[PATCH] kvm: Nested KVM MMUs need PAE root too

From: "Jiří Paleček" <jpalecek@web.de>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: kvm@vger.kernel.org
Subject: [PATCH] kvm: Nested KVM MMUs need PAE root too
Date: Sat, 22 Jun 2019 19:42:04 +0200	[thread overview]
Message-ID: <87lfvl5f28.fsf@debian> (raw)

On AMD processors, in PAE 32bit mode, nested KVM instances don't
work. The L0 host get a kernel OOPS, which is related to
arch.mmu->pae_root being NULL.

The reason for this is that when setting up nested KVM instance,
arch.mmu is set to &arch.guest_mmu (while normally, it would be
&arch.root_mmu). However, the initialization and allocation of
pae_root only creates it in root_mmu. KVM code (ie. in
mmu_alloc_shadow_roots) then accesses arch.mmu->pae_root, which is the
unallocated arch.guest_mmu->pae_root.

This fix just allocates (and frees) pae_root in both guest_mmu and
root_mmu (and also lm_root if it was allocated). The allocation is
subject to previous restrictions ie. it won't allocate anything on
64-bit and AFAIK not on Intel.

See bug 203923 for details.

Signed-off-by: Jiri Palecek <jpalecek@web.de>
Tested-by: Jiri Palecek <jpalecek@web.de>

---
 arch/x86/kvm/mmu.c | 30 ++++++++++++++++++++++--------
 1 file changed, 22 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 24843cf49579..efa8285bb56d 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -5592,13 +5592,13 @@ slot_handle_leaf(struct kvm *kvm, struct kvm_memory_slot *memslot,
 				 PT_PAGE_TABLE_LEVEL, lock_flush_tlb);
 }

-static void free_mmu_pages(struct kvm_vcpu *vcpu)
+static void free_mmu_pages(struct kvm_mmu *mmu)
 {
-	free_page((unsigned long)vcpu->arch.mmu->pae_root);
-	free_page((unsigned long)vcpu->arch.mmu->lm_root);
+	free_page((unsigned long)mmu->pae_root);
+	free_page((unsigned long)mmu->lm_root);
 }

-static int alloc_mmu_pages(struct kvm_vcpu *vcpu)
+static int alloc_mmu_pages(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu)
 {
 	struct page *page;
 	int i;
@@ -5619,9 +5619,9 @@ static int alloc_mmu_pages(struct kvm_vcpu *vcpu)
 	if (!page)
 		return -ENOMEM;

-	vcpu->arch.mmu->pae_root = page_address(page);
+	mmu->pae_root = page_address(page);
 	for (i = 0; i < 4; ++i)
-		vcpu->arch.mmu->pae_root[i] = INVALID_PAGE;
+		mmu->pae_root[i] = INVALID_PAGE;

 	return 0;
 }
@@ -5629,6 +5629,7 @@ static int alloc_mmu_pages(struct kvm_vcpu *vcpu)
 int kvm_mmu_create(struct kvm_vcpu *vcpu)
 {
 	uint i;
+	int ret;

 	vcpu->arch.mmu = &vcpu->arch.root_mmu;
 	vcpu->arch.walk_mmu = &vcpu->arch.root_mmu;
@@ -5646,7 +5647,19 @@ int kvm_mmu_create(struct kvm_vcpu *vcpu)
 		vcpu->arch.guest_mmu.prev_roots[i] = KVM_MMU_ROOT_INFO_INVALID;

 	vcpu->arch.nested_mmu.translate_gpa = translate_nested_gpa;
-	return alloc_mmu_pages(vcpu);
+
+	ret = alloc_mmu_pages(vcpu, &vcpu->arch.guest_mmu);
+	if (ret)
+		return ret;
+
+	ret = alloc_mmu_pages(vcpu, &vcpu->arch.root_mmu);
+	if (ret)
+		goto fail_allocate_root;
+
+	return ret;
+ fail_allocate_root:
+	free_mmu_pages(&vcpu->arch.guest_mmu);
+	return ret;
 }

 static void kvm_mmu_invalidate_zap_pages_in_memslot(struct kvm *kvm,
@@ -6102,7 +6115,8 @@ unsigned long kvm_mmu_calculate_default_mmu_pages(struct kvm *kvm)
 void kvm_mmu_destroy(struct kvm_vcpu *vcpu)
 {
 	kvm_mmu_unload(vcpu);
-	free_mmu_pages(vcpu);
+	free_mmu_pages(&vcpu->arch.root_mmu);
+	free_mmu_pages(&vcpu->arch.guest_mmu);
 	mmu_free_memory_caches(vcpu);
 }

--
2.23.0.rc1

