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    Subject[PATCH 4.4 184/193] x86/kvm: Update spectre-v1 mitigation
    Date
    4.4-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Dan Williams <dan.j.williams@intel.com>

    (cherry picked from commit 085331dfc6bbe3501fb936e657331ca943827600)

    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
    Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
    [jwang: cherry pick to 4.4]
    Signed-off-by: Jack Wang <jinpu.wang@profitbricks.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    arch/x86/kvm/vmx.c | 20 +++++++++-----------
    1 file changed, 9 insertions(+), 11 deletions(-)

    --- a/arch/x86/kvm/vmx.c
    +++ b/arch/x86/kvm/vmx.c
    @@ -32,6 +32,7 @@
    #include <linux/slab.h>
    #include <linux/tboot.h>
    #include <linux/hrtimer.h>
    +#include <linux/nospec.h>
    #include "kvm_cache_regs.h"
    #include "x86.h"

    @@ -827,21 +828,18 @@ static const unsigned short vmcs_field_t

    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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