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)

 {