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Subject: Re: [PATCH v2 10/13] KVM: x86: Protect memory accesses from
 Spectre-v1/L1TF attacks in x86.c
To:     Marios Pomonis <pomonis@google.com>, rkrcmar@redhat.com,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        Nick Finco <nifi@google.com>, Andrew Honig <ahonig@google.com>,
        stable@vger.kernel.org
References: <20191211204753.242298-1-pomonis@google.com>
 <20191211204753.242298-11-pomonis@google.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <d8f47c12-3301-cb70-8d08-fe93450d19eb@redhat.com>
Date:   Sat, 18 Jan 2020 21:13:53 +0100
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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