[PATCH] x86: clear XD_DISABLED flag on Intel to regain NX

[PATCH] x86: clear XD_DISABLED flag on Intel to regain NX

[Kees Cook]

This will clear the MSR_IA32_MISC_ENABLE_XD_DISABLE bit so that NX cannot
be inappropriately controlled by the BIOS on Intel CPUs.  If NX actually
needs to be disabled, "noexec=off" can be used.

Signed-off-by: Kees Cook <kees.cook@canonical.com>
---
 arch/x86/kernel/head_32.S |   19 +++++++++++++++++++
 arch/x86/kernel/head_64.S |   18 ++++++++++++++++++
 arch/x86/mm/setup_nx.c    |    2 +-
 3 files changed, 38 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
index 37c3d4b..111e434 100644
--- a/arch/x86/kernel/head_32.S
+++ b/arch/x86/kernel/head_32.S
@@ -309,6 +309,25 @@ ENTRY(startup_32_smp)
 	subl $0x80000001, %eax
 	cmpl $(0x8000ffff-0x80000001), %eax
 	ja 6f
+
+	/* Is this "GenuineIntel"? */
+	movl $0x0, %eax
+	cpuid
+	cmpl $0x756e6547, %ebx
+	jnz 5f
+	cmpl $0x49656e69, %edx
+	jnz 5f
+	cmpl $0x6c65746e, %ecx
+	jnz 5f
+
+	/* Clear MSR_IA32_MISC_ENABLE_XD_DISABLE if set */
+	movl $MSR_IA32_MISC_ENABLE, %ecx
+	rdmsr
+	btrl $2, %edx
+	jnc 5f
+	wrmsr
+
+5:
 	mov $0x80000001, %eax
 	cpuid
 	/* Execute Disable bit supported? */
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 3d1e6f1..e918e40 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -175,6 +175,24 @@ ENTRY(secondary_startup_64)
 	jmp	*%rax
 1:
 
+	/* Is this "GenuineIntel"? */
+	movl $0x0, %eax
+	cpuid
+	cmpl $0x756e6547, %ebx
+	jnz 5f
+	cmpl $0x49656e69, %edx
+	jnz 5f
+	cmpl $0x6c65746e, %ecx
+	jnz 5f
+
+	/* Clear MSR_IA32_MISC_ENABLE_XD_DISABLE if set */
+	movl $MSR_IA32_MISC_ENABLE, %ecx
+	rdmsr
+	btrl $2, %edx
+	jnc 5f
+	wrmsr
+
+5:
 	/* Check if nx is implemented */
 	movl	$0x80000001, %eax
 	cpuid
diff --git a/arch/x86/mm/setup_nx.c b/arch/x86/mm/setup_nx.c
index a3250aa..410531d 100644
--- a/arch/x86/mm/setup_nx.c
+++ b/arch/x86/mm/setup_nx.c
@@ -41,7 +41,7 @@ void __init x86_report_nx(void)
 {
 	if (!cpu_has_nx) {
 		printk(KERN_NOTICE "Notice: NX (Execute Disable) protection "
-		       "missing in CPU or disabled in BIOS!\n");
+		       "missing in CPU!\n");
 	} else {
 #if defined(CONFIG_X86_64) || defined(CONFIG_X86_PAE)
 		if (disable_nx) {
-- 
1.7.1

-- 
Kees Cook
Ubuntu Security Team

Re: [PATCH] x86: clear XD_DISABLED flag on Intel to regain NX

[H. Peter Anvin]

On 06/17/2010 03:13 PM, Kees Cook wrote:
> This will clear the MSR_IA32_MISC_ENABLE_XD_DISABLE bit so that NX cannot
> be inappropriately controlled by the BIOS on Intel CPUs.  If NX actually
> needs to be disabled, "noexec=off" can be used.
> 
> Signed-off-by: Kees Cook <kees.cook@canonical.com>
> ---
>  arch/x86/kernel/head_32.S |   19 +++++++++++++++++++
>  arch/x86/kernel/head_64.S |   18 ++++++++++++++++++
>  arch/x86/mm/setup_nx.c    |    2 +-
>  3 files changed, 38 insertions(+), 1 deletions(-)
> 
> diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
> index 37c3d4b..111e434 100644
> --- a/arch/x86/kernel/head_32.S
> +++ b/arch/x86/kernel/head_32.S
> @@ -309,6 +309,25 @@ ENTRY(startup_32_smp)
>  	subl $0x80000001, %eax
>  	cmpl $(0x8000ffff-0x80000001), %eax
>  	ja 6f
> +
> +	/* Is this "GenuineIntel"? */
> +	movl $0x0, %eax
> +	cpuid
> +	cmpl $0x756e6547, %ebx
> +	jnz 5f
> +	cmpl $0x49656e69, %edx
> +	jnz 5f
> +	cmpl $0x6c65746e, %ecx
> +	jnz 5f
> +
> +	/* Clear MSR_IA32_MISC_ENABLE_XD_DISABLE if set */
> +	movl $MSR_IA32_MISC_ENABLE, %ecx
> +	rdmsr
> +	btrl $2, %edx
> +	jnc 5f
> +	wrmsr
> +
> +5:
>  	mov $0x80000001, %eax
>  	cpuid
>  	/* Execute Disable bit supported? */

Multiple problems with this code.

a) Not all Intel CPUs with extended CPUID levels have
   MSR_IA32_MISC_ENABLE bit 34.  Since we can't take traps here we would
   have to know positively that we aren't going to trip on anything.
b) For 64 bits, this should go into verify_cpu_64.S, and since that is
   32-bit code anyway, it would be best if we could merge the 32- and
   64-bit code into that file; it already simply returns a value
   that could be ignored on 32 bits.

	-hpa

RE: [PATCH] x86: clear XD_DISABLED flag on Intel to regain NX

[Yuhong Bao]

> a) Not all Intel CPUs with extended CPUID levels have
> MSR_IA32_MISC_ENABLE bit 34. Since we can't take traps here we would
> have to know positively that we aren't going to trip on anything.
It won't as these older Intel CPUs are always going to read this bit as zero, and even if no checking for this is include all the wrmsr would do is write the same value back, which won't cause a trap.

Yuhong Bao
 		 	   		  
_________________________________________________________________
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RE: [PATCH] x86: clear XD_DISABLED flag on Intel to regain NX

[H. Peter Anvin]

Yes, I mentally flipped the sense of the bit... talked to Kees already.

"Yuhong Bao" <yuhongbao_386@hotmail.com> wrote:

>
>> a) Not all Intel CPUs with extended CPUID levels have
>> MSR_IA32_MISC_ENABLE bit 34. Since we can't take traps here we would
>> have to know positively that we aren't going to trip on anything.
>It won't as these older Intel CPUs are always going to read this bit as zero, and even if no checking for this is include all the wrmsr would do is write the same value back, which won't cause a trap.
>
>Yuhong Bao
> 		 	   		  
>_________________________________________________________________
>Hotmail has tools for the New Busy. Search, chat and e-mail from your inbox.
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-- 
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