lockdown: avoid crashing when getting system buid fails

When userpref_read_system_buid fails to retrieve the buid,
systembuid will be set to NULL. It was then unconditionally passed
to plist_new_string - which will crash with a NULL argument.

See https://bugs.debian.org/730756 for people reporting this
happening in the real world.

See https://github.com/libimobiledevice/libimobiledevice/commit/1331f6baa1799e41003aa812c0d1bf36193535ea
("lockdown: Make sure to set SystemBUID in generated pair records")
for the commit where this problem was introduced.

1 files changed, 3 insertions, 2 deletions

diff --git a/src/lockdown.c b/src/lockdown.c
index cf03e0f..b96ddc6 100644
--- a/src/lockdown.c
+++ b/src/lockdown.c
@@ -743,8 +743,9 @@ static lockdownd_error_t pair_record_generate(lockdownd_client_t client, plist_t
         }
 
         /* set SystemBUID */
-        userpref_read_system_buid(&system_buid);
-        plist_dict_set_item(*pair_record, USERPREF_SYSTEM_BUID_KEY, plist_new_string(system_buid));
+        if (userpref_read_system_buid(&system_buid)) {
+                plist_dict_set_item(*pair_record, USERPREF_SYSTEM_BUID_KEY, plist_new_string(system_buid));
+        }
 
         /* set HostID */
         host_id = generate_uuid();