The 07/29/2024 16:10, Willem de Bruijn wrote:

From: Willem de Bruijn <willemb@google.com>

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/20240723223109.2196886-1-kuba@kernel.org (local)
Fixes: e269d79c7d35 ("net: missing check virtio")
Cc: stable@vger.kernel.org
Signed-off-by: Willem de Bruijn <willemb@google.com>

---

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 [off-list ref]
AuthorDate: 2024-08-08 11:56:22 +0200
Commit:     Jakub Kicinski [off-list ref]
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 [off-list ref]
AuthorDate: 2024-08-19 17:06:21 +0200
Commit:     Jakub Kicinski [off-list ref]
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.