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>
diff --git a/kernel/printk.c b/kernel/printk.c
index ceb4a2f..572730b 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -414,7 +414,9 @@
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 @@
{
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 @@
error = -EFAULT;
goto out;
}
- error = wait_event_interruptible(log_wait,
- syslog_seq != log_next_seq);
+ error = mutex_lock_interruptible(&syslog_mutex);
if (error)
goto out;
+ error = wait_event_interruptible(log_wait,
+ syslog_seq != log_next_seq);
+ 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: