[PATCH 5.5 237/367] KVM: x86: Protect x86_decode_insn from Spectre-v1/L1TF attacks

Greg Kroah-Hartman
2020-02-10T12:59:14Z

From: Marios Pomonis <pomonis@google.com>

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@google.com>
Signed-off-by: Marios Pomonis <pomonis@google.com>
Reviewed-by: Andrew Honig <ahonig@google.com>
Cc: stable@vger.kernel.org
Reviewed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
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
@@ -5315,10 +5315,15 @@ 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;
case InstrDual:
if ((ctxt->modrm >> 6) == 3)