Patch "x86/cpu: Call verify_cpu() after having entered long mode too" has been added to the 4.1-stable tree

This is a note to let you know that I've just added the patch titled

x86/cpu: Call verify_cpu() after having entered long mode too

to the 4.1-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
x86-cpu-call-verify_cpu-after-having-entered-long-mode-too.patch
and it can be found in the queue-4.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Borislav Petkov <[email protected]>
Date: Thu, 5 Nov 2015 16:57:56 +0100
Subject: x86/cpu: Call verify_cpu() after having entered long mode too

From: Borislav Petkov <[email protected]>

commit 04633df0c43d710e5f696b06539c100898678235 upstream.

When we get loaded by a 64-bit bootloader, kernel entry point is startup_64 in head_64.S. We don't trust any and all bootloaders because some will fiddle with CPU configuration so we go ahead and massage each CPU into sanity again.

For example, some dell BIOSes have this XD disable feature which set IA32_MISC_ENABLE[34] and disable NX. This might be some dumb workaround for other OSes but Linux sure doesn't need it.

A similar thing is present in the Surface 3 firmware - see
https://bugzilla.kernel.org/show_bug.cgi?id=106051 - which sets this bit only on the BSP:

  # rdmsr -a 0x1a0
  400850089
  850089
  850089
  850089

I know, right?!

There's not even an off switch in there.

So fix all those cases by sanitizing the 64-bit entry point too. For that, make verify_cpu() callable in 64-bit mode also.

---
 arch/x86/kernel/head_64.S    |    8 ++++++++
 arch/x86/kernel/verify_cpu.S |   12 +++++++-----
 2 files changed, 15 insertions(+), 5 deletions(-)

@@ -65,6 +65,9 @@ startup_64:
 	 * tables and then reload them.
 	 */
 
+	/* Sanitize CPU configuration */
+	call verify_cpu
+
 	/*
 	 * Compute the delta between the address I am compiled to run at and the
 	 * address I am actually running at.

@@ -174,6 +177,9 @@ ENTRY(secondary_startup_64)
 	 * after the boot processor executes this code.
 	 */
 
+	/* Sanitize CPU configuration */
+	call verify_cpu
+
 	movq	$(init_level4_pgt - __START_KERNEL_map), %rax
 1:
 

@@ -288,6 +294,8 @@ ENTRY(secondary_startup_64)
 	pushq	%rax		# target address in negative space
 	lretq
 
+#include "verify_cpu.S"
+
 #ifdef CONFIG_HOTPLUG_CPU
 /*
  * Boot CPU0 entry point. It's called from play_dead(). Everything has been set

@@ -34,10 +34,11 @@
 #include <asm/msr-index.h>
 
 verify_cpu:
-	pushfl				# Save caller passed flags
-	pushl	$0			# Kill any dangerous flags
-	popfl
+	pushf				# Save caller passed flags
+	push	$0			# Kill any dangerous flags
+	popf
 
+#ifndef __x86_64__
 	pushfl				# standard way to check for cpuid
 	popl	%eax
 	movl	%eax,%ebx

@@ -48,6 +49,7 @@ verify_cpu:
 	popl	%eax
 	cmpl	%eax,%ebx
 	jz	verify_cpu_no_longmode	# cpu has no cpuid
+#endif
 
 	movl	$0x0,%eax		# See if cpuid 1 is implemented
 	cpuid

@@ -130,10 +132,10 @@ verify_cpu_sse_test:
 	jmp	verify_cpu_sse_test	# try again
 
 verify_cpu_no_longmode:
-	popfl				# Restore caller passed flags
+	popf				# Restore caller passed flags
 	movl $1,%eax
 	ret
 verify_cpu_sse_ok:
-	popfl				# Restore caller passed flags
+	popf				# Restore caller passed flags
 	xorl %eax, %eax
 	ret

Patches currently in stable-queue which might be from [email protected] are

queue-4.1/x86-cpu-call-verify_cpu-after-having-entered-long-mode-too.patch