TTY: ttyprintk, don't touch behind tty->write_buf

commit ee8b593 upstream.

If a user provides a buffer larger than a tty->write_buf chunk and
passes '\r' at the end of the buffer, we touch an out-of-bound memory.

Add a check there to prevent this.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Samo Pogacnik <samo_pogacnik@t-2.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>

drivers/char/ttyprintk.c

@@ -67,7 +67,7 @@ static int tpk_printk(const unsigned char *buf, int count)
  tmp[tpk_curr + 1] = '\0';
  printk(KERN_INFO "%s%s", tpk_tag, tmp);
  tpk_curr = 0;
- if (buf[i + 1] == '\n')
+ if ((i + 1) < count && buf[i + 1] == '\n')
  i++;
  break;
  case '\n':