[ 02/42] TTY: do not update atime/mtime on read/write

From: Greg Kroah-Hartman
Date: Mon Apr 29 2013 - 15:44:37 EST

3.8-stable 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>

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
@@ -977,8 +977,7 @@ static ssize_t tty_read(struct file *fil
i = -EIO;
- 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(
- if (written) {
- struct inode *inode = file->f_path.dentry->d_inode;
- inode->i_mtime = current_fs_time(inode->i_sb);
+ if (written)
ret = written;
- }
return ret;

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