High [CVE-2026-74583] fix fastmap use-after-free on filter
This high-severity Red Hat Linux advisory covers CVE-2026-74583 affecting Red Hat Enterprise Linux 7, Red Hat Enterprise Linux 8, Red Hat package: kernel-rt.
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Summary
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.
Affected versions
No affected-version range was extracted from the source record. The vendor advisory is authoritative — check it before change work.
Official advisory · high-confidence parse· fetched 13 days ago·verify at source
Fixed versions
No fixed release is recorded yet. That does not prove no patch exists — confirm against the vendor advisory.
Official advisory · high-confidence parse· fetched 13 days ago·verify at source
Mitigation checklist
- To mitigate this vulnerability, prevent the `cls_route` kernel module from loading. This can be achieved by creating a modprobe configuration file. 1. Create or edit `/etc/modprobe.d/disable-cls_route.conf` and add the following line: `install cls_route /bin/true` 2. Regenerate the initramfs to ensure the change takes effect on boot: `dracut -f -v` 3. Reboot the system for the changes to be applied. Note that disabling this module may impact network traffic classification functionality if it is actively used.
Official advisory · high-confidence parse· fetched 13 days ago·verify at source
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