[syzbot] [netfilter?] WARNING in nf_flow_table_extend_ct_timeout

[syzbot] [netfilter?] WARNING in nf_flow_table_extend_ct_timeout

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    50d05c7c76c9 Merge tag 'landlock-7.3-rc3' of git://git.ker..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=14ac428e580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=5e4e3a0e188a497e
dashboard link: https://syzkaller.appspot.com/bug?extid=521f7e7e2b974abde99a
compiler:       Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
userspace arch: i386

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/daf497619736/disk-50d05c7c.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/748fbd82f647/vmlinux-50d05c7c.xz
kernel image: https://storage.googleapis.com/syzbot-assets/e61c59099d07/bzImage-50d05c7c.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+521f7e7e2b974abde99a@syzkaller.appspotmail.com

------------[ cut here ]------------
1
WARNING: net/netfilter/nf_flow_table_core.c:546 at nf_flow_table_extend_ct_timeout+0x1ee/0x2f0 net/netfilter/nf_flow_table_core.c:546, CPU#0: kworker/0:2/3335
Modules linked in:
CPU: 0 UID: 0 PID: 3335 Comm: kworker/0:2 Tainted: G             L      syzkaller #0 PREEMPT(full) 
Tainted: [L]=SOFTLOCKUP
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/08/2026
Workqueue: events_power_efficient nf_flow_offload_work_gc
RIP: 0010:nf_flow_table_extend_ct_timeout+0x1ee/0x2f0 net/netfilter/nf_flow_table_core.c:546
Code: 00 00 00 41 83 ff 06 0f 84 9a 00 00 00 41 83 ff 05 0f 85 b0 00 00 00 e8 70 d1 ab f7 41 bf 30 75 00 00 eb 0f e8 63 d1 ab f7 90 <0f> 0b 90 41 bf 01 00 00 00 48 c7 c0 80 f2 a0 8e 48 c1 e8 03 42 80
RSP: 0000:ffffc900040ff938 EFLAGS: 00010293
RAX: ffffffff8a1bf92d RBX: ffff88807da5be00 RCX: ffff88803247ddc0
RDX: 0000000000000000 RSI: ffffffff902f6960 RDI: 000000000000002f
RBP: 0000000000039e60 R08: ffff88803247ddc0 R09: 0000000000000002
R10: 0000000000000011 R11: 0000000000000000 R12: dffffc0000000000
R13: 1ffff1100fb4b7d8 R14: ffff88807da5be48 R15: 000000000000002f
FS:  0000000000000000(0000) GS:ffff888124cd2000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000f7238c10 CR3: 000000007a894000 CR4: 00000000003526f0
DR0: ffffffffffffffff DR1: 00000000000001f7 DR2: 00000000ffffffff
DR3: ffffffffefffff15 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 nf_flow_offload_gc_step+0x545/0x7b0 net/netfilter/nf_flow_table_core.c:581
 nf_flow_table_iterate+0x13c/0x1f0 net/netfilter/nf_flow_table_core.c:452
 nf_flow_table_gc_run net/netfilter/nf_flow_table_core.c:604 [inline]
 nf_flow_offload_work_gc+0x22/0x70 net/netfilter/nf_flow_table_core.c:612
 process_one_work kernel/workqueue.c:3396 [inline]
 process_scheduled_works+0xc3d/0x1630 kernel/workqueue.c:3479
 worker_thread+0xa47/0xfb0 kernel/workqueue.c:3560
 kthread+0x38b/0x480 kernel/kthread.c:436
 ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </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@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want to overwrite report's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup

Re: [syzbot] [netfilter?] WARNING in nf_flow_table_extend_ct_timeout

[syzbot]

syzbot has found a reproducer for the following issue on:

