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 5D0F93A0E88; Tue, 8 Sep 2026 08:51:54 +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=1788857526; cv=none; b=gXBiBfgQstVIMlSGQJ+wxa/DJztN3VLeWZh6NUHwJ1rrhhDv8iT+JG6bqbE3NO2KjDXcz4b7IJ2vvC/YUMl+A+DgMZK//UEtBpJ3UjOYSjzlc4scOedkwJP44j+RgEkMdTQWEZPAG26S52Ct7ZHtB3UwYtwjHpSEHGhkrYIUBPI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788857526; c=relaxed/simple; bh=FazzNElmKFJLJo9AS3ULdfrFqkYreq4rHAvXggc7hgM=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=fnpZnWtKhDVwhBdB+/pG17/xIcdycEHBPtclUq4gUvlKnGxKUCUCGjK/m9l0Ijn0ehCfrSoSSy1Bi/EMWqta0r+JtN8FOfCEiclk+m/yhHyOckCaSyFpkRbC5V0SCNLmnjqSRUJLg/Vh9JC+Yk4zK8FjaBzaHzq/u7YvZ/PHsOg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=G13qovW2; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="G13qovW2" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 98DC81F00A3A; Tue, 8 Sep 2026 08:51:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788857512; bh=p3/uSeVxqiNU4n6pXcRtL4khCqdhtpTAmxXfeDkLjAU=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=G13qovW2Wfab5fm08euTsG4N5ouC4SzLiJ+LfI10d/BJ0WCPVp9WwxBJai8xzgpbU tjslxJWf0Wdminq2K7d7RKciu+2MTNZzD/CRkruJj5MTUiwlCMpUFl2D9BpbX+MO3P AM/RX/OXOstkvb5igHvLrG5V7HARBAHyZ+KIgLvAy0+xNF+vwBQZV9ZDOGIYAJ09IK I9WGsVF0Fbf5t/OllCcztkdjYQSH4X2hxITdRNSloZp9PfTPEkxIv2PlvnUNJbsmCa hqtbG9J2ARya3fq6MPogE41+QJKAraNbRVktRxmGTmnVnYIce65Zq8Wc5Hi59rifC3 +l3IaMSIAockg== From: Lee Jones To: lee@kernel.org, Alexander Viro , Greg Kroah-Hartman , Peter Zijlstra , Sasha Levin , Wentao Guan , Christian Brauner , Soheil Hassas Yeganeh , Eric Dumazet , Davidlohr Bueso , Paolo Abeni , Andrew Morton , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Cc: stable@vger.kernel.org, Jaeyoung Chung Subject: [STABLE v5.15.y 8/8] eventpoll: fix ep_remove struct eventpoll / struct file UAF Date: Tue, 8 Sep 2026 09:51:04 +0100 Message-ID: <20260908085113.3960814-8-lee@kernel.org> X-Mailer: git-send-email 2.55.0.979.g7e5102b832-goog In-Reply-To: <20260908085113.3960814-1-lee@kernel.org> References: <20260908085113.3960814-1-lee@kernel.org> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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 --- fs/eventpoll.c | 16 ++++++++++------ 1 file changed, 10 insertions(+), 6 deletions(-) diff --git a/fs/eventpoll.c b/fs/eventpoll.c index 88a7a8ead9cd..864959c045d1 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)) -- 2.55.0.979.g7e5102b832-goog