[PATCH 4.1 079/102] KVM: PPC: Book3S HV: Fix race in reading change bit when removing HPTE

Greg Kroah-Hartman
2015-09-19T18:23:44Z

From: Paul Mackerras <paulus@samba.org>

commit 1e5bf454f58731e360e504253e85bae7aaa2d298 upstream.

The reference (R) and change (C) bits in a HPT entry can be set by
hardware at any time up until the HPTE is invalidated and the TLB
invalidation sequence has completed. This means that when removing
a HPTE, we need to read the HPTE after the invalidation sequence has
completed in order to obtain reliable values of R and C. The code
in kvmppc_do_h_remove() used to do this. However, commit 6f22bd3265fb
("KVM: PPC: Book3S HV: Make HTAB code LE host aware") removed the
read after invalidation as a side effect of other changes. This
restores the read of the HPTE after invalidation.

The user-visible effect of this bug would be that when migrating a
guest, there is a small probability that a page modified by the guest
and then unmapped by the guest might not get re-transmitted and thus
the destination might end up with a stale copy of the page.

Fixes: 6f22bd3265fb
Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/powerpc/kvm/book3s_hv_rm_mmu.c | 18 ++++++++++++------
1 file changed, 12 insertions(+), 6 deletions(-)

--- a/arch/powerpc/kvm/book3s_hv_rm_mmu.c
+++ b/arch/powerpc/kvm/book3s_hv_rm_mmu.c
@@ -421,14 +421,20 @@ long kvmppc_do_h_remove(struct kvm *kvm,
rev = real_vmalloc_addr(&kvm->arch.revmap[pte_index]);
v = pte & ~HPTE_V_HVLOCK;
if (v & HPTE_V_VALID) {
- u64 pte1;
-
- pte1 = be64_to_cpu(hpte[1]);
hpte[0] &= ~cpu_to_be64(HPTE_V_VALID);
- rb = compute_tlbie_rb(v, pte1, pte_index);
+ rb = compute_tlbie_rb(v, be64_to_cpu(hpte[1]), pte_index);
do_tlbies(kvm, &rb, 1, global_invalidates(kvm, flags), true);
- /* Read PTE low word after tlbie to get final R/C values */
- remove_revmap_chain(kvm, pte_index, rev, v, pte1);
+ /*
+ * The reference (R) and change (C) bits in a HPT
+ * entry can be set by hardware at any time up until
+ * the HPTE is invalidated and the TLB invalidation
+ * sequence has completed. This means that when
+ * removing a HPTE, we need to re-read the HPTE after
+ * the invalidation sequence has completed in order to
+ * obtain reliable values of R and C.
+ */
+ remove_revmap_chain(kvm, pte_index, rev, v,
+ be64_to_cpu(hpte[1]));
}
r = rev->guest_rpte & ~HPTE_GR_RESERVED;
note_hpte_modification(kvm, rev);