CVE-2021-46990: powerpc/64s: Fix crashes when toggling entry flush barrier

Severity: medium (CVSS 5.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2024-02-28 · updated 2026-06-17

Description

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.

Affected products

Linux kernel: from 4.4.245 before 4.4.269; from 4.9.245 before 4.9.269; from 4.14.208 before 4.14.233; from 4.19.159 before 4.19.191; from 5.4.79 before 5.4.120; from 5.10 before 5.10.38 — fixed in 4.14.233, 4.19.191, 4.4.269, 4.9.269, 5.10.38, 5.11.22

How to fix

Upgrade to a fixed version: Linux kernel 4.14.233, 4.19.191, 4.4.269, 4.9.269. Check the vendor advisory for mitigations.

References

Get alerted when CVE-2021-46990 or the next vulnerability affects your stack