HEAD commit:    abeacca61eea Merge branch 'net-mlx5-hws-fix-modify-header-..
git tree:       net
console+strace: https://syzkaller.appspot.com/x/log.txt?x=111f4d0d580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=c4b098695335c89f
dashboard link: https://syzkaller.appspot.com/bug?extid=521f7e7e2b974abde99a
compiler:       Debian clang version 22.1.8 (1~deb13u4), Debian LLD 22.1.8
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1491e029580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=157d410d580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/22930d0e9391/disk-abeacca6.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/d2e18d51f288/vmlinux-abeacca6.xz
kernel image: https://storage.googleapis.com/syzbot-assets/69879bb16225/bzImage-abeacca6.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+521f7e7e2b974abde99a@syzkaller.appspotmail.com

------------[ cut here ]------------
1
WARNING: net/netfilter/nf_flow_table_core.c:553 at nf_flow_table_extend_ct_timeout+0x1ee/0x2f0 net/netfilter/nf_flow_table_core.c:553, CPU#1: kworker/1:2/123
Modules linked in:
CPU: 1 UID: 0 PID: 123 Comm: kworker/1:2 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/07/2026
Workqueue: events_power_efficient nf_flow_offload_work_gc
RIP: 0010:nf_flow_table_extend_ct_timeout+0x1ee/0x2f0 net/netfilter/nf_flow_table_core.c:553
Code: 00 00 00 41 83 ff 06 0f 84 9a 00 00 00 41 83 ff 05 0f 85 b0 00 00 00 e8 e0 10 a9 f7 41 bf 30 75 00 00 eb 0f e8 d3 10 a9 f7 90 <0f> 0b 90 41 bf 01 00 00 00 48 c7 c0 80 f2 a0 8f 48 c1 e8 03 42 80
RSP: 0018:ffffc90002dcf930 EFLAGS: 00010293
RAX: ffffffff8a1effad RBX: ffff888027a52900 RCX: ffff88801ffa1f40
RDX: 0000000000000000 RSI: ffffffff912fa640 RDI: 000000000000002f
RBP: 0000000000000737 R08: ffff88801ffa1f40 R09: 0000000000000002
R10: 0000000000000011 R11: 0000000000000000 R12: dffffc0000000000
R13: 1ffff11004f4a538 R14: ffff888027a52948 R15: 000000000000002f
FS:  0000000000000000(0000) GS:ffff888123bbd000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f926bd5bd3c CR3: 000000000fb48000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 nf_flow_offload_gc_step+0x5aa/0x840 net/netfilter/nf_flow_table_core.c:593
 nf_flow_table_iterate+0x13c/0x1f0 net/netfilter/nf_flow_table_core.c:459
 nf_flow_table_gc_run net/netfilter/nf_flow_table_core.c:616 [inline]
 nf_flow_offload_work_gc+0x22/0x70 net/netfilter/nf_flow_table_core.c:624
 process_one_work kernel/workqueue.c:3396 [inline]
 process_scheduled_works+0xc3d/0x1630 kernel/workqueue.c:3479
 worker_thread+0xa47/0xfb0 kernel/workqueue.c:3560
 kthread+0x38b/0x480 kernel/kthread.c:436
 ret_from_fork+0x514/0xb70 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.

Re: [syzbot] [netfilter?] WARNING in nf_flow_table_extend_ct_timeout

[mixalhs]

[-- Attachment #1: Type: text/plain, Size: 3531 bytes --]

