path: root/kernel/printk.c
diff options
authorYuanhan Liu <yuanhan.liu@linux.intel.com>2012-06-16 21:21:51 +0800
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2012-06-16 08:36:02 -0700
commit4a77a5a06ec66ed05199b301e7c25f42f979afdc (patch)
tree4b1897ff18501dae3a62c3f1618a761829eb64d7 /kernel/printk.c
parente2ae715d66bf4becfb85eb84b7150e23cf27df30 (diff)
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>
Diffstat (limited to 'kernel/printk.c')
1 files changed, 11 insertions, 2 deletions
diff --git a/kernel/printk.c b/kernel/printk.c
index ceb4a2f775a..572730bd8a5 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;
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);
/* Read/clear last kernel messages */