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Subject[PATCH v6 11/13] kvm, x86: update spectre-v1 mitigation
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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@google.com>
Cc: Jim Mattson <jmattson@google.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
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)
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