KVM: PPC: Book3S HV: Fix race in reading change bit when removing HPTE

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>

1 files changed, 12 insertions, 6 deletions

diff --git a/arch/powerpc/kvm/book3s_hv_rm_mmu.c b/arch/powerpc/kvm/book3s_hv_rm_mmu.c
index b027a89737b6..c6d601cc9764 100644
--- 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, unsigned long flags,
         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);