[PATCH] migration: Fix a potential guest memory corruption

[Zhenzhong Duan <zhenzhong.duan@intel.com>]

Imagine a rare case, after a dirty page is sent to compression threads's
ring, dirty bitmap sync trigger right away and mark the same page dirty
again and sent. Then the new page may be overwriten by stale page in
compression threads's ring in the destination.

So we need to ensure there is only one copy of the same dirty page either
by flushing the ring after every bitmap sync or avoiding processing same
dirty page continuously.

I choose the 2nd which avoids the time consuming flush operation.

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
---
 migration/ram.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/migration/ram.c b/migration/ram.c
index dc1de9ddbc68..67b2035586bd 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -1551,7 +1551,7 @@ static bool find_dirty_block(RAMState *rs, PageSearchStatus *pss, bool *again)
     pss->postcopy_requested = false;
     pss->postcopy_target_channel = RAM_CHANNEL_PRECOPY;
 
-    pss->page = migration_bitmap_find_dirty(rs, pss->block, pss->page);
+    pss->page = migration_bitmap_find_dirty(rs, pss->block, pss->page + 1);
     if (pss->complete_round && pss->block == rs->last_seen_block &&
         pss->page >= rs->last_page) {
         /*
@@ -1564,7 +1564,7 @@ static bool find_dirty_block(RAMState *rs, PageSearchStatus *pss, bool *again)
     if (!offset_in_ramblock(pss->block,
                             ((ram_addr_t)pss->page) << TARGET_PAGE_BITS)) {
         /* Didn't find anything in this RAM Block */
-        pss->page = 0;
+        pss->page = -1;
         pss->block = QLIST_NEXT_RCU(pss->block, next);
         if (!pss->block) {
             /*
@@ -2694,7 +2694,7 @@ static void ram_state_reset(RAMState *rs)
 {
     rs->last_seen_block = NULL;
     rs->last_sent_block = NULL;
-    rs->last_page = 0;
+    rs->last_page = -1;
     rs->last_version = ram_list.version;
     rs->xbzrle_enabled = false;
     postcopy_preempt_reset(rs);
@@ -2889,7 +2889,7 @@ void ram_postcopy_send_discard_bitmap(MigrationState *ms)
     /* Easiest way to make sure we don't resume in the middle of a host-page */
     rs->last_seen_block = NULL;
     rs->last_sent_block = NULL;
-    rs->last_page = 0;
+    rs->last_page = -1;
 
     postcopy_each_ram_send_discard(ms);
 
-- 
2.25.1