[PATCH] x86/CPU: Call verify_cpu() after having entered long mode too

From: Borislav Petkov
Date: Thu Nov 05 2015 - 10:58:10 EST


From: Borislav Petkov <bp@xxxxxxx>

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.

Requested-and-debugged-by: "H. Peter Anvin" <hpa@xxxxxxxxx>
Reported-and-tested-by: Bastien Nocera <bugzilla@xxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Cc: Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
---
arch/x86/kernel/head_64.S | 8 ++++++++
arch/x86/kernel/verify_cpu.S | 12 +++++++-----
2 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 1d40ca8a73f2..ffdc0e860390 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -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
diff --git a/arch/x86/kernel/verify_cpu.S b/arch/x86/kernel/verify_cpu.S
index b9242bacbe59..4cf401f581e7 100644
--- a/arch/x86/kernel/verify_cpu.S
+++ b/arch/x86/kernel/verify_cpu.S
@@ -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
--
2.3.5

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/