From: Christian Brauner <brauner@kernel.org> stable inclusion from stable-v6.18.33 commit ef4ca02e95363e78977ca04340d44fe3b4b2b81f category: bugfix bugzilla: https://atomgit.com/src-openeuler/kernel/issues/15497 CVE: CVE-2026-46242 Reference: https://web.git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/... -------------------------------- [ Upstream commit a6dc643c69311677c574a0f17a3f4d66a5f3744b ] ep_remove() (via ep_remove_file()) cleared file->f_ep under file->f_lock but then kept using @file inside the critical section (is_file_epoll(), hlist_del_rcu() through the head, spin_unlock). A concurrent __fput() taking the eventpoll_release() fastpath in that window observed the transient NULL, skipped eventpoll_release_file() and ran to f_op->release / file_free(). For the epoll-watches-epoll case, f_op->release is ep_eventpoll_release() -> ep_clear_and_put() -> ep_free(), which kfree()s the watched struct eventpoll. Its embedded ->refs hlist_head is exactly where epi->fllink.pprev points, so the subsequent hlist_del_rcu()'s "*pprev = next" scribbles into freed kmalloc-192 memory. In addition, struct file is SLAB_TYPESAFE_BY_RCU, so the slot backing @file could be recycled by alloc_empty_file() -- reinitializing f_lock and f_ep -- while ep_remove() is still nominally inside that lock. The upshot is an attacker-controllable kmem_cache_free() against the wrong slab cache. Pin @file via epi_fget() at the top of ep_remove() 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 hlist_del_rcu() and the f_lock 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 f_ep, that path takes the eventpoll_release() slow path into eventpoll_release_file() and blocks on ep->mtx until the waiter side's ep_clear_and_put() drops it. The bailed epi's share of ep->refcount stays intact, so the trailing ep_refcount_dec_and_test() in ep_clear_and_put() cannot free the eventpoll out from under eventpoll_release_file(); the orphaned epi is then cleaned up there. A successful pin also proves we are not racing eventpoll_release_file() on this epi, so drop the now-redundant re-check of epi->dying under f_lock. The cheap lockless READ_ONCE(epi->dying) fast-path bailout stays. Fixes: 58c9b016e128 ("epoll: use refcount to reduce ep_mutex contention") Reported-by: Jaeyoung Chung <jjy600901@snu.ac.kr> Link: https://patch.msgid.link/20260423-work-epoll-uaf-v1-6-2470f9eec0f5@kernel.or... Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org> Signed-off-by: Sasha Levin <sashal@kernel.org> Conflicts: fs/eventpoll.c [Cleanup argument(fput) not a function on file. Call fput() manually.] Signed-off-by: Zizhi Wo <wozizhi@huawei.com> --- fs/eventpoll.c | 18 ++++++++++++------ 1 file changed, 12 insertions(+), 6 deletions(-) diff --git a/fs/eventpoll.c b/fs/eventpoll.c index ded92b40fba6..fa63e96020da 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -799,30 +799,36 @@ static bool ep_remove_epi(struct eventpoll *ep, struct epitem *epi) /* * ep_remove variant for callers owing an additional reference to the ep */ static void ep_remove_safe(struct eventpoll *ep, struct epitem *epi) { - struct file *file = epi->ffd.file; + struct file *file = NULL; lockdep_assert_irqs_enabled(); lockdep_assert_held(&ep->mtx); ep_unregister_pollwait(ep, epi); - /* sync with eventpoll_release_file() */ + /* cheap sync with eventpoll_release_file() */ if (unlikely(READ_ONCE(epi->dying))) return; - spin_lock(&file->f_lock); - if (epi->dying) { - spin_unlock(&file->f_lock); + /* + * If we manage to grab a reference it means we're not in + * eventpoll_release_file() and aren't going to be. + */ + file = epi_fget(epi); + if (!file) return; - } + + spin_lock(&file->f_lock); ep_remove_file(ep, epi, file); if (ep_remove_epi(ep, epi)) WARN_ON_ONCE(ep_refcount_dec_and_test(ep)); + + fput(file); } static void ep_clear_and_put(struct eventpoll *ep) { struct rb_node *rbp, *next; -- 2.52.0