KVM: x86: hyper-v: Fix Hyper-V context null-ptr-deref

Reported by syzkaller:

    KASAN: null-ptr-deref in range [0x0000000000000140-0x0000000000000147]
    CPU: 1 PID: 8370 Comm: syz-executor859 Not tainted 5.11.0-syzkaller #0
    RIP: 0010:synic_get arch/x86/kvm/hyperv.c:165 [inline]
    RIP: 0010:kvm_hv_set_sint_gsi arch/x86/kvm/hyperv.c:475 [inline]
    RIP: 0010:kvm_hv_irq_routing_update+0x230/0x460 arch/x86/kvm/hyperv.c:498
    Call Trace:
     kvm_set_irq_routing+0x69b/0x940 arch/x86/kvm/../../../virt/kvm/irqchip.c:223
     kvm_vm_ioctl+0x12d0/0x2800 arch/x86/kvm/../../../virt/kvm/kvm_main.c:3959
     vfs_ioctl fs/ioctl.c:48 [inline]
     __do_sys_ioctl fs/ioctl.c:753 [inline]
     __se_sys_ioctl fs/ioctl.c:739 [inline]
     __x64_sys_ioctl+0x193/0x200 fs/ioctl.c:739
     do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
     entry_SYSCALL_64_after_hwframe+0x44/0xae

Hyper-V context is lazily allocated until Hyper-V specific MSRs are accessed
or SynIC is enabled. However, the syzkaller testcase sets irq routing table
directly w/o enabling SynIC. This results in null-ptr-deref when accessing
SynIC Hyper-V context. This patch fixes it.

syzkaller source: https://syzkaller.appspot.com/x/repro.c?x=163342ccd00000

Reported-by: syzbot+6987f3b2dbd9eda95f12@syzkaller.appspotmail.com
Fixes: 8f014550 ("KVM: x86: hyper-v: Make Hyper-V emulation enablement conditional")
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Message-Id: <1614326399-5762-1-git-send-email-wanpengli@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/hyperv.c

@@ -159,7 +159,7 @@ static struct kvm_vcpu_hv_synic *synic_get(struct kvm *kvm, u32 vpidx)
 	struct kvm_vcpu_hv_synic *synic;
 
 	vcpu = get_vcpu_by_vpidx(kvm, vpidx);
-	if (!vcpu)
+	if (!vcpu || !to_hv_vcpu(vcpu))
 		return NULL;
 	synic = to_hv_synic(vcpu);
 	return (synic->active) ? synic : NULL;