[syzbot] [mptcp?] WARNING in subflow_data_ready (4)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: b19a97d57c15 Merge tag 'pull-fixes' of git://git.kernel.or..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=174817a2580000
kernel config: https://syzkaller.appspot.com/x/.config?x=b7511150b112b9c3
dashboard link: https://syzkaller.appspot.com/bug?extid=0ff6b771b4f7a5bce83b
compiler: gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-b19a97d5.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/d6e98c49ae62/vmlinux-b19a97d5.xz
kernel image: https://storage.googleapis.com/syzbot-assets/c90ded1d8e17/bzImage-b19a97d5.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+0ff6b7...@syzkaller.appspotmail.com

------------[ cut here ]------------
WARNING: CPU: 3 PID: 33 at net/mptcp/subflow.c:1515 subflow_data_ready+0x40b/0x7c0 net/mptcp/subflow.c:1515
Modules linked in:
CPU: 3 UID: 0 PID: 33 Comm: ksoftirqd/3 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
RIP: 0010:subflow_data_ready+0x40b/0x7c0 net/mptcp/subflow.c:1515
Code: 89 ee e8 78 61 3c f6 40 84 ed 75 21 e8 8e 66 3c f6 44 89 fe bf 07 00 00 00 e8 c1 61 3c f6 41 83 ff 07 74 09 e8 76 66 3c f6 90 <0f> 0b 90 e8 6d 66 3c f6 48 89 df e8 e5 ad ff ff 31 ff 89 c5 89 c6
RSP: 0018:ffffc900006cf338 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff888031acd100 RCX: ffffffff8b7f2abf
RDX: ffff88801e6ea440 RSI: ffffffff8b7f2aca RDI: 0000000000000005
RBP: 0000000000000000 R08: 0000000000000005 R09: 0000000000000007
R10: 0000000000000004 R11: 0000000000002c10 R12: ffff88802ba69900
R13: 1ffff920000d9e67 R14: ffff888046f81800 R15: 0000000000000004
FS: 0000000000000000(0000) GS:ffff8880d69bc000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000560fc0ca1670 CR3: 0000000032c3a000 CR4: 0000000000352ef0
Call Trace:
<TASK>
tcp_data_queue+0x13b0/0x4f90 net/ipv4/tcp_input.c:5197
tcp_rcv_state_process+0xfdf/0x4ec0 net/ipv4/tcp_input.c:6922
tcp_v6_do_rcv+0x492/0x1740 net/ipv6/tcp_ipv6.c:1672
tcp_v6_rcv+0x2976/0x41e0 net/ipv6/tcp_ipv6.c:1918
ip6_protocol_deliver_rcu+0x188/0x1520 net/ipv6/ip6_input.c:438
ip6_input_finish+0x1e4/0x4b0 net/ipv6/ip6_input.c:489
NF_HOOK include/linux/netfilter.h:318 [inline]
NF_HOOK include/linux/netfilter.h:312 [inline]
ip6_input+0x105/0x2f0 net/ipv6/ip6_input.c:500
dst_input include/net/dst.h:471 [inline]
ip6_rcv_finish net/ipv6/ip6_input.c:79 [inline]
NF_HOOK include/linux/netfilter.h:318 [inline]
NF_HOOK include/linux/netfilter.h:312 [inline]
ipv6_rcv+0x264/0x650 net/ipv6/ip6_input.c:311
__netif_receive_skb_one_core+0x12d/0x1e0 net/core/dev.c:5979
__netif_receive_skb+0x1d/0x160 net/core/dev.c:6092
process_backlog+0x442/0x15e0 net/core/dev.c:6444
__napi_poll.constprop.0+0xba/0x550 net/core/dev.c:7494
napi_poll net/core/dev.c:7557 [inline]
net_rx_action+0xa9f/0xfe0 net/core/dev.c:7684
handle_softirqs+0x216/0x8e0 kernel/softirq.c:579
run_ksoftirqd kernel/softirq.c:968 [inline]
run_ksoftirqd+0x3a/0x60 kernel/softirq.c:960
smpboot_thread_fn+0x3f7/0xae0 kernel/smpboot.c:160
kthread+0x3c2/0x780 kernel/kthread.c:463
ret_from_fork+0x5d7/0x6f0 arch/x86/kernel/process.c:148
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
</TASK>


