TTY: fix atime/mtime regression

In commit b0de59b5 ("TTY: do not update atime/mtime on read/write")
we removed timestamps from tty inodes to fix a security issue and waited
if something breaks.  Well, 'w', the utility to find out logged users
and their inactivity time broke.  It shows that users are inactive since
the time they logged in.

To revert to the old behaviour while still preventing attackers to
guess the password length, we update the timestamps in one-minute
intervals by this patch.
Signed-off-by: NJiri Slaby <jslaby@suse.cz>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/tty/tty_io.c

@@ -941,6 +941,14 @@ void start_tty(struct tty_struct *tty)
 
 EXPORT_SYMBOL(start_tty);
 
+static void tty_update_time(struct timespec *time)
+{
+	unsigned long sec = get_seconds();
+	sec -= sec % 60;
+	if ((long)(sec - time->tv_sec) > 0)
+		time->tv_sec = sec;
+}
+
 /**
  *	tty_read	-	read method for tty device files
  *	@file: pointer to tty file
@@ -960,10 +968,11 @@ static ssize_t tty_read(struct file *file, char __user *buf, size_t count,
 			loff_t *ppos)
 {
 	int i;
+	struct inode *inode = file_inode(file);
 	struct tty_struct *tty = file_tty(file);
 	struct tty_ldisc *ld;
 
-	if (tty_paranoia_check(tty, file_inode(file), "tty_read"))
+	if (tty_paranoia_check(tty, inode, "tty_read"))
 		return -EIO;
 	if (!tty || (test_bit(TTY_IO_ERROR, &tty->flags)))
 		return -EIO;
@@ -977,6 +986,9 @@ static ssize_t tty_read(struct file *file, char __user *buf, size_t count,
 		i = -EIO;
 	tty_ldisc_deref(ld);
 
+	if (i > 0)
+		tty_update_time(&inode->i_atime);
+
 	return i;
 }
 
@@ -1077,8 +1089,10 @@ static inline ssize_t do_tty_write(
 			break;
 		cond_resched();
 	}
-	if (written)
+	if (written) {
+		tty_update_time(&file_inode(file)->i_mtime);
 		ret = written;
+	}
 out:
 	tty_write_unlock(tty);
 	return ret;