KVM: x86 emulator: limit instructions to 15 bytes

While we are never normally passed an instruction that exceeds 15 bytes,
smp games can cause us to attempt to interpret one, which will cause
large latencies in non-preempt hosts.

Cc: stable@kernel.org
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/include/asm/kvm_emulate.h

@@ -129,7 +129,7 @@ struct decode_cache {
 	u8 seg_override;
 	unsigned int d;
 	unsigned long regs[NR_VCPU_REGS];
-	unsigned long eip;
+	unsigned long eip, eip_orig;
 	/* modrm */
 	u8 modrm;
 	u8 modrm_mod;

arch/x86/kvm/emulate.c

@@ -622,6 +622,9 @@ static int do_insn_fetch(struct x86_emulate_ctxt *ctxt,
 {
 	int rc = 0;
 
+	/* x86 instructions are limited to 15 bytes. */
+	if (eip + size - ctxt->decode.eip_orig > 15)
+		return X86EMUL_UNHANDLEABLE;
 	eip += ctxt->cs_base;
 	while (size--) {
 		rc = do_fetch_insn_byte(ctxt, ops, eip++, dest++);
@@ -880,7 +883,7 @@ x86_decode_insn(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
 	/* Shadow copy of register state. Committed on successful emulation. */
 
 	memset(c, 0, sizeof(struct decode_cache));
-	c->eip = kvm_rip_read(ctxt->vcpu);
+	c->eip = c->eip_orig = kvm_rip_read(ctxt->vcpu);
 	ctxt->cs_base = seg_base(ctxt, VCPU_SREG_CS);
 	memcpy(c->regs, ctxt->vcpu->arch.regs, sizeof c->regs);