Issues have been identified in the way the Linux kernel’s TCP implementation processes Selective Acknowledgement (SACK) options and handles low Maximum Segment Size (MSS) values. These TCP SACK Panic vulnerabilities could expose servers to a denial of service attack, so it is crucial to have systems patched.
Updated versions of the Linux kernel packages are being published as part of the standard Ubuntu security maintenance of Ubuntu releases 16.04 LTS, 18.04 LTS, 18.10, 19.04 and as part of the extended security maintenance for Ubuntu 14.04 ESM users.
It is recommended to update to the latest kernel packages and consult Ubuntu Security Notices for further updates.
Ubuntu Advantage for Infrastructure subscription customers can find the latest status information in our Knowledge Base and file a support case with Canonical support for any additional questions or concerns around SACK Panic.
Canonical’s Kernel Livepatch updates for security vulnerabilities related to TCP SACK processing in the Linux kernel have been released and are described by CVEs 2019-11477 and 2019-11478, with details of the patch available in LSN-0052-1.
These CVEs have a Livepatch fix available, however, a minimum kernel version is required for Livepatch to install the fix as denoted by the table in LSN-0052-1, reproduced here:
| Kernel | Version | flavors | |--------------------------+----------+--------------------------| | 4.4.0-148.174 | 52.3 | generic, lowlatency | | 4.4.0-150.176 | 52.3 | generic, lowlatency | | 4.15.0-50.54 | 52.3 | generic, lowlatency | | 4.15.0-50.54~16.04.1 | 52.3 | generic, lowlatency | | 4.15.0-51.55 | 52.3 | generic, lowlatency | | 4.15.0-51.55~16.04.1 | 52.3 | generic, lowlatency |
Livepatch fixes for CVEs 2019-11477 and 2019-11478 are not available for prior kernels, and an upgrade and reboot to the appropriate minimum version is necessary. These kernel versions correspond to the availability of mitigations for the MDS series of CVEs (CVE-2018-12126, CVE-2018-12127, CVE-2018-12130 and CVE-2019-11091).
Additionally, a third SACK related issue, CVE-2019-11479, does not have a Livepatch fix available because it is not technically feasible to apply the changes via Livepatch. Mitigation information is available at the Ubuntu Security Team Wiki.
If you have any questions and want to learn more about these patches, please do not hesitate to get in touch.