[libvirt] [PATCH v3 08/18] snapshot: Avoid latent use-after-free when cleaning snapshots

[Eric Blake]

Right now, the only callers of qemuDomainSnapshotDiscardAllMetadata()
are right before freeing the virDomainSnapshotObjList, so it did not
matter if the list's metaroot (which points to all the defined root
snapshots) is left inconsistent. But an upcoming patch will want to
clear all snapshots if a bulk redefine fails partway through, in
which case things must be reset.  Make this work by teaching the
existing virDomainSnapshotUpdateRelations() to be safe regardless of
the incoming state of the metaroot (since we don't want to leak that
internal detail into qemu code), then fixing the qemu code to use
it after deleting all snapshots.

Signed-off-by: Eric Blake <eblake at redhat.com>
---
 src/conf/snapshot_conf.c | 7 +++++--
 src/qemu/qemu_domain.c   | 4 ++++
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/src/conf/snapshot_conf.c b/src/conf/snapshot_conf.c
index 206b05c172..386ec82d15 100644
--- a/src/conf/snapshot_conf.c
+++ b/src/conf/snapshot_conf.c
@@ -1209,13 +1209,16 @@ virDomainSnapshotSetRelations(void *payload,
 }

 /* Populate parent link and child count of all snapshots, with all
- * relations starting as 0/NULL.  Return 0 on success, -1 if a parent
- * is missing or if a circular relationship was requested.  */
+ * assigned defs having relations starting as 0/NULL.  Return 0 on
+ * success, -1 if a parent is missing or if a circular relationship
+ * was requested.  */
 int
 virDomainSnapshotUpdateRelations(virDomainSnapshotObjListPtr snapshots)
 {
     struct snapshot_set_relation act = { snapshots, 0 };

+    snapshots->metaroot.nchildren = 0;
+    snapshots->metaroot.first_child = NULL;
     virHashForEach(snapshots->objs, virDomainSnapshotSetRelations, &act);
     return act.err;
 }
diff --git a/src/qemu/qemu_domain.c b/src/qemu/qemu_domain.c
index db25e1596c..3ac79fa50b 100644
--- a/src/qemu/qemu_domain.c
+++ b/src/qemu/qemu_domain.c
@@ -8625,6 +8625,10 @@ qemuDomainSnapshotDiscardAllMetadata(virQEMUDriverPtr driver,
     rem.err = 0;
     virDomainSnapshotForEach(vm->snapshots, qemuDomainSnapshotDiscardAll,
                              &rem);
+    if (rem.current)
+        vm->current_snapshot = NULL;
+    if (virDomainSnapshotUpdateRelations(vm->snapshots) < 0 && !rem.err)
+        rem.err = -1;

     return rem.err;
 }
-- 
2.20.1