printk: guard the amount written per line by devkmsg_read()

devkmsg_read() uses 8k buffer and assumes that the formatted output
message won't overrun which seems safe given LOG_LINE_MAX, the current
use of dict and the escaping method being used; however, we're
planning to use devkmsg formatting wider and accounting for the buffer
size properly isn't that complicated.

This patch defines CONSOLE_EXT_LOG_MAX as 8192 and updates
devkmsg_read() so that it limits output accordingly.

Change-Id: Ic8579ddcd55294a38561e9e8b28449c067600db1
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Kay Sievers <kay@vrfy.org>
Cc: Petr Mladek <pmladek@suse.cz>

diff --git a/include/linux/printk.h b/include/linux/printk.h
index 9b30871..58b1fec 100644 (file)
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -30,6 +30,8 @@ static inline const char *printk_skip_level(const char *buffer)
        return buffer;
 }
 
+#define CONSOLE_EXT_LOG_MAX    8192
+
 /* printk's without a loglevel use this.. */
 #define MESSAGE_LOGLEVEL_DEFAULT CONFIG_MESSAGE_LOGLEVEL_DEFAULT
 

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 3c1aca0..9ed8ac3 100644 (file)
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -506,6 +506,11 @@ ok:
        return security_syslog(type);
 }
 
+static void append_char(char **pp, char *e, char c)
+{
+       if (*pp < e)
+               *(*pp)++ = c;
+}
 
 /* /dev/kmsg - userspace message inject/listen interface */
 struct devkmsg_user {
@@ -513,7 +518,7 @@ struct devkmsg_user {
        u32 idx;
        enum log_flags prev;
        struct mutex lock;
-       char buf[8192];
+       char buf[CONSOLE_EXT_LOG_MAX];
 };
 
 static ssize_t devkmsg_write(struct kiocb *iocb, struct iov_iter *from)
@@ -571,6 +576,7 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
 {
        struct devkmsg_user *user = file->private_data;
        struct printk_log *msg;
+       char *p, *e;
        u64 ts_usec;
        size_t i;
        char cont = '-';
@@ -580,6 +586,9 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
        if (!user)
                return -EBADF;
 
+       p = user->buf;
+       e = user->buf + sizeof(user->buf);
+
        ret = mutex_lock_interruptible(&user->lock);
        if (ret)
                return ret;
@@ -626,9 +635,9 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
                 ((user->prev & LOG_CONT) && !(msg->flags & LOG_PREFIX)))
                cont = '+';
 
-       len = sprintf(user->buf, "%u,%llu,%llu,%c;",
-                     (msg->facility << 3) | msg->level,
-                     user->seq, ts_usec, cont);
+       p += scnprintf(p, e - p, "%u,%llu,%llu,%c;",
+                      (msg->facility << 3) | msg->level,
+                      user->seq, ts_usec, cont);
        user->prev = msg->flags;
 
        /* escape non-printable characters */
@@ -636,11 +645,11 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
                unsigned char c = log_text(msg)[i];
 
                if (c < ' ' || c >= 127 || c == '\\')
-                       len += sprintf(user->buf + len, "\\x%02x", c);
+                       p += scnprintf(p, e - p, "\\x%02x", c);
                else
-                       user->buf[len++] = c;
+                       append_char(&p, e, c);
        }
-       user->buf[len++] = '\n';
+       append_char(&p, e, '\n');
 
        if (msg->dict_len) {
                bool line = true;
@@ -649,30 +658,31 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
                        unsigned char c = log_dict(msg)[i];
 
                        if (line) {
-                               user->buf[len++] = ' ';
+                               append_char(&p, e, ' ');
                                line = false;
                        }
 
                        if (c == '\0') {
-                               user->buf[len++] = '\n';
+                               append_char(&p, e, '\n');
                                line = true;
                                continue;
                        }
 
                        if (c < ' ' || c >= 127 || c == '\\') {
-                               len += sprintf(user->buf + len, "\\x%02x", c);
+                               p += scnprintf(p, e - p, "\\x%02x", c);
                                continue;
                        }
 
-                       user->buf[len++] = c;
+                       append_char(&p, e, c);
                }
-               user->buf[len++] = '\n';
+               append_char(&p, e, '\n');
        }
 
        user->idx = log_next(user->idx);
        user->seq++;
        raw_spin_unlock_irq(&logbuf_lock);
 
+       len = p - user->buf;
        if (len > count) {
                ret = -EINVAL;
                goto out;