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Subject[tip:x86/pti] x86/kvm: Update spectre-v1 mitigation
Commit-ID:  085331dfc6bbe3501fb936e657331ca943827600
Gitweb: https://git.kernel.org/tip/085331dfc6bbe3501fb936e657331ca943827600
Author: Dan Williams <dan.j.williams@intel.com>
AuthorDate: Wed, 31 Jan 2018 17:47:03 -0800
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitDate: Thu, 1 Feb 2018 10:59:10 +0100

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

---
arch/x86/kvm/vmx.c | 20 +++++++++-----------
1 file changed, 9 insertions(+), 11 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index a8b96dc..2894282 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"

@@ -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)
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