[PATCH] KVM: mmu: allow page tables to be in read-only slots

[Paolo Bonzini]

Page tables in a read-only memory slot will currently cause a triple
fault because the page walker uses gfn_to_hva and it fails on such a slot.

OVMF uses such a page table; however, real hardware seems to be fine with
that as long as the accessed/dirty bits are set.  Save whether the slot
is readonly, and later check it when updating the accessed and dirty bits.

Cc: stable@xxxxxxxxxxxxxxx
Cc: gleb@xxxxxxxxxx
Cc: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
	CCing to stable@ since the regression was introduced with
	support for readonly memory slots.

 arch/x86/kvm/paging_tmpl.h |  7 ++++++-
 include/linux/kvm_host.h   |  1 +
 virt/kvm/kvm_main.c        | 14 +++++++++-----
 3 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
index 0433301..dadc5c0 100644
--- a/arch/x86/kvm/paging_tmpl.h
+++ b/arch/x86/kvm/paging_tmpl.h
@@ -99,6 +99,7 @@ struct guest_walker {
 	pt_element_t prefetch_ptes[PTE_PREFETCH_NUM];
 	gpa_t pte_gpa[PT_MAX_FULL_LEVELS];
 	pt_element_t __user *ptep_user[PT_MAX_FULL_LEVELS];
+	bool pte_writable[PT_MAX_FULL_LEVELS];
 	unsigned pt_access;
 	unsigned pte_access;
 	gfn_t gfn;
@@ -235,6 +236,9 @@ static int FNAME(update_accessed_dirty_bits)(struct kvm_vcpu *vcpu,
 		if (pte == orig_pte)
 			continue;
 
+		if (unlikely(!walker->pte_writable[level - 1]))
+			return -EACCES;
+
 		ret = FNAME(cmpxchg_gpte)(vcpu, mmu, ptep_user, index, orig_pte, pte);
 		if (ret)
 			return ret;
@@ -309,7 +313,8 @@ retry_walk:
 			goto error;
 		real_gfn = gpa_to_gfn(real_gfn);
 
-		host_addr = gfn_to_hva(vcpu->kvm, real_gfn);
+		host_addr = gfn_to_hva_read(vcpu->kvm, real_gfn,
+					    &walker->pte_writable[walker->level - 1]);
 		if (unlikely(kvm_is_error_hva(host_addr)))
 			goto error;
 
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index ca645a0..22f9cdf 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -533,6 +533,7 @@ int gfn_to_page_many_atomic(struct kvm *kvm, gfn_t gfn, struct page **pages,
 
 struct page *gfn_to_page(struct kvm *kvm, gfn_t gfn);
 unsigned long gfn_to_hva(struct kvm *kvm, gfn_t gfn);
+unsigned long gfn_to_hva_read(struct kvm *kvm, gfn_t gfn, bool *writable);
 unsigned long gfn_to_hva_memslot(struct kvm_memory_slot *slot, gfn_t gfn);
 void kvm_release_page_clean(struct page *page);
 void kvm_release_page_dirty(struct page *page);
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index f7e4334..418d037 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1078,11 +1078,15 @@ unsigned long gfn_to_hva(struct kvm *kvm, gfn_t gfn)
 EXPORT_SYMBOL_GPL(gfn_to_hva);
 
 /*
- * The hva returned by this function is only allowed to be read.
- * It should pair with kvm_read_hva() or kvm_read_hva_atomic().
+ * If writable is set to false, the hva returned by this function is only
+ * allowed to be read.
  */
-static unsigned long gfn_to_hva_read(struct kvm *kvm, gfn_t gfn)
+unsigned long gfn_to_hva_read(struct kvm *kvm, gfn_t gfn, bool *writable)
 {
+	struct kvm_memory_slot *slot = gfn_to_memslot(kvm, gfn);
+	if (writable)
+		*writable = !memslot_is_readonly(slot);
+
 	return __gfn_to_hva_many(gfn_to_memslot(kvm, gfn), gfn, NULL, false);
 }
 
@@ -1450,7 +1454,7 @@ int kvm_read_guest_page(struct kvm *kvm, gfn_t gfn, void *data, int offset,
 	int r;
 	unsigned long addr;
 
-	addr = gfn_to_hva_read(kvm, gfn);
+	addr = gfn_to_hva_read(kvm, gfn, NULL);
 	if (kvm_is_error_hva(addr))
 		return -EFAULT;
 	r = kvm_read_hva(data, (void __user *)addr + offset, len);
@@ -1488,7 +1492,7 @@ int kvm_read_guest_atomic(struct kvm *kvm, gpa_t gpa, void *data,
 	gfn_t gfn = gpa >> PAGE_SHIFT;
 	int offset = offset_in_page(gpa);
 
-	addr = gfn_to_hva_read(kvm, gfn);
+	addr = gfn_to_hva_read(kvm, gfn, NULL);
 	if (kvm_is_error_hva(addr))
 		return -EFAULT;
 	pagefault_disable();
-- 
1.8.3.1

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