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: H. Peter Anvin <hpa@zytor.com>
LKML-Reference: <1258154897-6770-2-git-send-email-hpa@zytor.com>
Acked-by: Kees Cook <kees.cook@canonical.com>

1 files changed, 11 insertions, 7 deletions

diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
index 050c278481b1..7fd318bac59c 100644
--- a/arch/x86/kernel/head_32.S
+++ b/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