util: fix use-after-free in module_load_one

g_hash_table_add always retains ownership of the pointer passed in as
the key. Its return status merely indicates whether the added entry was
new, or replaced an existing entry. Thus key must never be freed after
this method returns.

Spotted by ASAN:

==2407186==ERROR: AddressSanitizer: heap-use-after-free on address 0x6020003ac4f0 at pc 0x7ffff766659c bp 0x7fffffffd1d0 sp 0x7fffffffc980
READ of size 1 at 0x6020003ac4f0 thread T0
    #0 0x7ffff766659b  (/lib64/libasan.so.6+0x8a59b)
    #1 0x7ffff6bfa843 in g_str_equal ../glib/ghash.c:2303
    qemu#2 0x7ffff6bf8167 in g_hash_table_lookup_node ../glib/ghash.c:493
    qemu#3 0x7ffff6bf9b78 in g_hash_table_insert_internal ../glib/ghash.c:1598
    qemu#4 0x7ffff6bf9c32 in g_hash_table_add ../glib/ghash.c:1689
    qemu#5 0x5555596caad4 in module_load_one ../util/module.c:233
    qemu#6 0x5555596ca949 in module_load_one ../util/module.c:225
    qemu#7 0x5555596ca949 in module_load_one ../util/module.c:225
    qemu#8 0x5555596cbdf4 in module_load_qom_all ../util/module.c:349

Typical C bug...

Fixes: 9062912 ("module: use g_hash_table_add()")
Cc: qemu-stable@nongnu.org
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20210316134456.3243102-1-marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

Author: elmarco

util/module.c

@@ -230,10 +230,11 @@ bool module_load_one(const char *prefix, const char *lib_name, bool mayfail)
         }
     }
 
-    if (!g_hash_table_add(loaded_modules, module_name)) {
+    if (g_hash_table_contains(loaded_modules, module_name)) {
         g_free(module_name);
         return true;
     }
+    g_hash_table_add(loaded_modules, module_name);
 
     search_dir = getenv("QEMU_MODULE_DIR");
     if (search_dir != NULL) {