Sean Christopherson [off-list ref] writes:

quoted

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: 93286261de1b ("x86/hyperv: Reenlightenment notifications support")
Cc: stable@vger.kernel.org
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/hyperv/hv_init.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
index 24f4a06ac46a..7d252a58fbe4 100644
--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c

@@ -177,6 +177,9 @@ void set_hv_tscchange_cb(void (*cb)(void))
 		return;
 	}
 
+	if (!hv_vp_index)
+		return;
+

Arguably, we could've merged this with 'if (!hv_reenlightenment_available())' 
above to get a message printed:

diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
index 24f4a06ac46a..4a2a091c2f0e 100644
--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c

@@ -172,7 +172,7 @@ void set_hv_tscchange_cb(void (*cb)(void))
        };
        struct hv_tsc_emulation_control emu_ctrl = {.enabled = 1};
 
-       if (!hv_reenlightenment_available()) {
+       if (!hv_reenlightenment_available() || !hv_vp_index) {
                pr_warn("Hyper-V: reenlightenment support is unavailable\n");
                return;
        }

just to have an indication that something is off.

quoted

 	hv_reenlightenment_cb = cb;
 
 	/* Make sure callback is registered before we write to MSRs */

With or without the change,

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>