[v5.15] stack segment fault in qdisc_tree_reduce_backlog

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 2f693b607545 Linux 5.15.187
git tree: linux-5.15.y
console output: https://syzkaller.appspot.com/x/log.txt?x=157f9bd4580000
kernel config: https://syzkaller.appspot.com/x/.config?x=c91c2d91ea1a4388
dashboard link: https://syzkaller.appspot.com/bug?extid=c64b45dfb7f143955076
compiler: Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/7769e50dbae7/disk-2f693b60.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/ca8639ca4ec3/vmlinux-2f693b60.xz
kernel image: https://storage.googleapis.com/syzbot-assets/34e626fbb90f/bzImage-2f693b60.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+c64b45...@syzkaller.appspotmail.com

stack segment: 0000 [#1] PREEMPT SMP KASAN
CPU: 0 PID: 5332 Comm: syz.2.248 Not tainted 5.15.187-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
RIP: 0010:qdisc_tree_reduce_backlog+0x1f9/0x430 net/sched/sch_api.c:794
Code: 80 3c 28 00 74 08 48 89 df e8 13 2a c8 f9 4d 89 e6 85 ed 74 07 e8 07 b1 82 f9 eb 7b 48 8b 1b 48 8d 43 20 48 89 c5 48 c1 ed 03 <42> 80 7c 2d 00 00 48 89 04 24 74 0d 48 8b 3c 24 e8 e2 29 c8 f9 48
RSP: 0018:ffffc9000393ef70 EFLAGS: 00010202
RAX: 0000000000000020 RBX: 0000000000000000 RCX: ffff888025efbb80
RDX: 0000000000000002 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000004 R08: ffff888025efbb80 R09: 0000000000000002
R10: 00000000ffffffff R11: 0000000000000002 R12: ffff888077561000
R13: dffffc0000000000 R14: ffff888077561000 R15: 00000000000bffe6
FS: 00007fe0d7e126c0(0000) GS:ffff8880b9000000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fbb1e9af2d8 CR3: 000000005ae5a000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
codel_change+0x8d1/0xc00 net/sched/sch_codel.c:183
codel_init+0x190/0x330 net/sched/sch_codel.c:202
qdisc_create+0x7bd/0x1170 net/sched/sch_api.c:1260
tc_modify_qdisc+0xaad/0x16c0 net/sched/sch_api.c:-1
rtnetlink_rcv_msg+0x9b9/0xe60 net/core/rtnetlink.c:5650
netlink_rcv_skb+0x1e0/0x430 net/netlink/af_netlink.c:2489
netlink_unicast_kernel net/netlink/af_netlink.c:1311 [inline]
netlink_unicast+0x77c/0x920 net/netlink/af_netlink.c:1337
netlink_sendmsg+0x8ab/0xbc0 net/netlink/af_netlink.c:1905
sock_sendmsg_nosec net/socket.c:704 [inline]
__sock_sendmsg net/socket.c:716 [inline]
____sys_sendmsg+0x5a2/0x8c0 net/socket.c:2436
___sys_sendmsg+0x1f0/0x260 net/socket.c:2490
__sys_sendmsg net/socket.c:2519 [inline]
__do_sys_sendmsg net/socket.c:2528 [inline]
__se_sys_sendmsg+0x190/0x250 net/socket.c:2526
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x4c/0xa0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x66/0xd0
RIP: 0033:0x7fe0d9fec929
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:00007fe0d7e12038 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00007fe0da214160 RCX: 00007fe0d9fec929
RDX: 0000000004008000 RSI: 0000200000000200 RDI: 0000000000000005
RBP: 00007fe0da06eb39 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000001 R14: 00007fe0da214160 R15: 00007ffcf65208c8
</TASK>
Modules linked in:
---[ end trace bbe66061f9da2a44 ]---
RIP: 0010:qdisc_tree_reduce_backlog+0x1f9/0x430 net/sched/sch_api.c:794
Code: 80 3c 28 00 74 08 48 89 df e8 13 2a c8 f9 4d 89 e6 85 ed 74 07 e8 07 b1 82 f9 eb 7b 48 8b 1b 48 8d 43 20 48 89 c5 48 c1 ed 03 <42> 80 7c 2d 00 00 48 89 04 24 74 0d 48 8b 3c 24 e8 e2 29 c8 f9 48
RSP: 0018:ffffc9000393ef70 EFLAGS: 00010202
RAX: 0000000000000020 RBX: 0000000000000000 RCX: ffff888025efbb80
RDX: 0000000000000002 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000004 R08: ffff888025efbb80 R09: 0000000000000002
R10: 00000000ffffffff R11: 0000000000000002 R12: ffff888077561000
R13: dffffc0000000000 R14: ffff888077561000 R15: 00000000000bffe6
FS: 00007fe0d7e126c0(0000) GS:ffff8880b9000000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fbb1e9af2d8 CR3: 000000005ae5a000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
----------------
Code disassembly (best guess):
0: 80 3c 28 00 cmpb $0x0,(%rax,%rbp,1)
4: 74 08 je 0xe
6: 48 89 df mov %rbx,%rdi
9: e8 13 2a c8 f9 call 0xf9c82a21
e: 4d 89 e6 mov %r12,%r14
11: 85 ed test %ebp,%ebp
13: 74 07 je 0x1c
15: e8 07 b1 82 f9 call 0xf982b121
1a: eb 7b jmp 0x97
1c: 48 8b 1b mov (%rbx),%rbx
1f: 48 8d 43 20 lea 0x20(%rbx),%rax
23: 48 89 c5 mov %rax,%rbp
26: 48 c1 ed 03 shr $0x3,%rbp
* 2a: 42 80 7c 2d 00 00 cmpb $0x0,0x0(%rbp,%r13,1) <-- trapping instruction
30: 48 89 04 24 mov %rax,(%rsp)
34: 74 0d je 0x43
36: 48 8b 3c 24 mov (%rsp),%rdi
3a: e8 e2 29 c8 f9 call 0xf9c82a21
3f: 48 rex.W


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[syzbot]

Auto-closing this bug as obsolete.
Crashes did not happen for a while, no reproducer and no activity.