Medium [CVE-2026-72321] Fix potential memory leaks in igmp_mod_timer and igmp_stop_timer

In the Linux kernel, the following vulnerability has been resolved: ipv4: igmp: Fix potential memory leaks in igmp_mod_timer() and igmp_stop_timer() When a timer is deleted and not re-armed in igmp_mod_timer(), or stopped in igmp_stop_timer(), the code currently decrements the reference counter of the multicast list entry @im using refcount_dec(&im->refcnt).

However, both functions can be called from the RCU reader path: - igmp_mod_timer() via igmp_heard_query() -> for_each_pmc_rcu() - igmp_stop_timer() via igmp_rcv() -> igmp_heard_report() If the group im was concurrently removed from the list by ip_mc_dec_group(), its reference count might have already been decremented to 1.

In this case, timer_delete() succeeds, and refcount_dec() decrements the refcount from 1 to 0. Since refcount_dec() does not free the object when it hits 0 (unlike ip_ma_put()), the im structure is leaked.

Fix this by using ip_ma_put(im) instead of refcount_dec(&im->refcnt), and deferring the put until after the spinlock is released. A flaw was found in the Linux kernel's IPv4 Internet Group Management Protocol (IGMP) component.

Incorrect handling of reference counts in the `igmp_mod_timer()` and `igmp_stop_timer()` functions can occur when a multicast list entry is concurrently removed. This can lead to a memory leak of the `im` structure, potentially impacting system stability.