Deep-dive writeup on a Linux kernel use-after-free in “fs/eventpoll.c”. A 2023 optimisation traded a global “epmutex” for per-instance reference counting in epoll’s graph-walking code, but left the walkers running under “rcu_read_lock()” while “ep_free()” kept calling plain “kfree(ep)” with no RCU deferral — opening a same-CPU preemption race that yields a constrained write through a freed “struct eventpoll”. Fixed in commit “07712db80857″by switching to “kfree_rcu(ep, rcu)”. Affects Linux 6.6+ including Android (Pixel 10 tested).
