[PATCH] KVM: X86: Fix the decoding of segment overrides in 64bit mode

[Wanpeng Li <kernellwp@gmail.com>]

From: Wanpeng Li <wanpengli@tencent.com>

Explicit segment overides other than %fs and %gs are documented as ignored by
both Intel and AMD.

In practice, this means that:

 * Explicit uses of %ss don't actually yield #SS[0] for non-canonical
   memory references.
 * Explicit uses of %{e,c,d}s don't override %rbp/%rsp-based memory references
   to yield #GP[0] for non-canonical memory references.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
 arch/x86/kvm/emulate.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index dd88158..5091255 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -5148,8 +5148,10 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len)
 		case 0x2e:	/* CS override */
 		case 0x36:	/* SS override */
 		case 0x3e:	/* DS override */
-			has_seg_override = true;
-			ctxt->seg_override = (ctxt->b >> 3) & 3;
+			if (mode != X86EMUL_MODE_PROT64) {
+				has_seg_override = true;
+				ctxt->seg_override = (ctxt->b >> 3) & 3;
+			}
 			break;
 		case 0x64:	/* FS override */
 		case 0x65:	/* GS override */
-- 
2.7.4