From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S972974AbdDXRrl (ORCPT ); Mon, 24 Apr 2017 13:47:41 -0400 Received: from shards.monkeyblade.net ([184.105.139.130]:55208 "EHLO shards.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S972535AbdDXRrd (ORCPT ); Mon, 24 Apr 2017 13:47:33 -0400 Date: Mon, 24 Apr 2017 13:47:26 -0400 (EDT) Message-Id: <20170424.134726.1314464394825506301.davem@davemloft.net> To: Jason@zx2c4.com Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, security@kernel.org Subject: Re: [PATCH] macsec: avoid heap overflow in skb_to_sgvec From: David Miller In-Reply-To: <20170421211448.16995-1-Jason@zx2c4.com> References: <20170421211448.16995-1-Jason@zx2c4.com> X-Mailer: Mew version 6.7 on Emacs 24.5 / Mule 6.0 (HANACHIRUSATO) Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Greylist: Sender succeeded SMTP AUTH, not delayed by milter-greylist-4.5.12 (shards.monkeyblade.net [149.20.54.216]); Mon, 24 Apr 2017 10:06:09 -0700 (PDT) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: "Jason A. Donenfeld" Date: Fri, 21 Apr 2017 23:14:48 +0200 > While this may appear as a humdrum one line change, it's actually quite > important. An sk_buff stores data in three places: > > 1. A linear chunk of allocated memory in skb->data. This is the easiest > one to work with, but it precludes using scatterdata since the memory > must be linear. > 2. The array skb_shinfo(skb)->frags, which is of maximum length > MAX_SKB_FRAGS. This is nice for scattergather, since these fragments > can point to different pages. > 3. skb_shinfo(skb)->frag_list, which is a pointer to another sk_buff, > which in turn can have data in either (1) or (2). > > The first two are rather easy to deal with, since they're of a fixed > maximum length, while the third one is not, since there can be > potentially limitless chains of fragments. Fortunately dealing with > frag_list is opt-in for drivers, so drivers don't actually have to deal > with this mess. For whatever reason, macsec decided it wanted pain, and > so it explicitly specified NETIF_F_FRAGLIST. > > Because dealing with (1), (2), and (3) is insane, most users of sk_buff > doing any sort of crypto or paging operation calls a convenient function > called skb_to_sgvec (which happens to be recursive if (3) is in use!). > This takes a sk_buff as input, and writes into its output pointer an > array of scattergather list items. Sometimes people like to declare a > fixed size scattergather list on the stack; othertimes people like to > allocate a fixed size scattergather list on the heap. However, if you're > doing it in a fixed-size fashion, you really shouldn't be using > NETIF_F_FRAGLIST too (unless you're also ensuring the sk_buff and its > frag_list children arent't shared and then you check the number of > fragments in total required.) > > Macsec specifically does this: > > size += sizeof(struct scatterlist) * (MAX_SKB_FRAGS + 1); > tmp = kmalloc(size, GFP_ATOMIC); > *sg = (struct scatterlist *)(tmp + sg_offset); > ... > sg_init_table(sg, MAX_SKB_FRAGS + 1); > skb_to_sgvec(skb, sg, 0, skb->len); > > Specifying MAX_SKB_FRAGS + 1 is the right answer usually, but not if you're > using NETIF_F_FRAGLIST, in which case the call to skb_to_sgvec will > overflow the heap, and disaster ensues. > > Signed-off-by: Jason A. Donenfeld This is definitely the simplest fix for now, applied, thanks.