Commit 4a77a5a0 authored by Yuanhan Liu's avatar Yuanhan Liu Committed by Greg Kroah-Hartman
Browse files

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 <>
Reviewed-by: default avatarFengguang Wu <>
Acked-By: default avatarKay Sievers <>
Signed-off-by: default avatarGreg Kroah-Hartman <>
parent e2ae715d
......@@ -414,7 +414,9 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
if (!user)
return -EBADF;
ret = mutex_lock_interruptible(&user->lock);
if (ret)
return ret;
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) {
goto out;
error = syslog_print(buf, len);
/* Read/clear last kernel messages */
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