fs/proc/base.c in the Linux kernel before 2.6.39.4 does not properly restrict access to /proc/#####/io files, which allows local users to obtain sensitive I/O statistics by polling a file, as demonstrated by discovering the length of another user's password.
https://bugzilla.redhat.com/show_bug.cgi?id=716825
http://www.openwall.com/lists/oss-security/2011/06/27/1
http://rhn.redhat.com/errata/RHSA-2011-1212.html
http://ftp.osuosl.org/pub/linux/kernel/v2.6/ChangeLog-2.6.39.4