From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-12.7 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 13385C4320A for ; Wed, 18 Aug 2021 05:13:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id F3E07606A5 for ; Wed, 18 Aug 2021 05:13:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237689AbhHRFNq (ORCPT ); Wed, 18 Aug 2021 01:13:46 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60398 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237618AbhHRFNn (ORCPT ); Wed, 18 Aug 2021 01:13:43 -0400 Received: from mail-pj1-x1034.google.com (mail-pj1-x1034.google.com [IPv6:2607:f8b0:4864:20::1034]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5E679C0613CF; Tue, 17 Aug 2021 22:13:09 -0700 (PDT) Received: by mail-pj1-x1034.google.com with SMTP id 28-20020a17090a031cb0290178dcd8a4d1so4561018pje.0; Tue, 17 Aug 2021 22:13:09 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=90dW0tJPb1k97SIWgHh2sABddZEFDaN0VEgYFRlfpPo=; b=DWQuRvO1G7ty2TbQBSl+NCROP3gNOYFX3N/mVUho4VsYn4Dbk9hGj9k19Ipkja4r68 VqFFQOMi/t4b6hwTqwE8WBN+ovYuvG8Mzqjuym7Pw464aurW013UvQpG/vzLOIP9Uk+0 Zmk9hksqAKV2QKiS59WIlSOOMVCip5owDycuPD2KbpP6PffT0GVGVpnzQE1pqgOOeunI mJwpGxHRSOzkcvQzsZawONzqyhqHsDzNn8FpN07NrKIGs26+YRCLDvq8caClU9imCRBl q3ek7UMQ3WFce3+TtmpRsmF1g1zbzg2Acxjnt7TO0QqAllMtDQIjNlSupuTMEF64daUl rY8Q== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=90dW0tJPb1k97SIWgHh2sABddZEFDaN0VEgYFRlfpPo=; b=ZC9rkaeGLABgVb2hVGWD8cXNtZasw2g0SMEhoJbXCSTGeWM8MQCekmIsYtja74eoVy PxkFf8qcXCjRRhX/FYeiE+Sh9/Z9ej4USaFKJ862Ob7txsvkjmvJxiOwclAmVSLswH+x qFroTNxW/4zazBO0QfdSDnn8+nY+qIAXFvCIyWP2XnnKYz8Yi98OtyXA6wwf12MmOR/T 5vRddPkxTo5uHLEdHDkKrlMK9cVWKWNi2IUhwp5nIkrRqx7YzMhxngrWBK3UHZAGAf+5 gAAwHM8yH67KPJEuzbVpigVb4PYnPsUuBU2PyMiO7IsuZdHPJNL2+AUwfEUuFY/EZHD7 n6kw== X-Gm-Message-State: AOAM532Zvgqy5bt33ZrNpmiZM56ALoQCyimjIJk8piJumIOKRspdwJvH gEm2bwJfLiH5Ryzsc7nMpf4= X-Google-Smtp-Source: ABdhPJzb/KFABKI6RX7nTPrqqWvjz7OPMOjIQNTHTdxBRvM5SyG25reArcEYDoXM4SSnQeXvPtNYPA== X-Received: by 2002:a17:90a:930e:: with SMTP id p14mr7379131pjo.132.1629263588776; Tue, 17 Aug 2021 22:13:08 -0700 (PDT) Received: from fedora ([112.79.153.28]) by smtp.gmail.com with ESMTPSA id gg6sm3718430pjb.46.2021.08.17.22.12.59 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 17 Aug 2021 22:13:08 -0700 (PDT) Date: Wed, 18 Aug 2021 10:42:57 +0530 From: Shreyansh Chouhan To: Willem de Bruijn Cc: David Miller , Jakub Kicinski , Eric Dumazet , Willem de Bruijn , Xie He , "Gustavo A . R . Silva" , Wang Hai , Tanner Love , Eyal Birger , Richard Sanger , jiapeng.chong@linux.alibaba.com, Network Development , LKML , Jason Wang Subject: Re: Can a valid vnet header have both csum_start and csum_offset 0? Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, Thanks a lot for this explanation. On Mon, Aug 16, 2021 at 08:17:08AM -0700, Willem de Bruijn wrote: > On Wed, Aug 11, 2021 at 10:09 PM Shreyansh Chouhan > wrote: > > > > On Wed, Aug 11, 2021 at 12:36:38AM +0530, Shreyansh Chouhan wrote: > > > Hi, > > > > > > When parsing the vnet header in __packet_snd_vnet_parse[1], we do not > > > check for if the values of csum_start and csum_offset given in the > > > header are both 0. > > > > > > Having both these values 0, however, causes a crash[2] further down the > > > gre xmit code path. In the function ipgre_xmit, we pull the ip header > > > and gre header from skb->data, this results in an invalid > > > skb->csum_start which was calculated from the vnet header. The > > > skb->csum_start offset in this case turns out to be lower than > > > skb->transport_header. This causes us to pass a negative number as an > > > argument to csum_partial[3] and eventually to do_csum[4], which then causes > > > a kernel oops in the while loop. > > > > > > I do not understand what should the correct behavior be in this > > > scenario, should we consider this vnet header as invalid? > > > > Something like the following diff: > > > > diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c > > index 57a1971f29e5..65bff1c8f75c 100644 > > --- a/net/packet/af_packet.c > > +++ b/net/packet/af_packet.c > > @@ -2479,13 +2479,17 @@ static void tpacket_destruct_skb(struct sk_buff *skb) > > > > static int __packet_snd_vnet_parse(struct