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().