From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 4961151C348; Wed, 30 Sep 2026 18:25:04 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790792705; cv=none; b=MW8o0+boAuU/vxq3cZEAOXFPz2O9ywVpQ87IBUmif3mZW1UjihCjTU3VRzztGSfxYyI0USumD8Qxkqedji3puvwsYUiHVCaXvnZDkSKr6Jg9HwXVEb3+suUaA8N7ypLuHVSh7ANL1nEkMx4wnO7qh9nyj+sllkeB37e6Ge8m6qg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790792705; c=relaxed/simple; bh=YneuByqO1KVlHO1saCs3e5Ys3CfDlQehbtR/UXOopc4=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=g7fgOuizs4dAbzCTn64FtDSKRdLfyga+a/0cSkTai3NZxzOXUt4UbGORu/X3vBSnWdKWd2v16xNCIVCGlUGN0x78/zlcLD2iHeaxqFMaPqpANRhouOAaLJdr6P5jCmF6WDF8JjVDNfouNZqh6A2iCD5+rMx04zEUpLvzlW2ZGFI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=RunheB91; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="RunheB91" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 98C0D1F000FF; Wed, 30 Sep 2026 18:25:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1790792704; bh=ldunblDlFl0SPn+wIVDQEtSXI04Fb/7ybd9jfR5Awyg=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=RunheB91VUdyBdvntzgXE8+IJQ68PBc5VEbDvOh9j6P07v9EsBLzRHGPYxA/OYi0/ FZSQvWU64XiksyDGbXZ6ehN4uGuLvddxc0/Et63o8CWwMifhLPYkdkTu1pbhgE3HZq KAstoBInm4vaRbeHRK1a56JaVqP+UzUroPyinCdA= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Jaeyoung Chung , "Christian Brauner (Amutable)" , Wentao Guan , Lee Jones , Sasha Levin Subject: [PATCH 5.15 747/752] eventpoll: fix ep_remove struct eventpoll / struct file UAF Date: Wed, 30 Sep 2026 17:30:17 +0200 Message-ID: <20260930152414.379605975@linuxfoundation.org> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260930152358.131179731@linuxfoundation.org> References: <20260930152358.131179731@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 5.15-stable review patch. If anyone has any objections, please let me know. ------------------ From: Christian Brauner [ 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 Link: https://patch.msgid.link/20260423-work-epoll-uaf-v1-6-2470f9eec0f5@kernel.org Signed-off-by: Christian Brauner (Amutable) (cherry picked from commit a6dc643c69311677c574a0f17a3f4d66a5f3744b) Signed-off-by: Wentao Guan Signed-off-by: Greg Kroah-Hartman (cherry picked from commit 3e1144d2515d28e4312e663ea05eac203101491d) Signed-off-by: Lee Jones Signed-off-by: Sasha Levin --- fs/eventpoll.c | 16 ++++++++++------ 1 file changed, 10 insertions(+), 6 deletions(-) diff --git a/fs/eventpoll.c b/fs/eventpoll.c index 95d31d23db60b..50df3717f908e 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -794,22 +794,26 @@ static bool ep_remove_epi(struct eventpoll *ep, struct epitem *epi) */ static void ep_remove(struct eventpoll *ep, struct epitem *epi) { - struct file *file = epi->ffd.file; + struct file *file __free(fput) = 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)) -- 2.53.0