x86/kvm: Update spectre-v1 mitigation

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'.

The lfence can be avoided in this path by using the array_index_nospec()
helper designed for these types of fixes.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: Andrew Honig <ahonig@google.com>
Cc: kvm@vger.kernel.org
Cc: Jim Mattson <jmattson@google.com>
Link: https://lkml.kernel.org/r/151744959670.6342.3001723920950249067.stgit@dwillia2-desk3.amr.corp.intel.com

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index a8b96dc..2894282 100644 (file)
--- 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"
 
@@ -898,21 +899,18 @@ 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))
+       BUILD_BUG_ON(size > SHRT_MAX);
+       if (field >= size)
                return -ENOENT;
 
-       /*
-        * FIXME: Mitigation for CVE-2017-5753.  To be replaced with a
-        * generic mechanism.
-        */
-       asm("lfence");
-
-       if (vmcs_field_to_offset_table[field] == 0)
+       field = array_index_nospec(field, size);
+       offset = vmcs_field_to_offset_table[field];
+       if (offset == 0)
                return -ENOENT;
-
-       return vmcs_field_to_offset_table[field];
+       return offset;
 }
 
 static inline struct vmcs12 *get_vmcs12(struct kvm_vcpu *vcpu)