CVE-2026-31680

Description

In the Linux kernel, the following vulnerability has been resolved: net: ipv6: flowlabel: defer exclusive option free until RCU teardown `ip6fl_seq_show()` walks the global flowlabel hash under the seq-file RCU read-side lock and prints `fl->opt->opt_nflen` when an option block is present. Exclusive flowlabels currently free `fl->opt` as soon as `fl->users` drops to zero in `fl_release()`. However, the surrounding `struct ip6_flowlabel` remains visible in the global hash table until later garbage collection removes it and `fl_free_rcu()` finally tears it down. A concurrent `/proc/net/ip6_flowlabel` reader can therefore race that early `kfree()` and dereference freed option state, triggering a crash in `ip6fl_seq_show()`. Fix this by keeping `fl->opt` alive until `fl_free_rcu()`. That matches the lifetime already required for the enclosing flowlabel while readers can still reach it under RCU.

References

• https://git.kernel.org/stable/c/3c54b66c83fb8fcbde8e6a7bf90b65856e39f827
• https://git.kernel.org/stable/c/414726b69921fe6355ae453f5b35e68dd078342a
• https://git.kernel.org/stable/c/4b6798024f7b2d535f3db1002c760143cdbd1bd3
• https://git.kernel.org/stable/c/572ce62778519a7d4d1c15f55dd2e45a474133c4
• https://git.kernel.org/stable/c/5a6b15f861b7c1304949e3350d23490a5fe429fd