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 51FE3433BCA; Tue, 21 Jul 2026 21:08:20 +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=1784668101; cv=none; b=WjYo6Vf1wW0AJTwNvnMyxjMwMVyRogeQW1xLenfc7F+IWHmVx4ZJnOI09KB/pbThRGaR+lGUgMj7zfVgx3xhLx57g8p3g7luPMMic64pSPcfyr7GdsO0p+owLcpTu3BTtGe3LJAQulx1g42mMmqhnb6bLmoCvLxgNoLRscnwznk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784668101; c=relaxed/simple; bh=YmY2gE+SFBn/OKEdvL1x3uy56DEaNvWtZ6WvhPYrgms=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=LrDBN7buAKkIsmjFSRcMhZI6UxsW7A7DbYfp/OicaC9HSl0DYzkdL0t6lPeWxfbPxZBT7Ej0isp53lqosHcbnX0FVb5w8BXCAx1kvhxluqDIjFhYroBdzxTIS+5ovKZhEIN+mlUREcJZW75IthBnD5yLZt753NSWu1BtbL6bwCM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=VLJi4pC9; 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="VLJi4pC9" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B7AAE1F000E9; Tue, 21 Jul 2026 21:08:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1784668100; bh=Xo0UqODDeblbTmVsKYUHZn610zYUgFcBQ2XNwOLfN/M=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=VLJi4pC9qyuYapyLmUadqQoGYqyy/v4243aTTlS9e7M0glXyw1gjysHeP78OZMDt6 3na1692qq/l7kOnJ2Dsf7B9SPy/4bTuSTrDjQJoUdid/ZZvvU9kewGHuWUKyN4Elrv E5mDbXoycg78QqN4lTsFwOvkGsR9UjcjATfMAK28= From: Greg Kroah-Hartman To: stable@vger.kernel.org, carnil@debian.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Jaeyoung Chung , "Christian Brauner (Amutable)" , Wentao Guan Subject: [PATCH 6.1 0046/1067] eventpoll: fix ep_remove struct eventpoll / struct file UAF Date: Tue, 21 Jul 2026 17:10:47 +0200 Message-ID: <20260721152425.590221009@linuxfoundation.org> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260721152424.521567757@linuxfoundation.org> References: <20260721152424.521567757@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 6.1-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 --- fs/eventpoll.c | 16 ++++++++++------ 1 file changed, 10 insertions(+), 6 deletions(-) --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -801,22 +801,26 @@ static bool ep_remove_epi(struct eventpo */ 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))