[PATCH v4] test-vmstate: fix bad GTree usage, use-after-free

[Eric Auger <eric.auger@redhat.com>]

According to g_tree_foreach() documentation:
"The tree may not be modified while iterating over it (you can't
add/remove items)."

compare_trees()/diff_tree() fail to respect this rule.
Historically GLib2 used a slice allocator for the GTree APIs
which did not immediately release the memory back to the system
allocator. As a result QEMU's use-after-free bug was not visible.
With GLib > 2.75.3 however, GLib2 has switched to using malloc
and now a SIGSEGV can be observed while running test-vmstate.

Get rid of the node removal within the tree traversal. Also
check the trees have the same number of nodes before the actual
diff.

Fixes: 9a85e4b8f6 ("migration: Support gtree migration")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1518
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reported-by: Richard W.M. Jones <rjones@redhat.com>
Tested-by: Richard W.M. Jones <rjones@redhat.com>
Reviewed-by: Richard W.M. Jones <rjones@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

---
v3 -> v4:
- Adopt Daniel's wording and add his R-b

v2 -> v3:
- Enhance the commit message with Rich's explanations
- Add Roch's T-b and R-b

v1 -> v2:
- respin of Marc-André's patch from Aug 2020, which can be
found at
https://lore.kernel.org/qemu-devel/20200827161826.1165971-1-marcandre.lureau@redhat.com/
This fell through the cracks and now we hit a SIGSEGV
---
 tests/unit/test-vmstate.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/tests/unit/test-vmstate.c b/tests/unit/test-vmstate.c
index 79357b29ca..0b7d5ecd68 100644
--- a/tests/unit/test-vmstate.c
+++ b/tests/unit/test-vmstate.c
@@ -1073,7 +1073,6 @@ static gboolean diff_tree(gpointer key, gpointer value, gpointer data)
     struct match_node_data d = {tp->tree2, key, value};
 
     g_tree_foreach(tp->tree2, tp->match_node, &d);
-    g_tree_remove(tp->tree1, key);
     return false;
 }
 
@@ -1082,9 +1081,9 @@ static void compare_trees(GTree *tree1, GTree *tree2,
 {
     struct tree_cmp_data tp = {tree1, tree2, function};
 
+    assert(g_tree_nnodes(tree1) == g_tree_nnodes(tree2));
     g_tree_foreach(tree1, diff_tree, &tp);
-    assert(g_tree_nnodes(tree1) == 0);
-    assert(g_tree_nnodes(tree2) == 0);
+    g_tree_destroy(g_tree_ref(tree1));
 }
 
 static void diff_domain(TestGTreeDomain *d1, TestGTreeDomain *d2)
-- 
2.38.1