CVE-2026-98103

Summary

In the Linux kernel, the following vulnerability has been resolved:

igmp: convert struct ip_sf_list to RCU

Commit 23d2b94043ca ("igmp: Add ip_mc_list lock in ip_check_mc_rcu") added spin_lock_bh(&im->lock) to ip_check_mc_rcu() to prevent a use-after-free while iterating im->sources during concurrent deletions.

However, ip_check_mc_rcu() is called from RCU read-side critical sections in packet receive and route lookup fast paths (e.g. __mkroute_output(), ip_route_input_rcu(), and __udp4_lib_rcv()).

When igmpv3_send_cr() or igmpv3_send_report() holds &pmc->lock and calls add_grec() -> igmpv3_newpack() -> ip_route_output_ports(), an XFRM policy matching a multicast destination triggers xfrm_tmpl_resolve_one() -> xfrm4_get_saddr() -> __mkroute_output() -> ip_check_mc_rcu(). This attempts to acquire &im->lock while &pmc->lock is already held on the same CPU, triggering a lockdep recursive locking warning / deadlock.

Fix this by converting IPv4 struct ip_sf_list to RCU, mirroring the IPv6 implementation in net/ipv6/mcast.c:

1. Add struct rcu_head to struct ip_sf_list and annotate sf_next, sources, and tomb as __rcu pointers.
2. Use rcu_assign_pointer() and kfree_rcu() for list updates and deletions.
3. Remove spin_lock_bh(&im->lock) from ip_check_mc_rcu() and traverse im->sources locklessly with for_each_psf_rcu(), reading and writing counter fields with READ_ONCE() and WRITE_ONCE().

Note: RCU conversion of /proc/net/mcfilter will be done in a separate patch.

Affected Software

Weaknesses

References

• https://git.kernel.org/stable/c/f5182dfad54277267a8cb0c004a9cd4cf35aeebb
• https://git.kernel.org/stable/c/d3011d1f293478c730aa0870490afa145c37600e
• https://git.kernel.org/stable/c/e3206419bdb04e2087d8cc6be324df8639b3fac5
• https://git.kernel.org/stable/c/2987ee196c88dbde0463dc87d5fb209c684e34a2

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