Race condition access to n_hdlc.tbuf causes double free in n_hdlc_release()
CVE: CVE-2017-2636
Component: kernel
CWE: CWE-362
Severity: Important
CVSS v3: 7.9/CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Release date: 2017-03-07T00:00:00+00:00

A race condition flaw was found in the N_HLDC Linux kernel driver when accessing n_hdlc.tbuf list that can lead to double free. A local, unprivileged user able to set the HDLC line discipline on the tty device could use this flaw to increase their privileges on the system.

Statement: This issue does not affect the Linux kernel packages as shipped with Red Hat Enterprise Linux 5.

This issue does affect the Linux kernel packages as shipped with Red Hat Enterprise Linux 6, 7 and Red Hat Enterprise MRG 2. As this issue is rated as Important, it has been scheduled to be fixed in future updates for the respective releases.

Remediation:
For details on how to apply this update, which includes the changes described in this advisory, refer to:

https://access.redhat.com/articles/11258

The system must be rebooted for this update to take effect.
For details on how to apply this update, which includes the changes described in this advisory, refer to:

https://access.redhat.com/articles/11258

The system must be rebooted for this update to take effect.
For details on how to apply this update, which includes the changes described in this advisory, refer to:

https://access.redhat.com/articles/11258

The system must be rebooted for this update to take effect.
For details on how to apply this update, which includes the changes described in this advisory, refer to:

https://access.redhat.com/articles/11258

The system must be rebooted for this update to take effect.
For details on how to apply this update, which includes the changes described in this advisory, refer to:

https://access.redhat.com/articles/11258

The system must be rebooted for this update to take effect.
For details on how to apply this update, which includes the changes described in this advisory, refer to:

https://access.redhat.com/articles/11258

The system must be rebooted for this update to take effect.
For details on how to apply this update, which includes the changes described in this advisory, refer to:

https://access.redhat.com/articles/11258

The system must be rebooted for this update to take effect.
For details on how to apply this update, which includes the changes
described in this advisory, refer to:

https://access.redhat.com/articles/11258

The system must be rebooted for this update to take effect.
For details on how to apply this update, which includes the changes described in this advisory, refer to:

https://access.redhat.com/articles/11258

The system must be rebooted for this update to take effect.
For details on how to apply this update, which includes the changes described in this advisory, refer to:

https://access.redhat.com/articles/11258

The system must be rebooted for this update to take effect.
The  n_hdlc kernel module will be automatically loaded when an application  attempts to use the HDLC line discipline from userspace.  This module  can be prevented from being loaded by using the system-wide modprobe  rules. The following command, run as root, will prevent accidental or  intentional loading of the module.  Red Hat Product Security believe  this method is a robust method to prevent accidental loading of the  module, even by privileged users.

​# echo "install n_hdlc /bin/true" >> /etc/modprobe.d/disable-n_hdlc.conf

The  system will need to be restarted if the n_hdlc modules are already  loaded.  In most circumstances, the n_hdlc kernel modules will be unable to be unloaded if in use and while any current process using this line  discipline is required.

Exploiting this flaw does not require Microgate or SyncLink hardware to be in use.

If further assistance is needed, see this KCS article ( https://access.redhat.com/solutions/41278 ) or contact Red Hat Global Support Services.