[patch] kvm: make cr3 loading more robust

[Ingo Molnar <mingo-X9Un+BFzKDI@public.gmane.org>]

From: Ingo Molnar <mingo-X9Un+BFzKDI@public.gmane.org>
Subject: [patch] kvm: make cr3 loading more robust

rmap_write_protect() has a BUG_ON() if a physical address is not found 
the the memslot. But this is a possible scenario if a buggy guest OS 
loads an invalid or corrupted cr3 value. So exit more gracefully.

[ Avi: this patch needs kvm_mmu_get_page() NULL return to not be ignored 
  by mmu_alloc_roots()/nonpaging_init_context()/paging_new_cr3()/etc. 
  and passed further down - i assume you are working on those
  issues already, or should i fix those too? ]

Signed-off-by: Ingo Molnar <mingo-X9Un+BFzKDI@public.gmane.org>

Index: linux/drivers/kvm/mmu.c
===================================================================
--- linux.orig/drivers/kvm/mmu.c
+++ linux/drivers/kvm/mmu.c
@@ -378,7 +378,7 @@ static void rmap_remove(struct kvm_vcpu 
 	}
 }
 
-static void rmap_write_protect(struct kvm_vcpu *vcpu, u64 gfn)
+static int rmap_write_protect(struct kvm_vcpu *vcpu, u64 gfn)
 {
 	struct kvm *kvm = vcpu->kvm;
 	struct page *page;
@@ -387,7 +387,8 @@ static void rmap_write_protect(struct kv
 	u64 *spte;
 
 	slot = gfn_to_memslot(kvm, gfn);
-	BUG_ON(!slot);
+	if (!slot)
+		return -1;
 	page = gfn_to_page(slot, gfn);
 
 	while (page->private) {
@@ -407,6 +408,7 @@ static void rmap_write_protect(struct kv
 		kvm_arch_ops->tlb_flush(vcpu);
 		*spte &= ~(u64)PT_WRITABLE_MASK;
 	}
+	return 0;
 }
 
 static int is_empty_shadow_page(hpa_t page_hpa)
@@ -592,12 +594,17 @@ static struct kvm_mmu_page *kvm_mmu_get_
 	page = kvm_mmu_alloc_page(vcpu, parent_pte);
 	if (!page)
 		return page;
+	if (!metaphysical) {
+		if (rmap_write_protect(vcpu, gfn) < 0) {
+			kvm_mmu_free_page(vcpu, page->page_hpa);
+			return NULL;
+		}
+	}
 	pgprintk("%s: adding gfn %lx role %x\n", __FUNCTION__, gfn, role.word);
 	page->gfn = gfn;
 	page->role = role;
 	hlist_add_head(&page->hash_link, bucket);
-	if (!metaphysical)
-		rmap_write_protect(vcpu, gfn);
+
 	return page;
 }
 

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