diff mbox series

[2/2] KVM: PPC: Book3S HV: Close race with page faults around memslot flushes

Message ID 20200528015410.GE307798@thinks.paulus.ozlabs.org
State Accepted
Headers show
Series [1/2] KVM: PPC: Book3S HV: Remove user-triggerable WARN_ON | expand

Commit Message

Paul Mackerras May 28, 2020, 1:54 a.m. UTC
There is a potential race condition between hypervisor page faults
and flushing a memslot.  It is possible for a page fault to read the
memslot before a memslot is updated and then write a PTE to the
partition-scoped page tables after kvmppc_radix_flush_memslot has
completed.  (Note that this race has never been explicitly observed.)

To close this race, it is sufficient to increment the MMU sequence
number while the kvm->mmu_lock is held.  That will cause
mmu_notifier_retry() to return true, and the page fault will then
return to the guest without inserting a PTE.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
 arch/powerpc/kvm/book3s_64_mmu_radix.c | 5 +++++
 1 file changed, 5 insertions(+)
diff mbox series


diff --git a/arch/powerpc/kvm/book3s_64_mmu_radix.c b/arch/powerpc/kvm/book3s_64_mmu_radix.c
index bc3f795..aa41183 100644
--- a/arch/powerpc/kvm/book3s_64_mmu_radix.c
+++ b/arch/powerpc/kvm/book3s_64_mmu_radix.c
@@ -1130,6 +1130,11 @@  void kvmppc_radix_flush_memslot(struct kvm *kvm,
 		gpa += PAGE_SIZE;
+	/*
+	 * Increase the mmu notifier sequence number to prevent any page
+	 * fault that read the memslot earlier from writing a PTE.
+	 */
+	kvm->mmu_notifier_seq++;