printk: use mutex lock to stop syslog_seq from going wild

Although syslog_seq and log_next_seq stuff are protected by logbuf_lock
spin log, it's not enough. Say we have two processes A and B, and let
syslog_seq = N, while log_next_seq = N + 1, and the two processes both
come to syslog_print at almost the same time. And No matter which
process get the spin lock first, it will increase syslog_seq by one,
then release spin lock; thus later, another process increase syslog_seq
by one again. In this case, syslog_seq is bigger than syslog_next_seq.
And latter, it would make:
   wait_event_interruptiable(log_wait, syslog != log_next_seq)
don't wait any more even there is no new write comes. Thus it introduce
a infinite loop reading.

I can easily see this kind of issue by the following steps:
  # cat /proc/kmsg # at meantime, I don't kill rsyslog
                   # So they are the two processes.
  # xinit          # I added drm.debug=6 in the kernel parameter line,
                   # so that it will produce lots of message and let that
                   # issue happen

It's 100% reproducable on my side. And my disk will be filled up by
/var/log/messages in a quite short time.

So, introduce a mutex_lock to stop syslog_seq from going wild just like
what devkmsg_read() does. It does fix this issue as expected.

v2: use mutex_lock_interruptiable() instead (comments from Kay)
Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: Fengguang Wu <fengguang.wu@intel.com>
Acked-By: Kay Sievers <kay@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

kernel/printk.c

@@ -414,7 +414,9 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
 	if (!user)
 		return -EBADF;
 
-	mutex_lock(&user->lock);
+	ret = mutex_lock_interruptible(&user->lock);
+	if (ret)
+		return ret;
 	raw_spin_lock(&logbuf_lock);
 	while (user->seq == log_next_seq) {
 		if (file->f_flags & O_NONBLOCK) {
@@ -976,6 +978,7 @@ int do_syslog(int type, char __user *buf, int len, bool from_file)
 {
 	bool clear = false;
 	static int saved_console_loglevel = -1;
+	static DEFINE_MUTEX(syslog_mutex);
 	int error;
 
 	error = check_syslog_permissions(type, from_file);
@@ -1002,11 +1005,17 @@ int do_syslog(int type, char __user *buf, int len, bool from_file)
 			error = -EFAULT;
 			goto out;
 		}
+		error = mutex_lock_interruptible(&syslog_mutex);
+		if (error)
+			goto out;
 		error = wait_event_interruptible(log_wait,
 						 syslog_seq != log_next_seq);
-		if (error)
+		if (error) {
+			mutex_unlock(&syslog_mutex);
 			goto out;
+		}
 		error = syslog_print(buf, len);
+		mutex_unlock(&syslog_mutex);
 		break;
 	/* Read/clear last kernel messages */
 	case SYSLOG_ACTION_READ_CLEAR: