From patchwork Thu Nov 9 04:04:44 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: "Jason A. Donenfeld" X-Patchwork-Id: 836164 X-Patchwork-Delegate: davem@davemloft.net Return-Path: X-Original-To: patchwork-incoming@ozlabs.org Delivered-To: patchwork-incoming@ozlabs.org Authentication-Results: ozlabs.org; spf=none (mailfrom) smtp.mailfrom=vger.kernel.org (client-ip=209.132.180.67; helo=vger.kernel.org; envelope-from=netdev-owner@vger.kernel.org; receiver=) Authentication-Results: ozlabs.org; dkim=pass (2048-bit key; unprotected) header.d=zx2c4.com header.i=@zx2c4.com header.b="06edjsKv"; dkim-atps=neutral Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by ozlabs.org (Postfix) with ESMTP id 3yXV1v0Nq1z9t2V for ; Thu, 9 Nov 2017 15:05:23 +1100 (AEDT) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751886AbdKIEFL (ORCPT ); Wed, 8 Nov 2017 23:05:11 -0500 Received: from frisell.zx2c4.com ([192.95.5.64]:41681 "EHLO frisell.zx2c4.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751472AbdKIEFK (ORCPT ); Wed, 8 Nov 2017 23:05:10 -0500 Received: by frisell.zx2c4.com (ZX2C4 Mail Server) with ESMTP id 960701b5; Thu, 9 Nov 2017 04:01:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha1; c=relaxed; d=zx2c4.com; h=from:to:cc :subject:date:message-id:in-reply-to:references; s=mail; bh=d6xg LJDcoNu0mBYbrpz7xZqE8yc=; b=06edjsKvS+xgJhu5W/mqA0IF/WMVw/qNwOq1 IT6+6jB3YWzyPWWwt8B3KImaOjiWRl7H740UfzYxEcruejHDKuIr7gL2CepPGeZf 4OVeVglNy/5Q3DZ7mlGUCIbFqB2nLrU0EzGYqH3/I732QytnmzhH0PLP8f0edbjY g37+X/zL5IyS+bzUihDx4GEykWxCR9s/AuXjMx6ssd+8dU89WiWM/rwloKVUwYyr fRRL1pm3j71rK11moScODX9HvkO6GWby8EVUZ12F6ROdhT4DoP5cXSpr3eH5P+Fa YNBn2GzXSuPu67CWvaL8kayxJMuaCBoDFAiL2gWO0vI4PWMMrw== Received: by frisell.zx2c4.com (ZX2C4 Mail Server) with ESMTPSA id 447316fb (TLSv1.2:ECDHE-RSA-AES256-GCM-SHA384:256:NO); Thu, 9 Nov 2017 04:01:43 +0000 (UTC) From: "Jason A. Donenfeld" To: Johannes Berg , davem@davemloft.net, Netdev , linux-kernel@vger.kernel.org Cc: "Jason A. Donenfeld" Subject: [PATCH v4] af_netlink: ensure that NLMSG_DONE never fails in dumps Date: Thu, 9 Nov 2017 13:04:44 +0900 Message-Id: <20171109040444.1728-1-Jason@zx2c4.com> In-Reply-To: <20171109014218.20562-1-Jason@zx2c4.com> References: <20171109014218.20562-1-Jason@zx2c4.com> Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org The way people generally use netlink_dump is that they fill in the skb as much as possible, breaking when nla_put returns an error. Then, they get called again and start filling out the next skb, and again, and so forth. The mechanism at work here is the ability for the iterative dumping function to detect when the skb is filled up and not fill it past the brim, waiting for a fresh skb for the rest of the data. However, if the attributes are small and nicely packed, it is possible that a dump callback function successfully fills in attributes until the skb is of size 4080 (libmnl's default page-sized receive buffer size). The dump function completes, satisfied, and then, if it happens to be that this is actually the last skb, and no further ones are to be sent, then netlink_dump will add on the NLMSG_DONE part: nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE, sizeof(len), NLM_F_MULTI); It is very important that netlink_dump does this, of course. However, in this example, that call to nlmsg_put_answer will fail, because the previous filling by the dump function did not leave it enough room. And how could it possibly have done so? All of the nla_put variety of functions simply check to see if the skb has enough tailroom, independent of the context it is in. In order to keep the important assumptions of all netlink dump users, it is therefore important to give them an skb that has this end part of the tail already reserved, so that the call to nlmsg_put_answer does not fail. Otherwise, library authors are forced to find some bizarre sized receive buffer that has a large modulo relative to the common sizes of messages received, which is ugly and buggy. This patch thus saves the NLMSG_DONE for an additional message, for the case that things are dangerously close to the brim. This requires keeping track of the errno from ->dump() across calls. Signed-off-by: Jason A. Donenfeld --- Can we get this into 4.14? Is there still time? It should also be queued up for stable. Changes v3->v4: - I like long lines. The kernel does not. Wrapped at 80 now. net/netlink/af_netlink.c | 17 +++++++++++------ net/netlink/af_netlink.h | 1 + 2 files changed, 12 insertions(+), 6 deletions(-) diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c index b93148e8e9fb..15c99dfa3d72 100644 --- a/net/netlink/af_netlink.c +++ b/net/netlink/af_netlink.c @@ -2136,7 +2136,7 @@ static int netlink_dump(struct sock *sk) struct sk_buff *skb = NULL; struct nlmsghdr *nlh; struct module *module; - int len, err = -ENOBUFS; + int err = -ENOBUFS; int alloc_min_size; int alloc_size; @@ -2183,9 +2183,11 @@ static int netlink_dump(struct sock *sk) skb_reserve(skb, skb_tailroom(skb) - alloc_size); netlink_skb_set_owner_r(skb, sk); - len = cb->dump(skb, cb); + if (nlk->dump_done_errno > 0) + nlk->dump_done_errno = cb->dump(skb, cb); - if (len > 0) { + if (nlk->dump_done_errno > 0 || + skb_tailroom(skb) < nlmsg_total_size(sizeof(nlk->dump_done_errno))) { mutex_unlock(nlk->cb_mutex); if (sk_filter(sk, skb)) @@ -2195,13 +2197,15 @@ static int netlink_dump(struct sock *sk) return 0; } - nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE, sizeof(len), NLM_F_MULTI); - if (!nlh) + nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE, + sizeof(nlk->dump_done_errno), NLM_F_MULTI); + if (WARN_ON(!nlh)) goto errout_skb; nl_dump_check_consistent(cb, nlh); - memcpy(nlmsg_data(nlh), &len, sizeof(len)); + memcpy(nlmsg_data(nlh), &nlk->dump_done_errno, + sizeof(nlk->dump_done_errno)); if (sk_filter(sk, skb)) kfree_skb(skb); @@ -2273,6 +2277,7 @@ int __netlink_dump_start(struct sock *ssk, struct sk_buff *skb, } nlk->cb_running = true; + nlk->dump_done_errno = INT_MAX; mutex_unlock(nlk->cb_mutex); diff --git a/net/netlink/af_netlink.h b/net/netlink/af_netlink.h index 028188597eaa..962de7b3c023 100644 --- a/net/netlink/af_netlink.h +++ b/net/netlink/af_netlink.h @@ -34,6 +34,7 @@ struct netlink_sock { wait_queue_head_t wait; bool bound; bool cb_running; + int dump_done_errno; struct netlink_callback cb; struct mutex *cb_mutex; struct mutex cb_def_mutex;