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Commit 4a77a5a0 authored by Yuanhan Liu's avatar Yuanhan Liu Committed by Greg Kroah-Hartman
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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: default avatarYuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: default avatarFengguang Wu <fengguang.wu@intel.com>
Acked-By: default avatarKay Sievers <kay@vrfy.org>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent e2ae715d

kernel/printk.c

+11 −2
Original line number Diff line number Diff line
@@ -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: