CVE-2017-2636
Description
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.
Mitigation
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.
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 7.8 | 7 | N/A |
| Attack Vector | Local | Local | N/A |
| Attack Complexity | Low | High | N/A |
| Privileges Required | Low | Low | N/A |
| User Interaction | None | None | N/A |
| Scope | Unchanged | Unchanged | N/A |
| Confidentiality | High | High | N/A |
| Integrity Impact | High | High | N/A |
| Availability Impact | High | High | N/A |
Vector
Red Hat: CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
NVD: CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
Acknowledgements
Red Hat would like to thank Alexander Popov for reporting this issue.
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