---
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[syzbot]

syzbot has found a reproducer for the following issue on:

HEAD commit: 8e621c9a3375 Merge tag 'net-6.18-rc7' of git://git.kernel...
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=12887a12580000
kernel config: https://syzkaller.appspot.com/x/.config?x=1cd7f786c0f5182f
dashboard link: https://syzkaller.appspot.com/bug?extid=0ff6b771b4f7a5bce83b
compiler: gcc (Debian 12.2.0-14+deb12u1) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=133c9a12580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=117a2a12580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/6be75789d60e/disk-8e621c9a.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/62e7a40cfe48/vmlinux-8e621c9a.xz
kernel image: https://storage.googleapis.com/syzbot-assets/3e523caa536d/bzImage-8e621c9a.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+0ff6b7...@syzkaller.appspotmail.com

------------[ cut here ]------------

WARNING: CPU: 0 PID: 15 at net/mptcp/subflow.c:1519 subflow_data_ready+0x40b/0x7c0 net/mptcp/subflow.c:1519
Modules linked in:
CPU: 0 UID: 0 PID: 15 Comm: ksoftirqd/0 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
RIP: 0010:subflow_data_ready+0x40b/0x7c0 net/mptcp/subflow.c:1519
Code: 89 ee e8 88 f8 72 f6 40 84 ed 75 21 e8 9e fd 72 f6 44 89 fe bf 07 00 00 00 e8 d1 f8 72 f6 41 83 ff 07 74 09 e8 86 fd 72 f6 90 <0f> 0b 90 e8 7d fd 72 f6 48 89 df e8 e5 ad ff ff 31 ff 89 c5 89 c6
RSP: 0018:ffffc90000147380 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff888030aa72c0 RCX: ffffffff8b4959ef
RDX: ffff88801d2cbc80 RSI: ffffffff8b4959fa RDI: 0000000000000005

RBP: 0000000000000000 R08: 0000000000000005 R09: 0000000000000007

R10: 0000000000000004 R11: 00000d230000000c R12: ffff888033463c00
R13: 1ffff92000028e70 R14: ffff88802f9bbc00 R15: 0000000000000004
FS: 0000000000000000(0000) GS:ffff888124a0d000(0000) knlGS:0000000000000000

CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033

CR2: 00007f841c04df98 CR3: 0000000078b7e000 CR4: 00000000003526f0
Call Trace:
<TASK>
tcp_data_ready+0x110/0x550 net/ipv4/tcp_input.c:5355
tcp_data_queue+0x1aa6/0x5000 net/ipv4/tcp_input.c:5445
tcp_rcv_state_process+0xfb6/0x6490 net/ipv4/tcp_input.c:7159
tcp_v4_do_rcv+0x68e/0x10a0 net/ipv4/tcp_ipv4.c:1954
tcp_v4_rcv+0x3077/0x4db0 net/ipv4/tcp_ipv4.c:2374
ip_protocol_deliver_rcu+0xba/0x4c0 net/ipv4/ip_input.c:205
ip_local_deliver_finish+0x3f2/0x720 net/ipv4/ip_input.c:239

NF_HOOK include/linux/netfilter.h:318 [inline]
NF_HOOK include/linux/netfilter.h:312 [inline]

ip_local_deliver+0x18e/0x1f0 net/ipv4/ip_input.c:260
dst_input include/net/dst.h:474 [inline]
ip_rcv_finish net/ipv4/ip_input.c:453 [inline]

NF_HOOK include/linux/netfilter.h:318 [inline]
NF_HOOK include/linux/netfilter.h:312 [inline]

ip_rcv+0x2e0/0x600 net/ipv4/ip_input.c:573
__netif_receive_skb_one_core+0x197/0x1e0 net/core/dev.c:6079
__netif_receive_skb+0x1d/0x160 net/core/dev.c:6192
process_backlog+0x439/0x15e0 net/core/dev.c:6544
__napi_poll.constprop.0+0xba/0x550 net/core/dev.c:7594
napi_poll net/core/dev.c:7657 [inline]
net_rx_action+0x97f/0xef0 net/core/dev.c:7784
handle_softirqs+0x219/0x8e0 kernel/softirq.c:622
run_ksoftirqd kernel/softirq.c:1063 [inline]
run_ksoftirqd+0x3a/0x60 kernel/softirq.c:1055
smpboot_thread_fn+0x3f7/0xae0 kernel/smpboot.c:160
kthread+0x3c5/0x780 kernel/kthread.c:463
ret_from_fork+0x675/0x7d0 arch/x86/kernel/process.c:158

ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
</TASK>


---

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.

