NX in head_32.S, and do a more rigorous test for the validity of
the response when probing for the extended CPUID range.
Signed-off-by: NH. Peter Anvin <hpa@zytor.com>
LKML-Reference: <1258154897-6770-2-git-send-email-hpa@zytor.com>
Acked-by: NKees Cook <kees.cook@canonical.com>

arch/x86/kernel/head_32.S

@@ -18,6 +18,8 @@
 #include <asm/asm-offsets.h>
 #include <asm/setup.h>
 #include <asm/processor-flags.h>
+#include <asm/msr-index.h>
+#include <asm/cpufeature.h>
 #include <asm/percpu.h>
 
 /* Physical address */
@@ -297,25 +299,27 @@ ENTRY(startup_32_smp)
 	orl %edx,%eax
 	movl %eax,%cr4
 
-	btl $5, %eax		# check if PAE is enabled
-	jnc 6f
+	testb $X86_CR4_PAE, %al		# check if PAE is enabled
+	jz 6f
 
 	/* Check if extended functions are implemented */
 	movl $0x80000000, %eax
 	cpuid
-	cmpl $0x80000000, %eax
-	jbe 6f
+	/* Value must be in the range 0x80000001 to 0x8000ffff */
+	subl $0x80000001, %eax
+	cmpl $(0x8000ffff-0x80000001), %eax
+	ja 6f
 	mov $0x80000001, %eax
 	cpuid
 	/* Execute Disable bit supported? */
-	btl $20, %edx
+	btl $(X86_FEATURE_NX & 31), %edx
 	jnc 6f
 
 	/* Setup EFER (Extended Feature Enable Register) */
-	movl $0xc0000080, %ecx
+	movl $MSR_EFER, %ecx
 	rdmsr
 
-	btsl $11, %eax
+	btsl $_EFER_NX, %eax
 	/* Make changes effective */
 	wrmsr