[PATCH -v2 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 three options:

* ratelimit - ratelimit logging from userspace.
* 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.

In addition, we can control the logging from a lower priority
sysctl interface - kernel.printk_kmsg={0,1,2} - with numeric values
corresponding to the options above.

That interface will succeed only if printk.kmsg *hasn't* been supplied
on the command line. If it has, then printk.kmsg is a one-time setting
which remains for the duration of the system lifetime.

Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 Documentation/kernel-parameters.txt |  6 +++
 Documentation/sysctl/kernel.txt     | 14 +++++++
 include/linux/printk.h              |  6 +++
 kernel/printk/printk.c              | 77 +++++++++++++++++++++++++++++++++----
 kernel/sysctl.c                     |  9 +++++
 5 files changed, 104 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/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
index a3683ce2a2f3..7823eb8c714f 100644
--- a/Documentation/sysctl/kernel.txt
+++ b/Documentation/sysctl/kernel.txt
@@ -752,6 +752,20 @@ send before ratelimiting kicks in.
 
 ==============================================================
 
+printk_kmsg:
+
+Control the logging to /dev/kmsg from userspace:
+
+0: default, ratelimited
+1: unlimited logging to /dev/kmsg from userspace
+2: logging to /dev/kmsg disabled
+
+The kernel command line parameter printk.kmsg= overrides this and is a
+one-time setting for the duration of the system lifetime: once set, it
+cannot be changed by this sysctl interface anymore.
+
+==============================================================
+
 randomize_va_space:
 
 This option can be used to select the type of process address
diff --git a/include/linux/printk.h b/include/linux/printk.h
index f4da695fd615..bcf72e756122 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -171,6 +171,12 @@ extern bool printk_timed_ratelimit(unsigned long *caller_jiffies,
 extern int printk_delay_msec;
 extern int dmesg_restrict;
 extern int kptr_restrict;
+extern unsigned int devkmsg_log;
+
+struct ctl_table;
+
+int devkmsg_sysctl_set_loglvl(struct ctl_table *table, int write,
+			      void __user *buffer, size_t *lenp, loff_t *ppos);
 
 extern void wake_up_klogd(void);
 
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 60cdf6386763..9f0a885c2718 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -86,6 +86,48 @@ static struct lockdep_map console_lock_dep_map = {
 };
 #endif
 
+#define DEVKMSG_LOG_RATELIMIT		0
+#define DEVKMSG_LOG_ON			1
+#define DEVKMSG_LOG_OFF			2
+#define DEVKMSG_LOCK			(1 << 8)
+#define DEVKMSG_LOG_MASK		(DEVKMSG_LOCK - 1)
+#define DEVKMSG_LOCKED_MASK		~DEVKMSG_LOG_MASK
+
+/* DEVKMSG_LOG_RATELIMIT by default */
+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 if (!strncmp(str, "ratelimit", 9))
+		devkmsg_log = DEVKMSG_LOG_RATELIMIT;
+	else
+		return -EINVAL;
+
+	/* Sysctl cannot change it anymore. */
+	devkmsg_log |= DEVKMSG_LOCK;
+
+	return 0;
+}
+__setup("printk.kmsg=", control_devkmsg);
+
+
+int devkmsg_sysctl_set_loglvl(struct ctl_table *table, int write,
+			      void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+	if (devkmsg_log & DEVKMSG_LOCKED_MASK) {
+		if (write)
+			return -EINVAL;
+	}
+
+	return proc_dointvec_minmax(table, write, buffer, lenp, ppos);
+}
+
 /*
  * Number of registered extended console drivers.
  *
@@ -614,6 +656,7 @@ 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 +666,25 @@ 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_MASK) == 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_MASK) != DEVKMSG_LOG_ON)) {
+		if (!___ratelimit(&user->rs, current->comm))
+			return ret;
+	}
+
 	buf = kmalloc(len+1, GFP_KERNEL);
 	if (buf == NULL)
 		return -ENOMEM;
@@ -801,18 +858,20 @@ 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 +890,8 @@ 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;
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 87b2fc38398b..a29d6c4fa86c 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -814,6 +814,15 @@ static struct ctl_table kern_table[] = {
 		.extra2		= &ten_thousand,
 	},
 	{
+		.procname	= "printk_kmsg",
+		.data		= &devkmsg_log,
+		.maxlen		= sizeof(unsigned int),
+		.mode		= 0644,
+		.proc_handler	= devkmsg_sysctl_set_loglvl,
+		.extra1		= &zero,
+		.extra2		= &two,
+	},
+	{
 		.procname	= "dmesg_restrict",
 		.data		= &dmesg_restrict,
 		.maxlen		= sizeof(int),
-- 
2.7.3