Re: [BUG] arch/x86/kvm/vmx/vmx_onhyperv.h:109:36: error: dereference of NULL ‘0’

[Sean Christopherson <seanjc@google.com>]

On Fri, Jul 19, 2024, Mirsad Todorovac wrote:
> Hi, all!
> 
> Here is another potential NULL pointer dereference in kvm subsystem of linux
> stable vanilla 6.10, as GCC 12.3.0 complains.
> 
> (Please don't throw stuff at me, I think this is the last one for today :-)
> 
> arch/x86/include/asm/mshyperv.h
> -------------------------------
>   242 static inline struct hv_vp_assist_page *hv_get_vp_assist_page(unsigned int cpu)
>   243 {
>   244         if (!hv_vp_assist_page)
>   245                 return NULL;
>   246 
>   247         return hv_vp_assist_page[cpu];
>   248 }
> 
> arch/x86/kvm/vmx/vmx_onhyperv.h
> -------------------------------
>   102 static inline void evmcs_load(u64 phys_addr)
>   103 {
>   104         struct hv_vp_assist_page *vp_ap =
>   105                 hv_get_vp_assist_page(smp_processor_id());
>   106 
>   107         if (current_evmcs->hv_enlightenments_control.nested_flush_hypercall)
>   108                 vp_ap->nested_control.features.directhypercall = 1;
>   109         vp_ap->current_nested_vmcs = phys_addr;
>   110         vp_ap->enlighten_vmentry = 1;
>   111 }
> 
> Now, this one is simple:

Nope :-)

> hv_vp_assist_page(cpu) can return NULL, and in line 104 it is assigned to
> wp_ap, which is dereferenced in lines 108, 109, and 110, which is not checked
> against returning NULL by hv_vp_assist_page().

When enabling eVMCS, and when onlining a CPU with eVMCS enabled, KVM verifies
that every CPU has a valid hv_vp_assist_page() and either aborts enabling eVMCS
or rejects CPU onlining.  So very subtly, it's impossible for hv_vp_assist_page()
to be NULL at evmcs_load().