Re: [PATCH v2 10/13] KVM: x86: Protect memory accesses from Spectre-v1/L1TF attacks in x86.c

Paolo Bonzini
2020-01-18T20:13:59Z

On 11/12/19 21:47, Marios Pomonis wrote:
> This fixes Spectre-v1/L1TF vulnerabilities in
> vmx_read_guest_seg_selector(), vmx_read_guest_seg_base(),
> vmx_read_guest_seg_limit() and vmx_read_guest_seg_ar().
> These functions contain index computations based on the
> (attacker-influenced) segment value.
>
> Fixes: commit 2fb92db1ec08 ("KVM: VMX: Cache vmcs segment fields")

I think we could instead do

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 2d4faefe8dd4..20c0cbdff1be 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -5195,16 +5195,28 @@ int x86_decode_insn(struct x86_emulate_ctxt
*ctxt, void *insn, int insn_len)
ctxt->ad_bytes = def_ad_bytes ^ 6;
break;
case 0x26: /* ES override */
+ has_seg_override = true;
+ ctxt->seg_override = VCPU_SREG_ES;
+ break;
case 0x2e: /* CS override */
+ has_seg_override = true;
+ ctxt->seg_override = VCPU_SREG_CS;
+ break;
case 0x36: /* SS override */
+ has_seg_override = true;
+ ctxt->seg_override = VCPU_SREG_SS;
+ break;
case 0x3e: /* DS override */
has_seg_override = true;
- ctxt->seg_override = (ctxt->b >> 3) & 3;
+ ctxt->seg_override = VCPU_SREG_DS;
break;
case 0x64: /* FS override */
+ has_seg_override = true;
+ ctxt->seg_override = VCPU_SREG_FS;
+ break;
case 0x65: /* GS override */
has_seg_override = true;
- ctxt->seg_override = ctxt->b & 7;
+ ctxt->seg_override = VCPU_SREG_GS;
break;
case 0x40 ... 0x4f: /* REX */
if (mode != X86EMUL_MODE_PROT64)

so that the segment is never calculated.

Paolo