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

On Thu, Oct 22, 2026 at 6:44 PM syzbot
<syzbot+521f7e7e2b974abde99a@syzkaller.appspotmail.com> wrote:
>
> syzbot has found a reproducer for the following issue on:
>
> HEAD commit:    abeacca61eea Merge branch 'net-mlx5-hws-fix-modify-header-..
> git tree:       net
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=111f4d0d580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=c4b098695335c89f
> dashboard link: https://syzkaller.appspot.com/bug?extid=521f7e7e2b974abde99a
> compiler:       Debian clang version 22.1.8 (1~deb13u4), Debian LLD 22.1.8
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1491e029580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=157d410d580000
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/22930d0e9391/disk-abeacca6.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/d2e18d51f288/vmlinux-abeacca6.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/69879bb16225/bzImage-abeacca6.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+521f7e7e2b974abde99a@syzkaller.appspotmail.com
>
> ------------[ cut here ]------------
> 1
> WARNING: net/netfilter/nf_flow_table_core.c:553 at nf_flow_table_extend_ct_timeout+0x1ee/0x2f0 net/netfilter/nf_flow_table_core.c:553, CPU#1: kworker/1:2/123
> Modules linked in:
> CPU: 1 UID: 0 PID: 123 Comm: kworker/1:2 Not tainted syzkaller #0 PREEMPT(full)
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/07/2026
> Workqueue: events_power_efficient nf_flow_offload_work_gc
> RIP: 0010:nf_flow_table_extend_ct_timeout+0x1ee/0x2f0 net/netfilter/nf_flow_table_core.c:553
> Code: 00 00 00 41 83 ff 06 0f 84 9a 00 00 00 41 83 ff 05 0f 85 b0 00 00 00 e8 e0 10 a9 f7 41 bf 30 75 00 00 eb 0f e8 d3 10 a9 f7 90 <0f> 0b 90 41 bf 01 00 00 00 48 c7 c0 80 f2 a0 8f 48 c1 e8 03 42 80
> RSP: 0018:ffffc90002dcf930 EFLAGS: 00010293
> RAX: ffffffff8a1effad RBX: ffff888027a52900 RCX: ffff88801ffa1f40
> RDX: 0000000000000000 RSI: ffffffff912fa640 RDI: 000000000000002f
> RBP: 0000000000000737 R08: ffff88801ffa1f40 R09: 0000000000000002
> R10: 0000000000000011 R11: 0000000000000000 R12: dffffc0000000000
> R13: 1ffff11004f4a538 R14: ffff888027a52948 R15: 000000000000002f
> FS:  0000000000000000(0000) GS:ffff888123bbd000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f926bd5bd3c CR3: 000000000fb48000 CR4: 00000000003526f0
> Call Trace:
>  <TASK>
>  nf_flow_offload_gc_step+0x5aa/0x840 net/netfilter/nf_flow_table_core.c:593
>  nf_flow_table_iterate+0x13c/0x1f0 net/netfilter/nf_flow_table_core.c:459
>  nf_flow_table_gc_run net/netfilter/nf_flow_table_core.c:616 [inline]
>  nf_flow_offload_work_gc+0x22/0x70 net/netfilter/nf_flow_table_core.c:624
>  process_one_work kernel/workqueue.c:3396 [inline]
>  process_scheduled_works+0xc3d/0x1630 kernel/workqueue.c:3479
>  worker_thread+0xa47/0xfb0 kernel/workqueue.c:3560
>  kthread+0x38b/0x480 kernel/kthread.c:436
>  ret_from_fork+0x514/0xb70 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.

[-- Attachment #2: 0001-netfilter-flowtable-extend-ct-timeout-of-offloaded-G.patch --]
[-- Type: application/octet-stream, Size: 3049 bytes --]

From: Michail Andreas Michalopoulos <mike.michalo@gmail.com>
Subject: [PATCH nf] netfilter: flowtable: extend ct timeout of offloaded GRE flows

Since commit 03428ca5cee9 ("netfilter: conntrack: rework offload nf_conn
timeout extension logic"), the flowtable gc worker extends the timeout
of an offloaded conntrack entry from nf_flow_table_extend_ct_timeout()
once it falls below 5 minutes. That function only knows about TCP and
UDP and hits WARN_ON_ONCE() for any other layer 4 protocol. However,
GRE flows can be offloaded too, both via nft_flow_offload and act_ct,
since commit 4e8d9584d154 ("netfilter: flowtable: Support GRE").

