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>

1 files changed, 11 insertions, 2 deletions

diff --git a/kernel/printk.c b/kernel/printk.c
index ceb4a2f775a1..572730bd8a5c 100644
--- a/kernel/printk.c
+++ b/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: