[PATCH 2/2] printk: Add kernel parameter to control writes to /dev/kmsg

[Borislav Petkov]

From: Borislav Petkov <bp@xxxxxxx>

Add a "printk.kmsg" kernel command line parameter which controls how
userspace writes into /dev/kmsg. It has two options:

* on  - unlimited logging from userspace
* off - logging from userspace gets ignored

The default setting is to ratelimit the messages written to it.

It additionally does not limit logging to /dev/kmsg while the system is
booting if we haven't disabled it on the command line.

This patch is based on previous patches from Linus and Steven.

Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 Documentation/kernel-parameters.txt |  6 ++++
 kernel/printk/printk.c              | 57 +++++++++++++++++++++++++++++++------
 2 files changed, 55 insertions(+), 8 deletions(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 82b42c958d1c..4799c88b7258 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -3150,6 +3150,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			Format: <bool>  (1/Y/y=enable, 0/N/n=disable)
 			default: disabled
 
+	printk.kmsg={on,off}
+			Control writing to /dev/kmsg.
+			on - unlimited logging to /dev/kmsg from userspace
+			off - logging to /dev/kmsg disabled
+			Default: ratelimited logging.
+
 	printk.time=	Show timing data prefixed to each printk message line
 			Format: <bool>  (1/Y/y=enable, 0/N/n=disable)
 
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 60cdf6386763..33701a166f26 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -86,6 +86,29 @@ static struct lockdep_map console_lock_dep_map = {
 };
 #endif
 
+#define DEVKMSG_LOG_RATELIMIT	0
+#define DEVKMSG_LOG_ON		1
+#define DEVKMSG_LOG_OFF		2
+
+/* DEVKMSG_LOG_RATELIMIT by default */
+static unsigned int __read_mostly devkmsg_log;
+static int __init control_devkmsg(char *str)
+{
+	if (!str)
+		return -EINVAL;
+
+	if (!strncmp(str, "on", 2))
+		devkmsg_log = DEVKMSG_LOG_ON;
+	else if (!strncmp(str, "off", 3))
+		devkmsg_log = DEVKMSG_LOG_OFF;
+	else
+		return -EINVAL;
+
+	return 0;
+}
+__setup("printk.kmsg=", control_devkmsg);
+
+
 /*
  * Number of registered extended console drivers.
  *
@@ -614,6 +637,7 @@ struct devkmsg_user {
 	u64 seq;
 	u32 idx;
 	enum log_flags prev;
+	struct ratelimit_state rs;
 	struct mutex lock;
 	char buf[CONSOLE_EXT_LOG_MAX];
 };
@@ -623,11 +647,24 @@ static ssize_t devkmsg_write(struct kiocb *iocb, struct iov_iter *from)
 	char *buf, *line;
 	int level = default_message_loglevel;
 	int facility = 1;	/* LOG_USER */
+	struct file *file = iocb->ki_filp;
+	struct devkmsg_user *user = file->private_data;
 	size_t len = iov_iter_count(from);
 	ssize_t ret = len;
 
-	if (len > LOG_LINE_MAX)
+	if (!user || len > LOG_LINE_MAX)
 		return -EINVAL;
+
+	/* Ignore when user logging is disabled. */
+	if (devkmsg_log == DEVKMSG_LOG_OFF)
+		return len;
+
+	/* Ratelimit when not explicitly enabled or when we're not booting. */
+	if ((system_state != SYSTEM_BOOTING) && (devkmsg_log != DEVKMSG_LOG_ON)) {
+		if (!___ratelimit(&user->rs, current->comm))
+			return ret;
+	}
+
 	buf = kmalloc(len+1, GFP_KERNEL);
 	if (buf == NULL)
 		return -ENOMEM;
@@ -801,18 +838,20 @@ static int devkmsg_open(struct inode *inode, struct file *file)
 	int err;
 
 	/* write-only does not need any file context */
-	if ((file->f_flags & O_ACCMODE) == O_WRONLY)
-		return 0;
-
-	err = check_syslog_permissions(SYSLOG_ACTION_READ_ALL,
-				       SYSLOG_FROM_READER);
-	if (err)
-		return err;
+	if ((file->f_flags & O_ACCMODE) != O_WRONLY) {
+		err = check_syslog_permissions(SYSLOG_ACTION_READ_ALL,
+					       SYSLOG_FROM_READER);
+		if (err)
+			return err;
+	}
 
 	user = kmalloc(sizeof(struct devkmsg_user), GFP_KERNEL);
 	if (!user)
 		return -ENOMEM;
 
+	ratelimit_default_init(&user->rs);
+	ratelimit_set_flags(&user->rs, RATELIMIT_MSG_ON_RELEASE);
+
 	mutex_init(&user->lock);
 
 	raw_spin_lock_irq(&logbuf_lock);
@@ -831,6 +870,8 @@ static int devkmsg_release(struct inode *inode, struct file *file)
 	if (!user)
 		return 0;
 
+	ratelimit_state_exit(&user->rs);
+
 	mutex_destroy(&user->lock);
 	kfree(user);
 	return 0;
-- 
2.7.3