linux-kernel - [PATCH 1/2] KVM: SVM: avoid infinite loop on NPF from bad address

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Message-Id: <20200417163843.71624-2-pbonzini@redhat.com>
Date:   Fri, 17 Apr 2020 12:38:42 -0400
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        stable@...r.kernel.org
Subject: [PATCH 1/2] KVM: SVM: avoid infinite loop on NPF from bad address

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.

Fixes: 05d5a4863525 ("KVM: SVM: Workaround errata#1096 (insn_len maybe zero on SMAP violation)")
Cc: stable@...r.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kvm/svm/svm.c | 7 +++++++
 virt/kvm/kvm_main.c    | 1 +
 2 files changed, 8 insertions(+)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index a91e397d6750..c86f7278509b 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3837,6 +3837,13 @@ static bool svm_need_emulation_on_page_fault(struct kvm_vcpu *vcpu)
 	bool smap = cr4 & X86_CR4_SMAP;
 	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.
 	 *
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index e2f60e313c87..e7436d054305 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1602,6 +1602,7 @@ struct kvm_memory_slot *kvm_vcpu_gfn_to_memslot(struct kvm_vcpu *vcpu, gfn_t gfn
 {
 	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)
 {
-- 
2.18.2