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Fencer Vulns
medium 5.5

CVE-2021-46990 json

powerpc/64s: Fix crashes when toggling entry flush barrier

published 2024-02-28 2 y ago modified 2026-06-17 4 mo ago also EUVD-2021-33638

cvss
5.5
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
0.2%
EPSS 0.2% chance of exploitation in 30 days, percentile 14%
public exploits
0
none known

Summary

In the Linux kernel, the following vulnerability has been resolved:

powerpc/64s: Fix crashes when toggling entry flush barrier

The entry flush mitigation can be enabled/disabled at runtime via a debugfs file (entry_flush), which causes the kernel to patch itself to enable/disable the relevant mitigations.

However depending on which mitigation we're using, it may not be safe to do that patching while other CPUs are active. For example the following crash:

sleeper[15639]: segfault (11) at c000000000004c20 nip c000000000004c20 lr c000000000004c20

Shows that we returned to userspace with a corrupted LR that points into the kernel, due to executing the partially patched call to the fallback entry flush (ie. we missed the LR restore).

Fix it by doing the patching under stop machine. The CPUs that aren't doing the patching will be spinning in the core of the stop machine logic. That is currently sufficient for our purposes, because none of the patching we do is to that code or anywhere in the vicinity.

Patch / fix

  • Linux Linux 4.4.269, 4.9.269, 4.14.233, 4.19.191, 5.4.120, 5.10, 5.10.38, 5.11.22, 5.12.5, 5.13
  • linux Debian 14, Debian 13, Debian 12 5.10.38-1

debian advisories DEBIAN-CVE-2021-46990

  • linux Ubuntu 18.04 LTS 4.15.0-151.157
  • linux Ubuntu 20.04 LTS 5.4.0-80.90
  • linux-aws Ubuntu 18.04 LTS 4.15.0-1109.116
  • linux-aws Ubuntu 20.04 LTS 5.4.0-1054.57
  • linux-gcp Ubuntu 20.04 LTS 5.4.0-1049.53
  • linux-kvm Ubuntu 18.04 LTS 4.15.0-1097.99
  • linux-kvm Ubuntu 20.04 LTS 5.4.0-1044.46
  • linux-azure Ubuntu 20.04 LTS 5.4.0-1055.57

ubuntu advisories UBUNTU-CVE-2021-46990

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • kernel-syms SLES for SAP 12 SP5, SLES 12 SP5 4.12.14-122.216.1
  • kernel-source SLES for SAP 12 SP5, SLES 12 SP5 4.12.14-122.216.1
  • kernel-default SLES for SAP 12 SP5, SLES 12 SP5, SLE Workstation Extension 12 SP5 +2 more 4.12.14-122.216.1
  • kernel-obs-build SLE Software Development Kit 12 SP5 4.12.14-122.216.1
  • kernel-rt SLE Real Time 12 SP5 4.12.14-10.182.1
  • kernel-syms-rt SLE Real Time 12 SP5 4.12.14-10.182.1
  • kernel-rt_debug SLE Real Time 12 SP5 4.12.14-10.182.1
  • kernel-source-rt SLE Real Time 12 SP5 4.12.14-10.182.1

suse advisories SUSE-SU-2024:1870-1SUSE-SU-2024:1646-1SUSE-SU-2024:1643-1SUSE-SU-2024:1489-1SUSE-SU-2024:1465-1SUSE-SU-2024:1454-1

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2021-46990 updated cvss 5.5

kernel: powerpc/64s: Fix crashes when toggling entry flush barrier

  • Red Hat Enterprise Linux 6 perf, kernel, python-perf, kernel-kdump, kernel-firmware, kernel-bootwrapper, +1 more Red Hat Enterprise Linux 7 perf, kernel, bpftool, kernel-rt, python-perf, kernel-kdump, +8 more
    affected: no fix planned “Out of support scope”
  • Red Hat Enterprise Linux 8 perf, kernel, bpftool, kernel-rt, kernel-core, kernel-tools, +29 more Red Hat Enterprise Linux 9 rv, perf, rtla, kernel, bpftool, libperf, +83 more
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
partial
Assessed
2 y ago

Exploit prediction

0.23% chance of exploitation in the next 30 days, higher than 14% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
NVD 3.1 5.5 CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Affected products

  • Linux Linux

Classification

Technology feeds

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.