KVM: VMX: hyper-v: Prevent impossible NULL pointer dereference in evmcs_load()

GCC 12.3.0 complains about a potential NULL pointer dereference in
evmcs_load() as hv_get_vp_assist_page() can return NULL. In fact, this
cannot happen because KVM verifies (hv_init_evmcs()) that every CPU has a
valid VP assist page and aborts enabling the feature otherwise. CPU
onlining path is also checked in vmx_hardware_enable().

To make the compiler happy and to future proof the code, add a KVM_BUG_ON()
sentinel. It doesn't seem to be possible (and logical) to observe
evmcs_load() happening without an active vCPU so it is presumed that
kvm_get_running_vcpu() can't return NULL.

No functional change intended.

Reported-by: Mirsad Todorovac <mtodorovac69@gmail.com>
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20240816130124.286226-1-vkuznets@redhat.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/kvm/vmx/vmx_onhyperv.h b/arch/x86/kvm/vmx/vmx_onhyperv.h
index eb48153bfd73c0d4a62420b21184712ec840e426..bba24ed99ee6cac80c528c5da0fc74c37c502f81 100644 (file)
--- a/arch/x86/kvm/vmx/vmx_onhyperv.h
+++ b/arch/x86/kvm/vmx/vmx_onhyperv.h
@@ -104,6 +104,14 @@ static inline void evmcs_load(u64 phys_addr)
        struct hv_vp_assist_page *vp_ap =
                hv_get_vp_assist_page(smp_processor_id());
 
+       /*
+        * When enabling eVMCS, KVM verifies that every CPU has a valid hv_vp_assist_page()
+        * and aborts enabling the feature otherwise. CPU onlining path is also checked in
+        * vmx_hardware_enable().
+        */
+       if (KVM_BUG_ON(!vp_ap, kvm_get_running_vcpu()->kvm))
+               return;
+
        if (current_evmcs->hv_enlightenments_control.nested_flush_hypercall)
                vp_ap->nested_control.features.directhypercall = 1;
        vp_ap->current_nested_vmcs = phys_addr;