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Commit ae84739c authored by Kees Cook's avatar Kees Cook Committed by H. Peter Anvin
Browse files

x86, cpu: Clear XD_DISABLED flag on Intel to regain NX 

Intel CPUs have an additional MSR bit to indicate if the BIOS was
configured to disable the NX cpu feature. This bit was traditionally
used for operating systems that did not understand how to handle the
NX bit. Since Linux understands this, this BIOS flag should be ignored
by default.

In a review[1] of reported hardware being used by Ubuntu bug reporters,
almost 10% of systems had an incorrectly configured BIOS, leaving their
systems unable to use the NX features of their CPU.

This change will clear the MSR_IA32_MISC_ENABLE_XD_DISABLE bit so that NX
cannot be inappropriately controlled by the BIOS on Intel CPUs. If, under
very strange hardware configurations, NX actually needs to be disabled,
"noexec=off" can be used to restore the prior behavior.

[1] http://www.outflux.net/blog/archives/2010/02/18/data-mining-for-nx-bit/



Signed-off-by: default avatarKees Cook <kees.cook@canonical.com>
LKML-Reference: <1289414154-7829-3-git-send-email-kees.cook@canonical.com>
Acked-by: default avatarPekka Enberg <penberg@kernel.org>
Acked-by: default avatarAlan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: default avatarH. Peter Anvin <hpa@linux.intel.com>
parent c5cbac69

arch/x86/kernel/verify_cpu.S

+40 −8
Original line number Diff line number Diff line
@@ -7,6 +7,7 @@ 
 *	Copyright (c) 2007  Andi Kleen (ak@suse.de)

 *	Copyright (c) 2007  Eric Biederman (ebiederm@xmission.com)

 *	Copyright (c) 2007  Vivek Goyal (vgoyal@in.ibm.com)

 *	Copyright (c) 2010  Kees Cook (kees.cook@canonical.com)

 *

 * 	This source code is licensed under the GNU General Public License,

 * 	Version 2.  See the file COPYING for more details.
@@ -14,18 +15,16 @@ 
 *	This is a common code for verification whether CPU supports

 * 	long mode and SSE or not. It is not called directly instead this

 *	file is included at various places and compiled in that context.

 * 	Following are the current usage.

 *	This file is expected to run in 32bit code.  Currently:

 *

 * 	This file is included by both 16bit and 32bit code.

 *	arch/x86_64/boot/compressed/head_64.S: Boot cpu verification

 *	arch/x86_64/kernel/trampoline_64.S: secondary processor verfication

 *

 *	arch/x86_64/boot/setup.S : Boot cpu verification (16bit)

 *	arch/x86_64/boot/compressed/head.S: Boot cpu verification (32bit)

 *	arch/x86_64/kernel/trampoline.S: secondary processor verfication (16bit)

 *	arch/x86_64/kernel/acpi/wakeup.S:Verfication at resume (16bit)

 *

 *	verify_cpu, returns the status of cpu check in register %eax.

 *	verify_cpu, returns the status of longmode and SSE in register %eax.

 *		0: Success    1: Failure

 *

 *	On Intel, the XD_DISABLE flag will be cleared as a side-effect.

 *

 * 	The caller needs to check for the error code and take the action

 * 	appropriately. Either display a message or halt.

 */
@@ -62,8 +61,41 @@ verify_cpu: 
	cmpl	$0x444d4163,%ecx

	jnz	verify_cpu_noamd

	mov	$1,%di			# cpu is from AMD

	jmp	verify_cpu_check



verify_cpu_noamd:

	cmpl	$0x756e6547,%ebx        # GenuineIntel?

	jnz	verify_cpu_check

	cmpl	$0x49656e69,%edx

	jnz	verify_cpu_check

	cmpl	$0x6c65746e,%ecx

	jnz	verify_cpu_check



	# only call IA32_MISC_ENABLE when:

	# family > 6 || (family == 6 && model >= 0xd)

	movl	$0x1, %eax		# check CPU family and model

	cpuid

	movl	%eax, %ecx



	andl	$0x0ff00f00, %eax	# mask family and extended family

	shrl	$8, %eax

	cmpl	$6, %eax

	ja	verify_cpu_clear_xd	# family > 6, ok

	jb	verify_cpu_check	# family < 6, skip



	andl	$0x000f00f0, %ecx	# mask model and extended model

	shrl	$4, %ecx

	cmpl	$0xd, %ecx

	jb	verify_cpu_check	# family == 6, model < 0xd, skip



verify_cpu_clear_xd:

	movl	$MSR_IA32_MISC_ENABLE, %ecx

	rdmsr

	btrl	$2, %edx		# clear MSR_IA32_MISC_ENABLE_XD_DISABLE

	jnc	verify_cpu_check	# only write MSR if bit was changed

	wrmsr



verify_cpu_check:

	movl    $0x1,%eax		# Does the cpu have what it takes

	cpuid

	andl	$REQUIRED_MASK0,%edx