[074/118] TTY: do not update atime/mtime on read/write

[Ben Hutchings]

3.2.45-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Jiri Slaby <jslaby@xxxxxxx>

commit b0de59b5733d18b0d1974a060860a8b5c1b36a2e upstream.

On http://vladz.devzero.fr/013_ptmx-timing.php, we can see how to find
out length of a password using timestamps of /dev/ptmx. It is
documented in "Timing Analysis of Keystrokes and Timing Attacks on
SSH". To avoid that problem, do not update time when reading
from/writing to a TTY.

I am afraid of regressions as this is a behavior we have since 0.97
and apps may expect the time to be current, e.g. for monitoring
whether there was a change on the TTY. Now, there is no change. So
this would better have a lot of testing before it goes upstream.

References: CVE-2013-0160

Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 drivers/tty/tty_io.c |    8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -976,8 +976,7 @@ static ssize_t tty_read(struct file *fil
 	else
 		i = -EIO;
 	tty_ldisc_deref(ld);
-	if (i > 0)
-		inode->i_atime = current_fs_time(inode->i_sb);
+
 	return i;
 }
 
@@ -1078,11 +1077,8 @@ static inline ssize_t do_tty_write(
 			break;
 		cond_resched();
 	}
-	if (written) {
-		struct inode *inode = file->f_path.dentry->d_inode;
-		inode->i_mtime = current_fs_time(inode->i_sb);
+	if (written)
 		ret = written;
-	}
 out:
 	tty_write_unlock(tty);
 	return ret;

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