On Mon, Jan 29, 2018 at 5:03 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> 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>
Hi Thomas,
I notice this one missing from -tip, I suspect because of the
collision with the raw 'lfence' that is in current mainline? No
worries, I'll send a rebased fixup to Paolo directly once
array_idx_nospec() goes upstream.
> ---
> 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)
>
