[v6.1] general protection fault in fq_codel_enqueue

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: bec0e10ee67e Linux 6.1.160
git tree: linux-6.1.y
console output: https://syzkaller.appspot.com/x/log.txt?x=16628dfa580000
kernel config: https://syzkaller.appspot.com/x/.config?x=6a5cbd268406bf43
dashboard link: https://syzkaller.appspot.com/bug?extid=2f101af2c24f14e89b93
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/ddf82d415346/disk-bec0e10e.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/11536559ad07/vmlinux-bec0e10e.xz
kernel image: https://storage.googleapis.com/syzbot-assets/fba42d912db2/bzImage-bec0e10e.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+2f101a...@syzkaller.appspotmail.com

general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
CPU: 1 PID: 6952 Comm: syz.3.956 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
RIP: 0010:dequeue_head net/sched/sch_fq_codel.c:120 [inline]
RIP: 0010:fq_codel_drop net/sched/sch_fq_codel.c:168 [inline]
RIP: 0010:fq_codel_enqueue+0xd2d/0x1590 net/sched/sch_fq_codel.c:230
Code: 24 08 48 8b 44 24 20 80 3c 10 00 74 12 4c 89 ef e8 18 ca 88 f9 48 ba 00 00 00 00 00 fc ff df 4d 8b 65 00 4c 89 e0 48 c1 e8 03 <80> 3c 10 00 74 12 4c 89 e7 e8 f5 c9 88 f9 48 ba 00 00 00 00 00 fc
RSP: 0018:ffffc90005ddf380 EFLAGS: 00010246
RAX: 0000000000000000 RBX: 1ffff92000bbbec0 RCX: 1ffff1100a93695a
RDX: dffffc0000000000 RSI: 0000000000000f7a RDI: 0000000000000f7b
RBP: ffffc90005ddf4d0 R08: dffffc0000000000 R09: fffffbfff211967d
R10: fffffbfff211967d R11: 1ffffffff211967c R12: 0000000000000000
R13: ffff88807e6b8000 R14: 0000000000000000 R15: 0000000000000000
FS: 00007f3a957916c0(0000) GS:ffff8880b8f00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000110c33fd8b CR3: 0000000078470000 CR4: 00000000003506e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
qdisc_enqueue include/net/sch_generic.h:816 [inline]
netem_dequeue+0xcfc/0x13c0 net/sched/sch_netem.c:737
dequeue_skb net/sched/sch_generic.c:295 [inline]
qdisc_restart net/sched/sch_generic.c:400 [inline]
__qdisc_run+0x24b/0x1560 net/sched/sch_generic.c:418
__dev_xmit_skb net/core/dev.c:3970 [inline]
__dev_queue_xmit+0x107e/0x3760 net/core/dev.c:4312
packet_snd net/packet/af_packet.c:3127 [inline]
packet_sendmsg+0x3be8/0x4ea0 net/packet/af_packet.c:3158
sock_sendmsg_nosec net/socket.c:718 [inline]
__sock_sendmsg net/socket.c:730 [inline]
__sys_sendto+0x44f/0x5c0 net/socket.c:2152
__do_sys_sendto net/socket.c:2164 [inline]
__se_sys_sendto net/socket.c:2160 [inline]
__x64_sys_sendto+0xda/0xf0 net/socket.c:2160
do_syscall_x64 arch/x86/entry/common.c:46 [inline]
do_syscall_64+0x4c/0xa0 arch/x86/entry/common.c:76
entry_SYSCALL_64_after_hwframe+0x68/0xd2
RIP: 0033:0x7f3a9498f749
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 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 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f3a95791038 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
RAX: ffffffffffffffda RBX: 00007f3a94be5fa0 RCX: 00007f3a9498f749
RDX: 000000000000000e RSI: 0000200000000240 RDI: 0000000000000009
RBP: 00007f3a94a13f91 R08: 00002000000001c0 R09: 0000000000000014
R10: 0000000000000040 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f3a94be6038 R14: 00007f3a94be5fa0 R15: 00007ffd2de588c8
</TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:dequeue_head net/sched/sch_fq_codel.c:120 [inline]
RIP: 0010:fq_codel_drop net/sched/sch_fq_codel.c:168 [inline]
RIP: 0010:fq_codel_enqueue+0xd2d/0x1590 net/sched/sch_fq_codel.c:230
Code: 24 08 48 8b 44 24 20 80 3c 10 00 74 12 4c 89 ef e8 18 ca 88 f9 48 ba 00 00 00 00 00 fc ff df 4d 8b 65 00 4c 89 e0 48 c1 e8 03 <80> 3c 10 00 74 12 4c 89 e7 e8 f5 c9 88 f9 48 ba 00 00 00 00 00 fc
RSP: 0018:ffffc90005ddf380 EFLAGS: 00010246
RAX: 0000000000000000 RBX: 1ffff92000bbbec0 RCX: 1ffff1100a93695a
RDX: dffffc0000000000 RSI: 0000000000000f7a RDI: 0000000000000f7b
RBP: ffffc90005ddf4d0 R08: dffffc0000000000 R09: fffffbfff211967d
R10: fffffbfff211967d R11: 1ffffffff211967c R12: 0000000000000000
R13: ffff88807e6b8000 R14: 0000000000000000 R15: 0000000000000000
FS: 00007f3a957916c0(0000) GS:ffff8880b8f00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000110c33fd8b CR3: 0000000078470000 CR4: 00000000003506e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
----------------
Code disassembly (best guess):
0: 24 08 and $0x8,%al
2: 48 8b 44 24 20 mov 0x20(%rsp),%rax
7: 80 3c 10 00 cmpb $0x0,(%rax,%rdx,1)
b: 74 12 je 0x1f
d: 4c 89 ef mov %r13,%rdi
10: e8 18 ca 88 f9 call 0xf988ca2d
15: 48 ba 00 00 00 00 00 movabs $0xdffffc0000000000,%rdx
1c: fc ff df
1f: 4d 8b 65 00 mov 0x0(%r13),%r12
23: 4c 89 e0 mov %r12,%rax
26: 48 c1 e8 03 shr $0x3,%rax
* 2a: 80 3c 10 00 cmpb $0x0,(%rax,%rdx,1) <-- trapping instruction
2e: 74 12 je 0x42
30: 4c 89 e7 mov %r12,%rdi
33: e8 f5 c9 88 f9 call 0xf988ca2d
38: 48 rex.W
39: ba 00 00 00 00 mov $0x0,%edx
3e: 00 fc add %bh,%ah


---
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 syzk...@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