Subject: Re: [PATCH net v2] net: drop bad gso csum_start and offset in virtio_net_hdr

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 8 Sep 2024 16:43:55 -0400

On Sun, Sep 08, 2024 at 04:09:43PM -0400, Willem de Bruijn wrote:
> Willem de Bruijn wrote:
> > Szabolcs Nagy wrote:
> > > The 07/29/2024 16:10, Willem de Bruijn wrote:
> > > > From: Willem de Bruijn <[email-masked]>
> > > > 
> > > > Tighten csum_start and csum_offset checks in virtio_net_hdr_to_skb
> > > > for GSO packets.
> > > > 
> > > > The function already checks that a checksum requested with
> > > > VIRTIO_NET_HDR_F_NEEDS_CSUM is in skb linear. But for GSO packets
> > > > this might not hold for segs after segmentation.
> > > > 
> > > > Syzkaller demonstrated to reach this warning in skb_checksum_help
> > > > 
> > > > 	offset = skb_checksum_start_offset(skb);
> > > > 	ret = -EINVAL;
> > > > 	if (WARN_ON_ONCE(offset >= skb_headlen(skb)))
> > > > 
> > > > By injecting a TSO packet:
> > > > 
> > > > WARNING: CPU: 1 PID: 3539 at net/core/dev.c:3284 skb_checksum_help+0x3d0/0x5b0
> > > >  ip_do_fragment+0x209/0x1b20 net/ipv4/ip_output.c:774
> > > >  ip_finish_output_gso net/ipv4/ip_output.c:279 [inline]
> > > >  __ip_finish_output+0x2bd/0x4b0 net/ipv4/ip_output.c:301
> > > >  iptunnel_xmit+0x50c/0x930 net/ipv4/ip_tunnel_core.c:82
> > > >  ip_tunnel_xmit+0x2296/0x2c70 net/ipv4/ip_tunnel.c:813
> > > >  __gre_xmit net/ipv4/ip_gre.c:469 [inline]
> > > >  ipgre_xmit+0x759/0xa60 net/ipv4/ip_gre.c:661
> > > >  __netdev_start_xmit include/linux/netdevice.h:4850 [inline]
> > > >  netdev_start_xmit include/linux/netdevice.h:4864 [inline]
> > > >  xmit_one net/core/dev.c:3595 [inline]
> > > >  dev_hard_start_xmit+0x261/0x8c0 net/core/dev.c:3611
> > > >  __dev_queue_xmit+0x1b97/0x3c90 net/core/dev.c:4261
> > > >  packet_snd net/packet/af_packet.c:3073 [inline]
> > > > 
> > > > The geometry of the bad input packet at tcp_gso_segment:
> > > > 
> > > > [   52.003050][ T8403] skb len=12202 headroom=244 headlen=12093 tailroom=0
> > > > [   52.003050][ T8403] mac=(168,24) mac_len=24 net=(192,52) trans=244
> > > > [   52.003050][ T8403] shinfo(txflags=0 nr_frags=1 gso(size=1552 type=3 segs=0))
> > > > [   52.003050][ T8403] csum(0x60000c7 start=199 offset=1536
> > > > ip_summed=3 complete_sw=0 valid=0 level=0)
> > > > 
> > > > Mitigate with stricter input validation.
> > > > 
> > > > csum_offset: for GSO packets, deduce the correct value from gso_type.
> > > > This is already done for USO. Extend it to TSO. Let UFO be:
> > > > udp[46]_ufo_fragment ignores these fields and always computes the
> > > > checksum in software.
> > > > 
> > > > csum_start: finding the real offset requires parsing to the transport
> > > > header. Do not add a parser, use existing segmentation parsing. Thanks
> > > > to SKB_GSO_DODGY, that also catches bad packets that are hw offloaded.
> > > > Again test both TSO and USO. Do not test UFO for the above reason, and
> > > > do not test UDP tunnel offload.
> > > > 
> > > > GSO packet are almost always CHECKSUM_PARTIAL. USO packets may be
> > > > CHECKSUM_NONE since commit 10154dbded6d6 ("udp: Allow GSO transmit
> > > > from devices with no checksum offload"), but then still these fields
> > > > are initialized correctly in udp4_hwcsum/udp6_hwcsum_outgoing. So no
> > > > need to test for ip_summed == CHECKSUM_PARTIAL first.
> > > > 
> > > > This revises an existing fix mentioned in the Fixes tag, which broke
> > > > small packets with GSO offload, as detected by kselftests.
> > > > 
> > > > Link: https://syzkaller.appspot.com/bug?extid=e1db31216c789f552871
> > > > Link: https://lore.kernel.org/netdev/[email-masked]
> > > > Fixes: e269d79c7d35 ("net: missing check virtio")
> > > > Cc: [email-masked]
> > > > Signed-off-by: Willem de Bruijn <[email-masked]>
> > > > 
> > > > ---
> > > > 
> > > > v1->v2
> > > >   - skb_transport_header instead of skb->transport_header (edumazet@)
> > > >   - typo: migitate -> mitigate
> > > > ---
> > > 
> > > this breaks booting from nfs root on an arm64 fvp
> > > model for me.
> > > 
> > > i see two fixup commits
> > > 
> > > commit 30b03f2a0592eee1267298298eac9dd655f55ab2
> > > Author:     Jakub Sitnicki <[email-masked]>
> > > AuthorDate: 2024-08-08 11:56:22 +0200
> > > Commit:     Jakub Kicinski <[email-masked]>
> > > CommitDate: 2024-08-09 21:58:08 -0700
> > > 
