KVM: MMU: flush TLBs on writable -> read-only spte overwrite

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>
Signed-off-by: Avi Kivity <avi@redhat.com>

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 11b9102..9943394 100644 (file)
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -2069,6 +2069,16 @@ static void mmu_set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
                                 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;
        }