[Paolo Abeni]

#syz test:
git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git main

I can't repro it locally. Running a debug patch to collect more status
which hopefully should help understanding the race.

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-by: syzbot+0ff6b7...@syzkaller.appspotmail.com
Tested-by: syzbot+0ff6b7...@syzkaller.appspotmail.com

Tested on:

commit: ab084f0b drivers: net: fbnic: Return the true error in..
git tree: git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git main
console output: https://syzkaller.appspot.com/x/log.txt?x=153a1e92580000
kernel config: https://syzkaller.appspot.com/x/.config?x=1cc75ad16015d162

dashboard link: https://syzkaller.appspot.com/bug?extid=0ff6b771b4f7a5bce83b
compiler: gcc (Debian 12.2.0-14+deb12u1) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40

patch: https://syzkaller.appspot.com/x/patch.diff?x=121ba57c580000

Note: testing is done by a robot and is best-effort only.

[Paolo Abeni]

On 11/20/25 9:58 PM, syzbot wrote:

The previous test was supposed to crash and dump some useful information.
I'm not sure if the result means the issue is no more present on net-next,
harder to reproduce or the repro is unstable.

Let's try again, on top of the same tag used to find the repro.

#syz test:
git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
8e621c9a3375

[syzbot]

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
WARNING in subflow_data_ready

MPTCP: fallaback under lock 0 req mpc 1 req mpj 0 state 16 parent state 7 send 0 received 0
------------[ cut here ]------------
WARNING: CPU: 1 PID: 23 at net/mptcp/subflow.c:1527 subflow_data_ready+0x59d/0x980 net/mptcp/subflow.c:1527
Modules linked in:
CPU: 1 UID: 0 PID: 23 Comm: ksoftirqd/1 Not tainted syzkaller #0 PREEMPT(full)

Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025

RIP: 0010:subflow_data_ready+0x59d/0x980 net/mptcp/subflow.c:1527
Code: e0 48 c7 c7 c0 71 ee 8c 44 89 f9 8b 54 24 1c 48 c1 ee 02 45 0f b7 c0 83 e6 01 e8 2e 6c 51 f6 48 8b 7c 24 10 e8 44 ee 16 00 90 <0f> 0b 90 58 5a e8 e9 fb 72 f6 48 89 df e8 51 ac ff ff 31 ff 89 c5
RSP: 0018:ffffc900001d7348 EFLAGS: 00010202
RAX: 0000000000000101 RBX: ffff8880343cd940 RCX: 0000000000000002
RDX: 0000000000000000 RSI: ffffffff8da29344 RDI: ffffffff8bf078c0
RBP: ffff8880312b1538 R08: 0000000000000001 R09: 0000000000000001
R10: ffffffff90824bd7 R11: 0000000000000001 R12: ffff8880312b0c00
R13: 1ffff9200003ae70 R14: 0000000000000004 R15: 0000000000000000
FS: 0000000000000000(0000) GS:ffff888124b0d000(0000) knlGS:0000000000000000

CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033

CR2: 00007f6ced887d58 CR3: 0000000064f00000 CR4: 00000000003526f0

Tested on:

commit: 8e621c9a Merge tag 'net-6.18-rc7' of git://git.kernel...
git tree: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
console output: https://syzkaller.appspot.com/x/log.txt?x=14916f42580000

kernel config: https://syzkaller.appspot.com/x/.config?x=1cd7f786c0f5182f
dashboard link: https://syzkaller.appspot.com/bug?extid=0ff6b771b4f7a5bce83b
compiler: gcc (Debian 12.2.0-14+deb12u1) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40

patch: https://syzkaller.appspot.com/x/patch.diff?x=158f657c580000