> > >     udp: Fall back to software USO if IPv6 extension headers are present
> > > 
> > > and
> > > 
> > > commit b128ed5ab27330deeeaf51ea8bb69f1442a96f7f
> > > Author:     Felix Fietkau <[email-masked]>
> > > AuthorDate: 2024-08-19 17:06:21 +0200
> > > Commit:     Jakub Kicinski <[email-masked]>
> > > CommitDate: 2024-08-21 17:15:05 -0700
> > > 
> > >     udp: fix receiving fraglist GSO packets
> > > 
> > > but they don't fix the issue for me,
> > > at the boot console i see
> > > 
> > > ...
> > > [    3.686846] Sending DHCP requests ., OK
> > > [    3.687302] IP-Config: Got DHCP answer from 172.20.51.254, my address is 172.20.51.1
> > > [    3.687423] IP-Config: Complete:
> > > [    3.687482]      device=eth0, hwaddr=ea:0d:79:71:af:cd, ipaddr=172.20.51.1, mask=255.255.255.0, gw=172.20.51.254
> > > [    3.687631]      host=172.20.51.1, domain=, nis-domain=(none)
> > > [    3.687719]      bootserver=172.20.51.254, rootserver=10.2.80.41, rootpath=
> > > [    3.687771]      nameserver0=172.20.51.254, nameserver1=172.20.51.252, nameserver2=172.20.51.251
> > > [    3.689075] clk: Disabling unused clocks
> > > [    3.689167] PM: genpd: Disabling unused power domains
> > > [    3.689258] ALSA device list:
> > > [    3.689330]   No soundcards found.
> > > [    3.716297] VFS: Mounted root (nfs4 filesystem) on device 0:24.
> > > [    3.716843] devtmpfs: mounted
> > > [    3.734352] Freeing unused kernel memory: 10112K
> > > [    3.735178] Run /sbin/init as init process
> > > [    3.743770] eth0: bad gso: type: 1, size: 1440
> > > [    3.744186] eth0: bad gso: type: 1, size: 1440
> > > ...
> > > [  154.610991] eth0: bad gso: type: 1, size: 1440
> > > [  185.330941] nfs: server 10.2.80.41 not responding, still trying
> > > ...
> > > 
> > > the "bad gso" message keeps repeating and init
> > > is not executed.
> > > 
> > > if i revert the 3 patches above on 6.11-rc6 then
> > > init runs without "bad gso" error.
> > > 
> > > this affects testing the arm64-gcs patches on
> > > top of 6.11-rc3 and 6.11-rc6
> > > 
> > > not sure if this is an fvp or kernel bug.
> > 
> > Thanks for the report, sorry that you're encountering this breakage.
> > 
> > Makes sense that this commit introduced it
> > 
> >         if (virtio_net_hdr_to_skb(skb, &hdr->hdr,
> >                                   virtio_is_little_endian(vi->vdev))) {
> >                 net_warn_ratelimited("%s: bad gso: type: %u, size: %u\n",
> >                                      dev->name, hdr->hdr.gso_type,
> >                                      hdr->hdr.gso_size);
> >                 goto frame_err;
> >         }
> >         
> > Type 1 is VIRTIO_NET_HDR_GSO_TCPV4
> > 
> > Most likely this application is inserting a packet with flag
> > VIRTIO_NET_HDR_F_NEEDS_CSUM and a wrong csum_start. Or is requesting
> > TSO without checksum offload at all. In which case the kernel goes out
> > of its way to find the right offset, but may fail.
> > 
> > Which nfs-client is this? I'd like to take a look at the sourcecode.
> > 
> > Unfortunately the kernel warning lacks a few useful pieces of data,
> > such as the other virtio_net_hdr fields and the packet length.
> 
> This happens on the virtio-net receive path, so the bad data is
> received from the hypervisor.
> 
> >From what I gather that arm64 fvp (Fixed Virtual Platforms) hypervisor
> is closed source?
> 
> Disabling GRO on this device will likely work as temporary workaround.
> 
> What we can do is instead of dropping packets to correct their offset:
> 
>                 case SKB_GSO_TCPV4:
>                 case SKB_GSO_TCPV6:
> -                        if (skb->csum_offset != offsetof(struct tcphdr, check))
> -                                return -EINVAL;
> +                        if (skb->csum_offset != offsetof(struct tcphdr, check)) {
> +                                DEBUG_NET_WARN_ON_ONCE(1);
> +                                skb->csum_offset = offsetof(struct tcphdr, check);
> +                        }
> 
> If the issue is in csum_offset. If the new csum_start check fails,
> that won't help.
> 
> It would be helpful to see these values at this point, e.g., with
> skb_dump(KERN_INFO, skb, false);

It's an iteresting question whether when VIRTIO_NET_F_GUEST_HDRLEN
is not negotiated, csum_offset can be relied upon for GRO.

Jason, WDYT?

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MST