[PATCH v6 11/13] kvm, x86: update spectre-v1 mitigation

[Dan Williams]

Commit 75f139aaf896 "KVM: x86: Add memory barrier on vmcs field lookup"
added a raw 'asm("lfence");' to prevent a bounds check bypass of
'vmcs_field_to_offset_table'. We can save an lfence in this path and
just use the common array_index_nospec() helper designed for these types
of fixes.

Cc: Andrew Honig <ahonig@xxxxxxxxxx>
Cc: Jim Mattson <jmattson@xxxxxxxxxx>
Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
 arch/x86/kvm/vmx.c |   14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 924589c53422..8514186241d2 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -34,6 +34,7 @@
 #include <linux/tboot.h>
 #include <linux/hrtimer.h>
 #include <linux/frame.h>
+#include <linux/nospec.h>
 #include "kvm_cache_regs.h"
 #include "x86.h"
 
@@ -883,13 +884,18 @@ static const unsigned short vmcs_field_to_offset_table[] = {
 
 static inline short vmcs_field_to_offset(unsigned long field)
 {
-	BUILD_BUG_ON(ARRAY_SIZE(vmcs_field_to_offset_table) > SHRT_MAX);
+	const size_t size = ARRAY_SIZE(vmcs_field_to_offset_table);
+	unsigned short offset;
 
-	if (field >= ARRAY_SIZE(vmcs_field_to_offset_table) ||
-	    vmcs_field_to_offset_table[field] == 0)
+	BUILD_BUG_ON(size > SHRT_MAX);
+	if (field >= size)
 		return -ENOENT;
 
-	return vmcs_field_to_offset_table[field];
+	field = array_index_nospec(field, size);
+	offset = vmcs_field_to_offset_table[field];
+	if (offset == 0)
+		return -ENOENT;
+	return offset;
 }
 
 static inline struct vmcs12 *get_vmcs12(struct kvm_vcpu *vcpu)