x86/hyperv: Fix NULL deref in set_hv_tscchange_cb() if Hyper-V setup fails

Check for a valid hv_vp_index array prior to derefencing hv_vp_index when
setting Hyper-V's TSC change callback.  If Hyper-V setup failed in
hyperv_init(), the kernel will still report that it's running under
Hyper-V, but will have silently disabled nearly all functionality.

  BUG: kernel NULL pointer dereference, address: 0000000000000010
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  PGD 0 P4D 0
  Oops: 0000 [#1] SMP
  CPU: 4 PID: 1 Comm: swapper/0 Not tainted 5.15.0-rc2+ #75
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
  RIP: 0010:set_hv_tscchange_cb+0x15/0xa0
  Code: <8b> 04 82 8b 15 12 17 85 01 48 c1 e0 20 48 0d ee 00 01 00 f6 c6 08
  ...
  Call Trace:
   kvm_arch_init+0x17c/0x280
   kvm_init+0x31/0x330
   vmx_init+0xba/0x13a
   do_one_initcall+0x41/0x1c0
   kernel_init_freeable+0x1f2/0x23b
   kernel_init+0x16/0x120
   ret_from_fork+0x22/0x30

Fixes: 93286261 ("x86/hyperv: Reenlightenment notifications support")
Cc: stable@vger.kernel.org
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20211104182239.1302956-2-seanjc@google.comSigned-off-by: Wei Liu <wei.liu@kernel.org>

arch/x86/hyperv/hv_init.c

@@ -177,6 +177,9 @@ void set_hv_tscchange_cb(void (*cb)(void))
 		return;
 	}
 
+	if (!hv_vp_index)
+		return;
+
 	hv_reenlightenment_cb = cb;
 
 	/* Make sure callback is registered before we write to MSRs */