[PATCH 3.16 76/99] KVM: x86: Protect x86_decode_insn from Spectre-v1/L1TF attacks

[Ben Hutchings]

3.16.84-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Marios Pomonis <pomonis@xxxxxxxxxx>

commit 3c9053a2cae7ba2ba73766a34cea41baa70f57f7 upstream.

This fixes a Spectre-v1/L1TF vulnerability in x86_decode_insn().
kvm_emulate_instruction() (an ancestor of x86_decode_insn()) is an exported
symbol, so KVM should treat it conservatively from a security perspective.

Fixes: 045a282ca415 ("KVM: emulator: implement fninit, fnstsw, fnstcw")

Signed-off-by: Nick Finco <nifi@xxxxxxxxxx>
Signed-off-by: Marios Pomonis <pomonis@xxxxxxxxxx>
Reviewed-by: Andrew Honig <ahonig@xxxxxxxxxx>
Reviewed-by: Jim Mattson <jmattson@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
[bwh: Backported to 3.16: Add #include <linux/nospec.h>]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 arch/x86/kvm/emulate.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -26,6 +26,7 @@
 #include <asm/kvm_emulate.h>
 #include <linux/stringify.h>
 #include <asm/nospec-branch.h>
+#include <linux/nospec.h>
 
 #include "x86.h"
 #include "tss.h"
@@ -4487,10 +4488,15 @@ done_prefixes:
 			}
 			break;
 		case Escape:
-			if (ctxt->modrm > 0xbf)
-				opcode = opcode.u.esc->high[ctxt->modrm - 0xc0];
-			else
+			if (ctxt->modrm > 0xbf) {
+				size_t size = ARRAY_SIZE(opcode.u.esc->high);
+				u32 index = array_index_nospec(
+					ctxt->modrm - 0xc0, size);
+
+				opcode = opcode.u.esc->high[index];
+			} else {
 				opcode = opcode.u.esc->op[(ctxt->modrm >> 3) & 7];
+			}
 			break;
 		default:
 			return EMULATION_FAILED;