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

This patchset updates netconsole so that it can emit messages with the
same header as used in /dev/kmsg which gives neconsole receiver full log
information which enables things like structured logging and detection
of lost messages.

This patch (of 7):

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.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: David Miller <davem@davemloft.net>
Cc: Kay Sievers <kay@vrfy.org>
Reviewed-by: Petr Mladek <pmladek@suse.cz>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 22 insertions, 12 deletions

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index c099b082cd02..a11549034e07 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -505,6 +505,11 @@ int check_syslog_permissions(int type, bool from_file)
         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 {
@@ -512,7 +517,7 @@ 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)
@@ -570,6 +575,7 @@ 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 = '-';
@@ -579,6 +585,9 @@ 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;
@@ -625,9 +634,9 @@ 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 */
@@ -635,11 +644,11 @@ 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;
@@ -648,30 +657,31 @@ 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;