Commit 4a77a5a0 authored Jun 16, 2012 by Yuanhan Liu Committed by Greg Kroah-Hartman Jun 16, 2012

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>

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:

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