Re: [syzbot] [net?] WARNING in skb_checksum_help (2)

[Willem de Bruijn]

On Thu, Sep 21, 2023 at 9:28 PM syzbot
<syzbot+01cdbc31e9c0ae9b33ac@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    489fa31ea873 Merge branch 'work.misc' of git://git.kernel...
> git tree:       upstream
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=1012dbf0c80000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=a8fa629ede9e7b9
> dashboard link: https://syzkaller.appspot.com/bug?extid=01cdbc31e9c0ae9b33ac
> compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=130cfe18c80000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=10561350c80000
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/2f2344129720/disk-489fa31e.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/e1be11fcb953/vmlinux-489fa31e.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/4549005e4fa5/bzImage-489fa31e.xz
>
> The issue was bisected to:
>
> commit 860b7f27b8f78564ca5a2f607e0820b2d352a562
> Author: Andrew Melnychenko <andrew@xxxxxxxxxx>
> Date:   Wed Dec 7 11:35:57 2022 +0000
>
>     linux/virtio_net.h: Support USO offload in vnet header.
>
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=15cb9d50c80000
> final oops:     https://syzkaller.appspot.com/x/report.txt?x=17cb9d50c80000
> console output: https://syzkaller.appspot.com/x/log.txt?x=13cb9d50c80000
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+01cdbc31e9c0ae9b33ac@xxxxxxxxxxxxxxxxxxxxxxxxx
> Fixes: 860b7f27b8f7 ("linux/virtio_net.h: Support USO offload in vnet header.")
>
> ------------[ cut here ]------------
> WARNING: CPU: 1 PID: 5082 at net/core/dev.c:3260 skb_checksum_help+0x4a2/0x600 net/core/dev.c:3260
> Modules linked in:
> CPU: 1 PID: 5082 Comm: syz-executor228 Not tainted 6.2.0-syzkaller-10827-g489fa31ea873 #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/16/2023
> RIP: 0010:skb_checksum_help+0x4a2/0x600 net/core/dev.c:3260
> Code: e0 48 83 c4 28 5b 5d 41 5c 41 5d 41 5e 41 5f c3 e8 63 15 70 f9 48 89 ef 41 bc ea ff ff ff e8 35 f9 ff ff eb d4 e8 4e 15 70 f9 <0f> 0b 0f b6 1d 88 29 4e 06 31 ff 89 de e8 4c 11 70 f9 84 db 0f 84
> RSP: 0018:ffffc90003b6f660 EFLAGS: 00010293
> RAX: 0000000000000000 RBX: 000000000000034a RCX: 0000000000000000
> RDX: ffff888024171d40 RSI: ffffffff8814b1f2 RDI: 0000000000000004
> RBP: ffff888079090780 R08: 0000000000000004 R09: 000000000000019a
> R10: 0000000000000348 R11: 0000000000000000 R12: ffff8880790907f4
> R13: 0000000000000000 R14: ffff888079090800 R15: 0000000000000348
> FS:  00005555559ed300(0000) GS:ffff8880b9900000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 000000002000f000 CR3: 0000000072c0b000 CR4: 00000000003506e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  <TASK>
>  netem_enqueue+0x2b57/0x35b0 net/sched/sch_netem.c:510
>  netem_enqueue+0x2050/0x35b0 net/sched/sch_netem.c:483
>  dev_qdisc_enqueue+0x46/0x3b0 net/core/dev.c:3773
>  __dev_xmit_skb net/core/dev.c:3862 [inline]
>  __dev_queue_xmit+0x236a/0x3db0 net/core/dev.c:4210
>  packet_snd net/packet/af_packet.c:3073 [inline]
>  packet_sendmsg+0x33aa/0x55d0 net/packet/af_packet.c:3104
>  sock_sendmsg_nosec net/socket.c:722 [inline]
>  sock_sendmsg+0xde/0x190 net/socket.c:745
>  __sys_sendto+0x23a/0x340 net/socket.c:2145
>  __do_sys_sendto net/socket.c:2157 [inline]
>  __se_sys_sendto net/socket.c:2153 [inline]
>  __x64_sys_sendto+0xe1/0x1b0 net/socket.c:2153
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x39/0xb0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
> RIP: 0033:0x7fa19dae7699
> Code: 28 c3 e8 4a 15 00 00 66 2e 0f 1f 84 00 00 00 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 c0 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007fffb3262e28 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
> RAX: ffffffffffffffda RBX: 00007fffb3262e48 RCX: 00007fa19dae7699
> RDX: 000000000000fc13 RSI: 0000000020000280 RDI: 0000000000000003
> RBP: 0000000000000003 R08: 0000000000000000 R09: 000000000000002f
> R10: 0000000000000800 R11: 0000000000000246 R12: 00007fffb3262e50
> R13: 00007fffb3262e70 R14: 0000000000000000 R15: 0000000000000000
>  </TASK>
>
>
> ---
> This report is generated by a bot. It may contain errors.
> See https://goo.gl/tpsmEJ for more information about syzbot.
> syzbot engineers can be reached at syzkaller@xxxxxxxxxxxxxxxx.
>
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> For information about bisection process see: https://goo.gl/tpsmEJ#bisection
>
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>
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>
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One of two reports due to the introduction of SKB_GSO_UDP_L4.

I will make checks on udp gso packets coming in over virtio more strict.

[   25.057760] skb len=64521 headroom=2 headlen=1355 tailroom=0
[   25.057760] mac=(2,14) net=(16,826) trans=842
[   25.057760] shinfo(txflags=0 nr_frags=2 gso(size=344 type=131074 segs=0))
[   25.057760] csum(0x201034a ip_summed=3 complete_sw=0 valid=0
level=0 start=842 off=513)
[   25.057760] hash(0x0 sw=0 l4=0) proto=0x0800 pkttype=0 iif=0
[   25.075806] dev name=lo feat=0x00000516401d7c69
[   25.079134] sk family=17 type=3 proto=0

This is clearly a bogus packet.

Checks to add, for starters:

1. udp gso packets coming over virtio must be held to the same
UDP_MAX_SEGMENTS limit as those generated by the local stack.

2. csum_offset should always be offsetof(struct udphdr, check).

3. checksum offload is mandatory.

I needed to patch skb_dump to get the csum_start and _offset fields.