audit: convert PPIDs to the inital PID namespace.

commit c92cdeb4 upstream.

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: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
[bwh: Backported to 3.2: sys_getppid() is used by audit_exit() but not
 audit_log_task_info()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

kernel/auditsc.c

@@ -473,7 +473,7 @@ 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;
@@ -1335,7 +1335,7 @@ static void audit_log_exit(struct audit_context *context, struct task_struct *ts
 	/* tsk == current */
 	context->pid = tsk->pid;
 	if (!context->ppid)
-		context->ppid = sys_getppid();
+		context->ppid = task_ppid_nr(tsk);
 	cred = current_cred();
 	context->uid   = cred->uid;
 	context->gid   = cred->gid;