audit: get comm using lock to avoid race in string printing

When task->comm is passed directly to audit_log_untrustedstring() without
getting a copy or using the task_lock, there is a race that could happen that
would output a NULL (\0) in the output string that would effectively truncate
the rest of the report text after the comm= field in the audit, losing fields.

Use get_task_comm() to get a copy while acquiring the task_lock to prevent
this and to prevent the result from being a mixture of old and new values of
comm.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Richard Guy Briggs <rgb@redhat.com>

kernel/audit.c

@@ -1850,7 +1850,7 @@ EXPORT_SYMBOL(audit_log_task_context);
 void audit_log_task_info(struct audit_buffer *ab, struct task_struct *tsk)
 {
 	const struct cred *cred;
-	char name[sizeof(tsk->comm)];
+	char comm[sizeof(tsk->comm)];
 	struct mm_struct *mm = tsk->mm;
 	char *tty;
 
@@ -1884,9 +1884,8 @@ void audit_log_task_info(struct audit_buffer *ab, struct task_struct *tsk)
 			 from_kgid(&init_user_ns, cred->fsgid),
 			 tty, audit_get_sessionid(tsk));
 
-	get_task_comm(name, tsk);
 	audit_log_format(ab, " comm=");
-	audit_log_untrustedstring(ab, name);
+	audit_log_untrustedstring(ab, get_task_comm(comm, tsk));
 
 	if (mm) {
 		down_read(&mm->mmap_sem);

kernel/auditsc.c

@@ -2424,6 +2424,7 @@ static void audit_log_task(struct audit_buffer *ab)
 	kgid_t gid;
 	unsigned int sessionid;
 	struct mm_struct *mm = current->mm;
+	char comm[sizeof(current->comm)];
 
 	auid = audit_get_loginuid(current);
 	sessionid = audit_get_sessionid(current);
@@ -2436,7 +2437,7 @@ static void audit_log_task(struct audit_buffer *ab)
 			 sessionid);
 	audit_log_task_context(ab);
 	audit_log_format(ab, " pid=%d comm=", task_pid_nr(current));
-	audit_log_untrustedstring(ab, current->comm);
+	audit_log_untrustedstring(ab, get_task_comm(comm, current));
 	if (mm) {
 		down_read(&mm->mmap_sem);
 		if (mm->exe_file)