The timeout of an offloaded GRE entry drops below that threshold as soon
as a packet of the flow still goes through conntrack. With ip6gre
tunnels using the default encapsulation limit this is ordinary traffic:
the flowtable does not parse the Destination Options header in front of
GRE, so every packet of the flow takes the classic forwarding path. It
also happens for a packet with ttl <= 1, which the flowtable fast path
passes up to the classic forwarding path. nf_conntrack_gre_packet() then
refreshes ct->timeout to the GRE stream timeout (180 seconds by default),
and the next gc run triggers, as reported by syzbot:

  WARNING: net/netfilter/nf_flow_table_core.c:553 at nf_flow_table_extend_ct_timeout+0x1ee/0x2f0
  Workqueue: events_power_efficient nf_flow_offload_work_gc
  Call Trace:
   nf_flow_offload_gc_step+0x545/0x7b0
   nf_flow_table_iterate+0x13c/0x1f0
   nf_flow_offload_work_gc+0x22/0x70
   process_scheduled_works+0xc3d/0x1630
   worker_thread+0xa47/0xfb0
   kthread+0x38b/0x480

Besides the splat, which panics the machine with panic_on_warn, the
default branch sets the new timeout to 1 jiffy, so the conntrack entry
expires right away and the offloaded flow is torn down.

Handle GRE like UDP and extend the timeout to NF_CT_DAY. GRE conntrack
has no protocol state to take into account, and NF_CT_DAY is also what
flow_offload_add() already sets for every offloaded entry, GRE included.
This also restores the behaviour from before commit 03428ca5cee9, when
the conntrack gc worker extended offloaded entries of any protocol to
NF_CT_DAY.

Fixes: 03428ca5cee9 ("netfilter: conntrack: rework offload nf_conn timeout extension logic")
Reported-by: syzbot+521f7e7e2b974abde99a@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=521f7e7e2b974abde99a
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Michail Andreas Michalopoulos <mike.michalo@gmail.com>
---
 net/netfilter/nf_flow_table_core.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/netfilter/nf_flow_table_core.c b/net/netfilter/nf_flow_table_core.c
index 36bbc7be2f..94502c97ba 100644
--- a/net/netfilter/nf_flow_table_core.c
+++ b/net/netfilter/nf_flow_table_core.c
@@ -544,6 +544,7 @@ static void nf_flow_table_extend_ct_timeout(struct nf_conn *ct)
 
 		switch (l4proto) {
 		case IPPROTO_UDP:
+		case IPPROTO_GRE:
 			new_timeout = NF_CT_DAY;
 			break;
 		case IPPROTO_TCP:
-- 
2.43.0

Re: [syzbot] [netfilter?] WARNING in nf_flow_table_extend_ct_timeout

[syzbot]

Hello,

syzbot tried to test the proposed patch but the build/boot failed:

failed to checkout kernel repo git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git/main: failed to run ["git" "-c" "core.hooksPath=/dev/null" "fetch" "--force" "675adff2843877c2da27b36b2517f827bc9915ea" "main"]: exit status 128


Tested on:

commit:         [unknown 
git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git main
kernel config:  https://syzkaller.appspot.com/x/.config?x=c4b098695335c89f
dashboard link: https://syzkaller.appspot.com/bug?extid=521f7e7e2b974abde99a
compiler:       
patch:          https://syzkaller.appspot.com/x/patch.diff?x=115f4d0d580000

Forwarded: Re: [syzbot] [netfilter?] WARNING in nf_flow_table_extend_ct_timeout

[syzbot]

For archival purposes, forwarding an incoming command email to
linux-kernel@vger.kernel.org.

***

Subject: Re: [syzbot] [netfilter?] WARNING in nf_flow_table_extend_ct_timeout
Author: mike.michalo@gmail.com

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

Forwarded: Re: [syzbot] [netfilter?] WARNING in nf_flow_table_extend_ct_timeout

[syzbot]

For archival purposes, forwarding an incoming command email to
linux-kernel@vger.kernel.org.

***

Subject: Re: [syzbot] [netfilter?] WARNING in nf_flow_table_extend_ct_timeout
Author: mike.michalo@gmail.com

#syz test: https://kernel.googlesource.com/pub/scm/linux/kernel/git/netdev/net
main