A linux security update has been released for Debian GNU/Linux 9 LTS to address several vulnerabilities.

DLA 2713-2: linux security update

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Debian LTS Advisory DLA-2713-2 debian-lts@lists.debian.org
  https://www.debian.org/lts/security/ Ben Hutchings
July 20, 2021   https://wiki.debian.org/LTS
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Package : linux
Version : 4.9.272-2
CVE ID : CVE-2021-3609 CVE-2021-21781 CVE-2021-33909 CVE-2021-34693
Debian Bug : 990072

Several vulnerabilities have been discovered in the Linux kernel that
may lead to a privilege escalation, denial of service or information
leaks.

This updated advisory text fills in information omitted in the
original advisory.

CVE-2021-3609

Norbert Slusarek reported a race condition vulnerability in the CAN
BCM networking protocol, allowing a local attacker to escalate
privileges.

CVE-2021-21781

"Lilith >_>" of Cisco Talos discovered that the Arm initialisation
code does not fully initialise the "sigpage" that is mapped into
user-space processes to support signal handling. This could
result in leaking sensitive information, particularly when the
system is rebooted.

CVE-2021-33909

The Qualys Research Labs discovered a size_t-to-int conversion
vulnerability in the Linux kernel's filesystem layer. An
unprivileged local attacker able to create, mount, and then delete a
deep directory structure whose total path length exceeds 1GB, can
take advantage of this flaw for privilege escalation.

Details can be found in the Qualys advisory at
  https://www.qualys.com/2021/07/20/cve-2021-33909/sequoia-local-privilege-escalation-linux.txt

CVE-2021-34693

Norbert Slusarek discovered an information leak in the CAN BCM
networking protocol. A local attacker can take advantage of this
flaw to obtain sensitive information from kernel stack memory.

For Debian 9 stretch, these problems have been fixed in version
4.9.272-2. This additionally fixes a regression in the previous
update (#990072) that affected LXC.

We recommend that you upgrade your linux packages.

For the detailed security status of linux please refer to
its security tracker page at:
  https://security-tracker.debian.org/tracker/linux

Further information about Debian LTS security advisories, how to apply
these updates to your system and frequently asked questions can be
found at:   https://wiki.debian.org/LTS

Ben Hutchings - Debian developer, member of kernel, installer and LTS teams