Re: [syzbot] [sctp?] kernel BUG in __pskb_pull_tail (2)

[Xin Long]

On Wed, Nov 4, 2026 at 12:03 AM syzbot
<syzbot+f15a6eff7a53f6eb7dc3@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> syzbot has found a reproducer for the following issue on:
>
> HEAD commit:    05441d697f7f Merge tag 'cocci-7.4-rc1' of git://git.kernel..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=162b8add580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=1cf189394a856f11
> dashboard link: https://syzkaller.appspot.com/bug?extid=f15a6eff7a53f6eb7dc3
> compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11b49b29580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15b49b29580000
>
> Downloadable assets:
> disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-05441d69.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/0edd3964be19/vmlinux-05441d69.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/84d6d8dd5648/bzImage-05441d69.xz
>
> AI-generated root cause summary:
> When receiving an SCTP packet that is nonlinear (such as an SCTP GSO packet
> constructed with fragments in frag_list), sctp_rcv attempts to linearize the
> packet using skb_linearize() if the sk_buff is cloned (e.g., tapped by a packet
> socket or shared) or not recognized as GSO. During linearization via
> __skb_linearize() and __pskb_pull_tail(), skb_copy_bits() fails with -EFAULT due
> to a discrepancy in frag_list fragment lengths or readable offsets, triggering a
> BUG_ON assertion failure. In production builds, this BUG() results in an invalid
> opcode oops and kernel panic, causing an immediate denial of service.
>
> full analysis:  https://syzkaller.appspot.com/x/ai_root_cause?x=f15a6eff7a53f6eb7dc3
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+f15a6eff7a53f6eb7dc3@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> ------------[ cut here ]------------
> kernel BUG at net/core/skbuff.c:2903!
> Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI
> CPU: 2 UID: 0 PID: 6002 Comm: syz.0.20 Not tainted syzkaller #0 PREEMPT(full)
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> RIP: 0010:__pskb_pull_tail+0x1335/0x1920 net/core/skbuff.c:2903
> Code: 5b f8 48 89 df e8 2b 03 fe ff 31 ff 41 89 c4 89 c6 e8 7f 58 5b f8 45 84 e4 0f 84 1c f2 ff ff e9 84 f2 ff ff e8 5c 5e 5b f8 90 <0f> 0b e8 54 5e 5b f8 48 8b 54 24 30 48 b8 00 00 00 00 00 fc ff df
> RSP: 0018:ffffc90000648870 EFLAGS: 00010246
> RAX: 0000000000000000 RBX: 00000000fffffff2 RCX: 0000000000000101
> RDX: ffff88802ce60000 RSI: ffffffff89b30684 RDI: 0000000000000005
> RBP: ffffc900006488f0 R08: 0000000000000000 R09: 00000000fffffff2
> R10: ffff888046a12f77 R11: 0000000000000000 R12: 00000000000000e4
> R13: 0000000000000180 R14: 0000000000000000 R15: ffff888030406780
> FS:  000055558120b500(0000) GS:ffff8880d467c000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 000020000001a000 CR3: 00000000385f5000 CR4: 0000000000352ef0
> Call Trace:
>  <IRQ>
>  __skb_linearize include/linux/skbuff.h:4025 [inline]
>  skb_linearize include/linux/skbuff.h:4037 [inline]
>  sctp_rcv+0x143e/0x3cc0 net/sctp/input.c:120
>  ip_protocol_deliver_rcu+0x44c/0x4d0 net/ipv4/ip_input.c:207
>  ip_local_deliver_finish+0x3fb/0x6e0 net/ipv4/ip_input.c:241
>  NF_HOOK include/linux/netfilter.h:325 [inline]
>  NF_HOOK include/linux/netfilter.h:319 [inline]
>  ip_local_deliver+0x1a3/0x200 net/ipv4/ip_input.c:262
>  dst_input include/net/dst.h:481 [inline]
>  ip_rcv_finish net/ipv4/ip_input.c:492 [inline]
>  NF_HOOK include/linux/netfilter.h:325 [inline]
>  NF_HOOK include/linux/netfilter.h:319 [inline]
>  ip_rcv+0x33a/0x3c0 net/ipv4/ip_input.c:612
>  __netif_receive_skb_one_core+0x197/0x1e0 net/core/dev.c:6433
>  __netif_receive_skb+0x1f/0x120 net/core/dev.c:6546
>  process_backlog+0x487/0x1600 net/core/dev.c:6897
>  __napi_poll.constprop.0+0xaf/0x450 net/core/dev.c:7966
>  napi_poll net/core/dev.c:8029 [inline]
>  net_rx_action+0xa40/0xf20 net/core/dev.c:8186
