[patch 3/4] KVM: MMU: flush TLBs on writable -> read-only spte overwrite

[Marcelo Tosatti <mtosatti@redhat.com>]

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This can happen in the following scenario:

vcpu0			vcpu1
read fault
gup(.write=0)		
			gup(.write=1)
			reuse swap cache, no COW
			set writable spte
			use writable spte
set read-only spte

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

Index: kvm/arch/x86/kvm/mmu.c
===================================================================
--- kvm.orig/arch/x86/kvm/mmu.c
+++ kvm/arch/x86/kvm/mmu.c
@@ -2068,6 +2068,16 @@ static void mmu_set_spte(struct kvm_vcpu
 				 spte_to_pfn(*sptep), pfn);
 			drop_spte(vcpu->kvm, sptep, shadow_trap_nonpresent_pte);
 			kvm_flush_remote_tlbs(vcpu->kvm);
+		/*
+ 		 * If we overwrite a writable spte with a read-only one,
+ 		 * drop it and flush remote TLBs. Otherwise rmap_write_protect
+ 		 * will find a read-only spte, even though the writable spte
+ 		 * might be cached on a CPU's TLB.
+ 		 */
+		} else if (is_writable_pte(*sptep) &&
+			  (!(pte_access & ACC_WRITE_MASK) || !dirty)) {
+			drop_spte(vcpu->kvm, sptep, shadow_trap_nonpresent_pte);
+			kvm_flush_remote_tlbs(vcpu->kvm);
 		} else
 			was_rmapped = 1;
 	}