[Qemu-devel] [PULL 3/3] migration: avoid segmentfault when take a snapshot of a VM which being migrated

["Dr. David Alan Gilbert (git)" <dgilbert@redhat.com>]

From: Jia Lina <jialina01@baidu.com>

During an active background migration, snapshot will trigger a
segmentfault. As snapshot clears the "current_migration" struct
and updates "to_dst_file" before it finds out that there is a
migration task, Migration accesses the null pointer in
"current_migration" struct and qemu crashes eventually.

Signed-off-by: Jia Lina <jialina01@baidu.com>
Signed-off-by: Chai Wen <chaiwen@baidu.com>
Signed-off-by: Zhang Yu <zhangyu31@baidu.com>
Message-Id: <20181026083620.10172-1-jialina01@baidu.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
---
 migration/migration.c |  2 +-
 migration/migration.h |  2 ++
 migration/savevm.c    | 19 +++++++++++--------
 3 files changed, 14 insertions(+), 9 deletions(-)

diff --git a/migration/migration.c b/migration/migration.c
index 8b36e7f184..b261c1e4ce 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -742,7 +742,7 @@ MigrationParameters *qmp_query_migrate_parameters(Error **errp)
  * Return true if we're already in the middle of a migration
  * (i.e. any of the active or setup states)
  */
-static bool migration_is_setup_or_active(int state)
+bool migration_is_setup_or_active(int state)
 {
     switch (state) {
     case MIGRATION_STATUS_ACTIVE:
diff --git a/migration/migration.h b/migration/migration.h
index f7813f8261..e413d4d8b6 100644
--- a/migration/migration.h
+++ b/migration/migration.h
@@ -241,6 +241,8 @@ void migrate_fd_error(MigrationState *s, const Error *error);
 
 void migrate_fd_connect(MigrationState *s, Error *error_in);
 
+bool migration_is_setup_or_active(int state);
+
 void migrate_init(MigrationState *s);
 bool migration_is_blocked(Error **errp);
 /* True if outgoing migration has entered postcopy phase */
diff --git a/migration/savevm.c b/migration/savevm.c
index 9992af4db4..ef707b8c43 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -1327,21 +1327,25 @@ static int qemu_savevm_state(QEMUFile *f, Error **errp)
     MigrationState *ms = migrate_get_current();
     MigrationStatus status;
 
-    migrate_init(ms);
-
-    ms->to_dst_file = f;
+    if (migration_is_setup_or_active(ms->state) ||
+        ms->state == MIGRATION_STATUS_CANCELLING ||
+        ms->state == MIGRATION_STATUS_COLO) {
+        error_setg(errp, QERR_MIGRATION_ACTIVE);
+        return -EINVAL;
+    }
 
     if (migration_is_blocked(errp)) {
-        ret = -EINVAL;
-        goto done;
+        return -EINVAL;
     }
 
     if (migrate_use_block()) {
         error_setg(errp, "Block migration and snapshots are incompatible");
-        ret = -EINVAL;
-        goto done;
+        return -EINVAL;
     }
 
+    migrate_init(ms);
+    ms->to_dst_file = f;
+
     qemu_mutex_unlock_iothread();
     qemu_savevm_state_header(f);
     qemu_savevm_state_setup(f);
@@ -1363,7 +1367,6 @@ static int qemu_savevm_state(QEMUFile *f, Error **errp)
         error_setg_errno(errp, -ret, "Error while writing VM state");
     }
 
-done:
     if (ret != 0) {
         status = MIGRATION_STATUS_FAILED;
     } else {
-- 
2.19.1