From: Paolo Bonzini <pbonzini@redhat.com>
Date: Fri, 17 Apr 2020 12:21:06 -0400
Subject: KVM: SVM: avoid infinite loop on NPF from bad address
Git-commit: e72436bc3a5206f95bb384e741154166ddb3202e
Patch-mainline: v5.8-rc1
References: CVE-2020-36310 bsc#1184512

When a nested page fault is taken from an address that does not have
a memslot associated to it, kvm_mmu_do_page_fault returns RET_PF_EMULATE
(via mmu_set_spte) and kvm_mmu_page_fault then invokes svm_need_emulation_on_page_fault.

The default answer there is to return false, but in this case this just
causes the page fault to be retried ad libitum.  Since this is not a
fast path, and the only other case where it is taken is an erratum,
just stick a kvm_vcpu_gfn_to_memslot check in there to detect the
common case where the erratum is not happening.

This fixes an infinite loop in the new set_memory_region_test.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Joerg Roedel <jroedel@suse.de>
---
 arch/x86/kvm/svm.c  |    7 +++++++
 virt/kvm/kvm_main.c |    1 +
 2 files changed, 8 insertions(+)

--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -7204,6 +7204,13 @@ static bool svm_need_emulation_on_page_f
 	bool is_user = svm_get_cpl(vcpu) == 3;
 
 	/*
+	 * If RIP is invalid, go ahead with emulation which will cause an
+	 * internal error exit.
+	 */
+	if (!kvm_vcpu_gfn_to_memslot(vcpu, kvm_rip_read(vcpu) >> PAGE_SHIFT))
+		return true;
+
+	/*
 	 * Detect and workaround Errata 1096 Fam_17h_00_0Fh.
 	 *
 	 * Errata:
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1369,6 +1369,7 @@ struct kvm_memory_slot *kvm_vcpu_gfn_to_
 {
 	return __gfn_to_memslot(kvm_vcpu_memslots(vcpu), gfn);
 }
+EXPORT_SYMBOL_GPL(kvm_vcpu_gfn_to_memslot);
 
 bool kvm_is_visible_gfn(struct kvm *kvm, gfn_t gfn)
 {