>  handle_softirqs+0x216/0xa20 kernel/softirq.c:669
>  do_softirq kernel/softirq.c:569 [inline]
>  do_softirq+0xaf/0xf0 kernel/softirq.c:556
>  </IRQ>
>  <TASK>
>  __local_bh_enable_ip+0xff/0x120 kernel/softirq.c:473
>  local_bh_enable include/linux/bottom_half.h:33 [inline]
>  ipt_do_table+0xd71/0x1b10 net/ipv4/netfilter/ip_tables.c:357
>  nf_hook_entry_hookfn include/linux/netfilter.h:165 [inline]
>  nf_hook_slow+0xbf/0x220 net/netfilter/core.c:633
>  nf_hook include/linux/netfilter.h:280 [inline]
>  __ip_local_out+0x541/0xb80 net/ipv4/ip_output.c:120
>  ip_local_out+0x2a/0x1f0 net/ipv4/ip_output.c:129
>  __ip_queue_xmit+0x876/0x1f50 net/ipv4/ip_output.c:533
>  sctp_v4_xmit+0x91a/0x1070 net/sctp/protocol.c:1061
>  sctp_packet_transmit+0x22fe/0x30b0 net/sctp/output.c:659
>  sctp_outq_flush_transports net/sctp/outqueue.c:1174 [inline]
>  sctp_outq_flush+0xb2b/0x3380 net/sctp/outqueue.c:1222
>  sctp_cmd_interpreter net/sctp/sm_sideeffect.c:1826 [inline]
>  sctp_side_effects net/sctp/sm_sideeffect.c:1211 [inline]
>  sctp_do_sm+0xce1/0x5c30 net/sctp/sm_sideeffect.c:1182
>  sctp_assoc_bh_rcv+0x392/0x740 net/sctp/associola.c:1048
>  sctp_inq_push+0x25c/0x320 net/sctp/inqueue.c:91
>  sctp_backlog_rcv+0x169/0x590 net/sctp/input.c:331
>  sk_backlog_rcv include/net/sock.h:1194 [inline]
>  __release_sock+0x39d/0x440 net/core/sock.c:3341
>  release_sock+0x1e5/0x280 net/core/sock.c:3940
>  sctp_sendmsg+0x1166/0x22e0 net/sctp/socket.c:2091
>  inet_sendmsg+0x11c/0x140 net/ipv4/af_inet.c:864
>  sock_sendmsg_nosec net/socket.c:800 [inline]
>  __sock_sendmsg net/socket.c:818 [inline]
>  ____sys_sendmsg+0xa23/0xc30 net/socket.c:2715
>  ___sys_sendmsg+0x190/0x1e0 net/socket.c:2769
>  __sys_sendmmsg+0x20c/0x440 net/socket.c:2858
>  __do_sys_sendmmsg net/socket.c:2885 [inline]
>  __se_sys_sendmmsg net/socket.c:2882 [inline]
>  __x64_sys_sendmmsg+0x9c/0x100 net/socket.c:2882
>  do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
>  do_syscall_64+0x123/0x790 arch/x86/entry/syscall_64.c:84
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7ff80879fb19
> Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 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 e8 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007fff70bcd848 EFLAGS: 00000246 ORIG_RAX: 0000000000000133
> RAX: ffffffffffffffda RBX: 00007ff808a25fa0 RCX: 00007ff80879fb19
> RDX: 0000000000000001 RSI: 0000200000000e40 RDI: 0000000000000004
> RBP: 00007ff808837b70 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
> R13: 00007ff808a25fac R14: 00007ff808a25fa0 R15: 00007ff808a25fa0
>  </TASK>
> Modules linked in:
> ---[ end trace 0000000000000000 ]---
> RIP: 0010:__pskb_pull_tail+0x1335/0x1920 net/core/skbuff.c:2903
> Code: 5b f8 48 89 df e8 2b 03 fe ff 31 ff 41 89 c4 89 c6 e8 7f 58 5b f8 45 84 e4 0f 84 1c f2 ff ff e9 84 f2 ff ff e8 5c 5e 5b f8 90 <0f> 0b e8 54 5e 5b f8 48 8b 54 24 30 48 b8 00 00 00 00 00 fc ff df
> RSP: 0018:ffffc90000648870 EFLAGS: 00010246
> RAX: 0000000000000000 RBX: 00000000fffffff2 RCX: 0000000000000101
> RDX: ffff88802ce60000 RSI: ffffffff89b30684 RDI: 0000000000000005
> RBP: ffffc900006488f0 R08: 0000000000000000 R09: 00000000fffffff2
> R10: ffff888046a12f77 R11: 0000000000000000 R12: 00000000000000e4
> R13: 0000000000000180 R14: 0000000000000000 R15: ffff888030406780
> FS:  000055558120b500(0000) GS:ffff8880d467c000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 000020000001a000 CR3: 00000000385f5000 CR4: 0000000000352ef0
>
>
> ---
> If you want syzbot to run the reproducer, reply with:
> #syz test: git://repo/address.git branch-or-commit-hash
> If you attach or paste a git patch, syzbot will apply it before testing.

Thanks for the reproducer.

The issue was caused by iptables TEE target (`pskb_copy`), where the new
skb doesn't copy the skbs in frag_list, so modifying the skbs from frag_list
in sctp stack is not safe.

I will prepare a patch for this.