audit: convert PPIDs to the inital PID namespace.

sys_getppid() returns the parent pid of the current process in its own pid
namespace.  Since audit filters are based in the init pid namespace, a process
could avoid a filter or trigger an unintended one by being in an alternate pid
namespace or log meaningless information.

Switch to task_ppid_nr() for PPIDs to anchor all audit filters in the
init_pid_ns.

(informed by ebiederman's 6c621b7e)
Cc: stable@vger.kernel.org
Cc: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Richard Guy Briggs <rgb@redhat.com>

kernel/audit.c

@@ -1819,10 +1819,10 @@ void audit_log_task_info(struct audit_buffer *ab, struct task_struct *tsk)
 	spin_unlock_irq(&tsk->sighand->siglock);
 
 	audit_log_format(ab,
-			 " ppid=%ld pid=%d auid=%u uid=%u gid=%u"
+			 " ppid=%d pid=%d auid=%u uid=%u gid=%u"
 			 " euid=%u suid=%u fsuid=%u"
 			 " egid=%u sgid=%u fsgid=%u tty=%s ses=%u",
-			 sys_getppid(),
+			 task_ppid_nr(tsk),
 			 tsk->pid,
 			 from_kuid(&init_user_ns, audit_get_loginuid(tsk)),
 			 from_kuid(&init_user_ns, cred->uid),

kernel/auditsc.c

@@ -465,7 +465,7 @@ static int audit_filter_rules(struct task_struct *tsk,
 		case AUDIT_PPID:
 			if (ctx) {
 				if (!ctx->ppid)
-					ctx->ppid = sys_getppid();
+					ctx->ppid = task_ppid_nr(tsk);
 				result = audit_comparator(ctx->ppid, f->op, f->val);
 			}
 			break;