AZL-89159

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

eventpoll: fix ep_remove struct eventpoll / struct file UAF

epremove() (via epremovefile()) cleared file->fep under file->flock but then kept using @file inside the critical section (isfileepoll(), hlistdelrcu() through the head, spinunlock). A concurrent _fput() taking the eventpollrelease() fastpath in that window observed the transient NULL, skipped eventpollreleasefile() and ran to fop->release / filefree().

For the epoll-watches-epoll case, fop->release is epeventpollrelease() -> epclearandput() -> epfree(), which kfree()s the watched struct eventpoll. Its embedded ->refs hlisthead is exactly where epi->fllink.pprev points, so the subsequent hlistdelrcu()'s "*pprev = next" scribbles into freed kmalloc-192 memory.

In addition, struct file is SLABTYPESAFEBYRCU, so the slot backing @file could be recycled by allocemptyfile() -- reinitializing flock and fep -- while epremove() is still nominally inside that lock. The upshot is an attacker-controllable kmemcachefree() against the wrong slab cache.

Pin @file via epifget() at the top of epremove() and gate the critical section on the pin succeeding. With the pin held @file cannot reach refcount zero, which holds _fput() off and transitively keeps the watched struct eventpoll alive across the hlistdelrcu() and the flock use, closing both UAFs.

If the pin fails @file has already reached refcount zero and its __fput() is in flight. Because we bailed before clearing fep, that path takes the eventpollrelease() slow path into eventpollreleasefile() and blocks on ep->mtx until the waiter side's epclearandput() drops it. The bailed epi's share of ep->refcount stays intact, so the trailing eprefcountdecandtest() in epclearandput() cannot free the eventpoll out from under eventpollreleasefile(); the orphaned epi is then cleaned up there.

A successful pin also proves we are not racing eventpollreleasefile() on this epi, so drop the now-redundant re-check of epi->dying under flock. The cheap lockless READONCE(epi->dying) fast-path bailout stays.