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>

1 files changed, 10 insertions, 0 deletions

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 11b9102f4113..99433943170c 100644
--- 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;
         }