4.16-stable review patch. If anyone has any objections, please let me know. ------------------ From: Sean Christopherson <sean.j.christopherson@xxxxxxxxx> commit 2c151b25441ae5c2da66472abd165af785c9ecd2 upstream. The bug that led to commit 95e057e25892eaa48cad1e2d637b80d0f1a4fac5 was a benign warning (no adverse affects other than the warning itself) that was detected by syzkaller. Further inspection shows that the WARN_ON in question, in handle_ept_misconfig(), is unnecessary and flawed (this was also briefly discussed in the original patch: https://patchwork.kernel.org/patch/10204649). * The WARN_ON is unnecessary as kvm_mmu_page_fault() will WARN if reserved bits are set in the SPTEs, i.e. it covers the case where an EPT misconfig occurred because of a KVM bug. * The WARN_ON is flawed because it will fire on any system error code that is hit while handling the fault, e.g. -ENOMEM can be returned by mmu_topup_memory_caches() while handling a legitmate MMIO EPT misconfig. The original behavior of returning -EFAULT when userspace munmaps an HVA without first removing the memslot is correct and desirable, i.e. KVM is letting userspace know it has generated a bad address. Returning RET_PF_EMULATE masks the WARN_ON in the EPT misconfig path, but does not fix the underlying bug, i.e. the WARN_ON is bogus. Furthermore, returning RET_PF_EMULATE has the unwanted side effect of causing KVM to attempt to emulate an instruction on any page fault with an invalid HVA translation, e.g. a not-present EPT violation on a VM_PFNMAP VMA whose fault handler failed to insert a PFN. * There is no guarantee that the fault is directly related to the instruction, i.e. the fault could have been triggered by a side effect memory access in the guest, e.g. while vectoring a #DB or writing a tracing record. This could cause KVM to effectively mask the fault if KVM doesn't model the behavior leading to the fault, i.e. emulation could succeed and resume the guest. * If emulation does fail, KVM will return EMULATION_FAILED instead of -EFAULT, which is a red herring as the user will either debug a bogus emulation attempt or scratch their head wondering why we were attempting emulation in the first place. TL;DR: revert to returning -EFAULT and remove the bogus WARN_ON in handle_ept_misconfig in a future patch. This reverts commit 95e057e25892eaa48cad1e2d637b80d0f1a4fac5. Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- arch/x86/kvm/mmu.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) --- a/arch/x86/kvm/mmu.c +++ b/arch/x86/kvm/mmu.c @@ -3031,7 +3031,7 @@ static int kvm_handle_bad_page(struct kv return RET_PF_RETRY; } - return RET_PF_EMULATE; + return -EFAULT; } static void transparent_hugepage_adjust(struct kvm_vcpu *vcpu,