landlock: Log the TGID of the domain creator

As for other Audit's "pid" fields, Landlock should use the task's TGID
instead of its TID.  Fix this issue by keeping a reference to the TGID
of the domain creator.

Existing tests already check for the PID but only with the thread group
leader, so always the TGID.  A following patch adds dedicated tests for
non-leader thread.

Remove the current_real_cred() check which does not make sense because
we only reference a struct pid, whereas a previous version did reference
a struct cred instead.

Cc: Christian Brauner <brauner@kernel.org>
Cc: Paul Moore <paul@paul-moore.com>
Reviewed-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20250410171725.1265860-1-mic@digikod.net
Signed-off-by: Mickaël Salaün <mic@digikod.net>

Author: l0kod

security/landlock/domain.c

@@ -16,6 +16,7 @@
 #include <linux/path.h>
 #include <linux/pid.h>
 #include <linux/sched.h>
+#include <linux/signal.h>
 #include <linux/uidgid.h>
 
 #include "access.h"
@@ -99,8 +100,7 @@ static struct landlock_details *get_current_details(void)
 		return ERR_PTR(-ENOMEM);
 
 	memcpy(details->exe_path, path_str, path_size);
-	WARN_ON_ONCE(current_cred() != current_real_cred());
-	details->pid = get_pid(task_pid(current));
+	details->pid = get_pid(task_tgid(current));
 	details->uid = from_kuid(&init_user_ns, current_uid());
 	get_task_comm(details->comm, current);
 	return details;