From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 09B2F305692 for ; Fri, 15 May 2026 15:40:17 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778859618; cv=none; b=LenA+FwR18v1Y+oqWRvzJ1GgystinBWSdezpGvsO0aKoOCjSI7/W7iNMjdufCoM13RhaLLIUmSRhnvIbiSS/+F1pvsJIrhrl2SPvdBRQmuVn/lKttnCWWzH2ox4JtGz+i51q3uTxleAWkHMTqefB6HKcHFyhGaso8LrxvrbMv8Q= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778859618; c=relaxed/simple; bh=1WjY5S6xF/i5n6aVgHlX1w5PBzrviKCWlxkeSLrSlI4=; h=Subject:To:Cc:From:Date:Message-ID:MIME-Version:Content-Type; b=tG3FpEy9TxPRpd3cdkdB9XFXCa7H2BHlfQxwM2UyzF/JNa4KJulN+KzuDFqnGBTZ/Am1pa9H5eSjL/YAzxB0eY3SJIhyhgjduIKFkN4Uvo9pCKjLV2MU1ewwezavqG4cGWnRIENY7j0SBfmEogEa6ybPC2bmrKMKospCbXrIz5M= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=elV5hEmN; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="elV5hEmN" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 78290C2BCB0; Fri, 15 May 2026 15:40:17 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1778859617; bh=1WjY5S6xF/i5n6aVgHlX1w5PBzrviKCWlxkeSLrSlI4=; h=Subject:To:Cc:From:Date:From; b=elV5hEmNjqNZ+lx6V++p+S7plStKH8PC5ovfnJYP4OBqrvfmxRC4ht3Gt6Qxhw6kO QtKAz9qoqt2eEtXft//fQWQ1UsK7XwTViyZp0FePcry/W4VRSSHvl7ONvySCh9HAoy o1XVudvfupyRBvX/bRVt6qG3QgS9Fs5LYX9G4Amc= Subject: FAILED: patch "[PATCH] eventpoll: fix ep_remove struct eventpoll / struct file UAF" failed to apply to 6.1-stable tree To: brauner@kernel.org,jjy600901@snu.ac.kr Cc: From: Date: Fri, 15 May 2026 17:40:22 +0200 Message-ID: <2026051522-phobia-rebate-b7c8@gregkh> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=ANSI_X3.4-1968 Content-Transfer-Encoding: 8bit The patch below does not apply to the 6.1-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to . To reproduce the conflict and resubmit, you may use the following commands: git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y git checkout FETCH_HEAD git cherry-pick -x a6dc643c69311677c574a0f17a3f4d66a5f3744b # git commit -s git send-email --to '' --in-reply-to '2026051522-phobia-rebate-b7c8@gregkh' --subject-prefix 'PATCH 6.1.y' HEAD^.. Possible dependencies: thanks, greg k-h ------------------ original commit in Linus's tree ------------------ >From a6dc643c69311677c574a0f17a3f4d66a5f3744b Mon Sep 17 00:00:00 2001 From: Christian Brauner Date: Thu, 23 Apr 2026 11:56:09 +0200 Subject: [PATCH] eventpoll: fix ep_remove struct eventpoll / struct file UAF 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) diff --git a/fs/eventpoll.c b/fs/eventpoll.c index 5ee4398a6cb8..0f785c0a1544 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -912,22 +912,26 @@ 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))