CVE-2026-74583

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.

Predictions

Heuristic predictions, AS-IS, for prioritization only.

Mitigations

References

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