TTY: do not update atime/mtime on read/write

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@suse.cz>
Cc: stable <stable@vger.kernel.org> # after 3.9 is out
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 2 insertions, 6 deletions

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 54a254ab85c7..8f44d627ccea 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -977,8 +977,7 @@ static ssize_t tty_read(struct file *file, char __user *buf, size_t count,
         else
                 i = -EIO;
         tty_ldisc_deref(ld);
-        if (i > 0)
-                inode->i_atime = current_fs_time(inode->i_sb);
+
         return i;
 }
 
@@ -1079,11 +1078,8 @@ 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;