[syzbot] [net?] general protection fault in qdisc_tree_reduce_backlog

[syzbot <syzbot+1261670bbdefc5485a06@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    bd475eeaaf3c Merge branch '200GbE' of git://git.kernel.org..
git tree:       net
console+strace: https://syzkaller.appspot.com/x/log.txt?x=12f0b3d4580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=36b0e72cad5298f8
dashboard link: https://syzkaller.appspot.com/bug?extid=1261670bbdefc5485a06
compiler:       Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=164d8c8c580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=14839ebc580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/d59bc82a55e0/disk-bd475eea.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/2a83759fceb6/vmlinux-bd475eea.xz
kernel image: https://storage.googleapis.com/syzbot-assets/07576fd8e432/bzImage-bd475eea.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+1261670bbdefc5485a06@syzkaller.appspotmail.com

Oops: general protection fault, probably for non-canonical address 0xdffffc0000000004: 0000 [#1] SMP KASAN PTI
KASAN: null-ptr-deref in range [0x0000000000000020-0x0000000000000027]
CPU: 0 UID: 0 PID: 5835 Comm: syz-executor374 Not tainted 6.16.0-rc3-syzkaller-00144-gbd475eeaaf3c #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
RIP: 0010:qdisc_tree_reduce_backlog+0x223/0x480 net/sched/sch_api.c:806
Code: 89 ef e8 40 80 b3 f8 4d 89 ef 85 db 74 0d e8 74 fc 4f f8 4c 89 f5 e9 88 00 00 00 48 8b 6d 00 48 8d 45 20 48 89 c3 48 c1 eb 03 <42> 80 3c 33 00 48 89 04 24 74 0d 48 8b 3c 24 e8 09 80 b3 f8 48 8b
RSP: 0018:ffffc900040bf0a8 EFLAGS: 00010202
RAX: 0000000000000020 RBX: 0000000000000004 RCX: 0000000000000000
RDX: ffff888031da3c00 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000000 R08: ffff888031da3c00 R09: 0000000000000002
R10: 00000000ffffffff R11: 0000000000000000 R12: 00000000000afff2
R13: ffff88802875a000 R14: dffffc0000000000 R15: ffff88802875a000
FS:  000055555b6b1380(0000) GS:ffff888125c50000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000200000000240 CR3: 00000000726c2000 CR4: 00000000003526f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 fq_codel_change+0xa96/0xef0 net/sched/sch_fq_codel.c:450
 fq_codel_init+0x355/0x960 net/sched/sch_fq_codel.c:487
 qdisc_create+0x7ac/0xea0 net/sched/sch_api.c:1324
 __tc_modify_qdisc net/sched/sch_api.c:1749 [inline]
 tc_modify_qdisc+0x1426/0x2010 net/sched/sch_api.c:1813
 rtnetlink_rcv_msg+0x779/0xb70 net/core/rtnetlink.c:6953
 netlink_rcv_skb+0x208/0x470 net/netlink/af_netlink.c:2534
 netlink_unicast_kernel net/netlink/af_netlink.c:1313 [inline]
 netlink_unicast+0x75b/0x8d0 net/netlink/af_netlink.c:1339
 netlink_sendmsg+0x805/0xb30 net/netlink/af_netlink.c:1883
 sock_sendmsg_nosec net/socket.c:712 [inline]
 __sock_sendmsg+0x21c/0x270 net/socket.c:727
 ____sys_sendmsg+0x505/0x830 net/socket.c:2566
 ___sys_sendmsg+0x21f/0x2a0 net/socket.c:2620
 __sys_sendmsg net/socket.c:2652 [inline]
 __do_sys_sendmsg net/socket.c:2657 [inline]
 __se_sys_sendmsg net/socket.c:2655 [inline]
 __x64_sys_sendmsg+0x19b/0x260 net/socket.c:2655
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f79d710a6a9
Code: 48 83 c4 28 c3 e8 37 17 00 00 0f 1f 80 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 b8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fff2ee70668 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00007fff2ee70838 RCX: 00007f79d710a6a9
RDX: 0000000000000800 RSI: 0000200000000100 RDI: 0000000000000003
RBP: 00007f79d717d610 R08: 0000000000000004 R09: 00007fff2ee70838
R10: 0000000000000001 R11: 0000000000000246 R12: 0000000000000001
R13: 00007fff2ee70828 R14: 0000000000000001 R15: 0000000000000001
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:qdisc_tree_reduce_backlog+0x223/0x480 net/sched/sch_api.c:806
Code: 89 ef e8 40 80 b3 f8 4d 89 ef 85 db 74 0d e8 74 fc 4f f8 4c 89 f5 e9 88 00 00 00 48 8b 6d 00 48 8d 45 20 48 89 c3 48 c1 eb 03 <42> 80 3c 33 00 48 89 04 24 74 0d 48 8b 3c 24 e8 09 80 b3 f8 48 8b
RSP: 0018:ffffc900040bf0a8 EFLAGS: 00010202
RAX: 0000000000000020 RBX: 0000000000000004 RCX: 0000000000000000
RDX: ffff888031da3c00 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000000 R08: ffff888031da3c00 R09: 0000000000000002
R10: 00000000ffffffff R11: 0000000000000000 R12: 00000000000afff2
R13: ffff88802875a000 R14: dffffc0000000000 R15: ffff88802875a000
FS:  000055555b6b1380(0000) GS:ffff888125c50000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000200000000240 CR3: 00000000726c2000 CR4: 00000000003526f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
----------------
Code disassembly (best guess):
   0:	89 ef                	mov    %ebp,%edi
   2:	e8 40 80 b3 f8       	call   0xf8b38047
   7:	4d 89 ef             	mov    %r13,%r15
   a:	85 db                	test   %ebx,%ebx
   c:	74 0d                	je     0x1b
   e:	e8 74 fc 4f f8       	call   0xf84ffc87
  13:	4c 89 f5             	mov    %r14,%rbp
  16:	e9 88 00 00 00       	jmp    0xa3
  1b:	48 8b 6d 00          	mov    0x0(%rbp),%rbp
  1f:	48 8d 45 20          	lea    0x20(%rbp),%rax
  23:	48 89 c3             	mov    %rax,%rbx
  26:	48 c1 eb 03          	shr    $0x3,%rbx
* 2a:	42 80 3c 33 00       	cmpb   $0x0,(%rbx,%r14,1) <-- trapping instruction
  2f:	48 89 04 24          	mov    %rax,(%rsp)
  33:	74 0d                	je     0x42
  35:	48 8b 3c 24          	mov    (%rsp),%rdi
  39:	e8 09 80 b3 f8       	call   0xf8b38047
  3e:	48                   	rex.W
  3f:	8b                   	.byte 0x8b


---
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