[3.19.y-ckt stable] Patch "KVM: PPC: Book3S HV: Fix race in reading change bit when removing HPTE" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

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

to the linux-3.19.y-queue branch of the 3.19.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.19.y-queue

This patch is scheduled to be released in version 3.19.8-ckt8.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.19.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From 3f6c0dbb8cb7a395c6436d641b5423f4a440ac9e Mon Sep 17 00:00:00 2001
From: Paul Mackerras <paulus at samba.org>
Date: Wed, 24 Jun 2015 21:18:05 +1000
Subject: KVM: PPC: Book3S HV: Fix race in reading change bit when removing
 HPTE

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 at samba.org>
Signed-off-by: Alexander Graf <agraf at suse.de>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 arch/powerpc/kvm/book3s_hv_rm_mmu.c | 18 ++++++++++++------
 1 file 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 510bdfb..4f88195 100644
--- a/arch/powerpc/kvm/book3s_hv_rm_mmu.c
+++ b/arch/powerpc/kvm/book3s_hv_rm_mmu.c
@@ -419,14 +419,20 @@ 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);
--
1.9.1