virtio_net_hdr *vnet_hdr, size_t len) > > { > > + __u16 csum_start = __virtio16_to_cpu(vio_le(), vnet_hdr->csum_start); > > + __u16 csum_offset = __virtio16_to_cpu(vio_le(), vnet_hdr->csum_offset); > > + __u16 hdr_len = __virtio16_to_cpu(vio_le(), vnet_hdr->hdr_len); > > + > > + if (csum_start + csum_offset == 0) > > + return -EINVAL; > > + > > Yet another virtio_net_hdr validation issue. > > The issue is not unique to value 0, but true for any csum_start < > skb->transport_header at the time of the call to lco_csum. That > function not unreasonably assumes that csum_start >= l4_hdr. > I thought of that, but then when I went further back in the code execution path, and saw the code that parses the virtio net header, I thought that if VIRTIO_NET_HDR_F_NEEDS_CSUM then the csum_start and csum_offset together must point to some valid location inside the packet. Thanks a lot for explaining this. Looks like I backtracked more than necessary. > The packet socket code is protocol agnostic. It does not know that > this is an IP GRE packet with greh->flags & TUNNEL_CSUM. We cannot add > checks there. > > This does appear to be specific to the GRE checksum field, so it won't > be relevant to other ip tunnel devices: > > if (flags & TUNNEL_CSUM && > !(skb_shinfo(skb)->gso_type & > (SKB_GSO_GRE | SKB_GSO_GRE_CSUM))) { > *ptr = 0; > if (skb->ip_summed == CHECKSUM_PARTIAL) { > *(__sum16 *)ptr = csum_fold(lco_csum(skb)); > > Then the check can be performed between where the ip header is pulled, > in ipgre_xmit: > > if (dev->header_ops) { > if (skb_cow_head(skb, 0)) > goto free_skb; > > tnl_params = (const struct iphdr *)skb->data; > > /* Pull skb since ip_tunnel_xmit() needs skb->data pointing > * to gre header. > */ > skb_pull(skb, tunnel->hlen + sizeof(struct iphdr)); <-------- > skb_reset_mac_header(skb); > } else { > if (skb_cow_head(skb, dev->needed_headroom)) > goto free_skb; > > tnl_params = &tunnel->parms.iph; > } > > if (gre_handle_offloads(skb, !!(tunnel->parms.o_flags & TUNNEL_CSUM))) > goto free_skb; > > __gre_xmit(skb, dev, tnl_params, skb->protocol); > > and when gre_build_header resets the transport header and calls > lco_csum. gre_build_header is called from __gre_xmit in the above. > > gre_handle_offloads might then be a good location. Perhaps: > > static int gre_handle_offloads(struct sk_buff *skb, bool csum) > { > + if (csum && skb->csum_start < skb->data) > + return -EINVAL; > + > return iptunnel_handle_offloads(skb, csum ? > SKB_GSO_GRE_CSUM : SKB_GSO_GRE); > } > > And same for ip6gre. > > Having to add branches inside the hotpath that normally sees well > behaved packets coming from the kernel stack, only to be robust > against with malformed packets with bad virtio_net_hdr, is > unfortunate. This way, the check is at least limited to GRE packets > with TUNNEL_CSUM. > I see, I had no idea about this being a hotpath as well. Thank you for clearing all of this out. This does help in isolating the branches to gre packets that require a checksum. I will make a patch according to the diff that you suggested, and send it. Regards, Shreyansh Chouhan > > if ((vnet_hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) && > > - (__virtio16_to_cpu(vio_le(), vnet_hdr->csum_start) + > > - __virtio16_to_cpu(vio_le(), vnet_hdr->csum_offset) + 2 > > > - __virtio16_to_cpu(vio_le(), vnet_hdr->hdr_len))) > > + csum_start + csum_offset + 2 > hdr_len) > > vnet_hdr->hdr_len = __cpu_to_virtio16(vio_le(), > > - __virtio16_to_cpu(vio_le(), vnet_hdr->csum_start) + > > - __virtio16_to_cpu(vio_le(), vnet_hdr->csum_offset) + 2); > > + csum_start + csum_offset + 2); > > > > if (__virtio16_to_cpu(vio_le(), vnet_hdr->hdr_len) > len) > > return -EINVAL; > > > > > Or should we rather accomodate for both csum_start > > > and csum_offset values to be 0 in ipgre_xmit? > > > > > > Regards, > > > Shreyansh Chouhan > > > > > > -- > > > > > > [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/net/packet/af_packet.c#n2480 > > > [2] https://syzkaller.appspot.com/bug?id=c391f74aac26dd8311c45743ae618f9d5e38b674 > > > [3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/skbuff.h#n4662 > > > [4] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/lib/csum-partial_64.c#n35