[Android 6.1] general protection fault in qdisc_tree_reduce_backlog

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 82186ba31010 ANDROID: vendor_hooks: Add hooks in reweight_..
git tree: android14-6.1
console+strace: https://syzkaller.appspot.com/x/log.txt?x=100c5842580000
kernel config: https://syzkaller.appspot.com/x/.config?x=d0b51d1a351532ed
dashboard link: https://syzkaller.appspot.com/bug?extid=3abd3513dd6eb316dbf3
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=16a1e5a2580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=16c3bc34580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/9475b5728a93/disk-82186ba3.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/9e7dcc6be5c3/vmlinux-82186ba3.xz
kernel image: https://storage.googleapis.com/syzbot-assets/fd06a664af96/bzImage-82186ba3.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+3abd35...@syzkaller.appspotmail.com

general protection fault, probably for non-canonical address 0xdffffc0000000004: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x0000000000000020-0x0000000000000027]
CPU: 0 PID: 290 Comm: syz-executor423 Not tainted 6.1.145-syzkaller-00004-g82186ba31010 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
RIP: 0010:qdisc_tree_reduce_backlog+0x181/0x360 net/sched/sch_api.c:803
Code: e8 24 b3 b4 fd 4d 89 ef 83 7d d0 00 74 0a e8 c6 4f 70 fd e9 a2 00 00 00 4d 8b 36 49 8d 46 20 48 89 c1 48 c1 e9 03 48 89 4d b8 <80> 3c 19 00 48 89 45 d0 74 0d 48 8b 7d d0 e8 ec b2 b4 fd 48 8b 45
RSP: 0018:ffffc90000e36e68 EFLAGS: 00010202
RAX: 0000000000000020 RBX: dffffc0000000000 RCX: 0000000000000004
RDX: ffff88811218a880 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffffc90000e36eb0 R08: ffff88811218a880 R09: 0000000000000002
R10: 00000000ffffffff R11: 0000000000000000 R12: 00000000000afff2
R13: ffff8881115ef800 R14: 0000000000000000 R15: ffff8881115ef800
FS: 0000555584a72380(0000) GS:ffff8881f7000000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f856a9e8fe0 CR3: 00000001221db000 CR4: 00000000003506b0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
codel_change+0x8b8/0xbf0 net/sched/sch_codel.c:177
codel_init+0x1ea/0x410 net/sched/sch_codel.c:196
qdisc_create+0x7f3/0x1040 net/sched/sch_api.c:1311
tc_modify_qdisc+0x8ea/0x1900 net/sched/sch_api.c:-1
rtnetlink_rcv_msg+0x9f4/0xcf0 net/core/rtnetlink.c:6153
netlink_rcv_skb+0x1f2/0x440 net/netlink/af_netlink.c:2503
rtnetlink_rcv+0x1c/0x20 net/core/rtnetlink.c:6171
netlink_unicast_kernel net/netlink/af_netlink.c:1312 [inline]
netlink_unicast+0x8b1/0xa30 net/netlink/af_netlink.c:1338
netlink_sendmsg+0x8aa/0xbc0 net/netlink/af_netlink.c:1860
sock_sendmsg_nosec net/socket.c:716 [inline]
__sock_sendmsg net/socket.c:728 [inline]
____sys_sendmsg+0x5a9/0x990 net/socket.c:2509
___sys_sendmsg+0x21c/0x290 net/socket.c:2563
__sys_sendmsg net/socket.c:2592 [inline]
__do_sys_sendmsg net/socket.c:2601 [inline]
__se_sys_sendmsg net/socket.c:2599 [inline]
__x64_sys_sendmsg+0x1f0/0x2c0 net/socket.c:2599
x64_sys_call+0x171/0x9a0 arch/x86/include/generated/asm/syscalls_64.h:47
do_syscall_x64 arch/x86/entry/common.c:51 [inline]
do_syscall_64+0x4c/0xa0 arch/x86/entry/common.c:81
entry_SYSCALL_64_after_hwframe+0x68/0xd2
RIP: 0033:0x7f856a9b05e9
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:00007fffa8236df8 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00007fffa8236fc8 RCX: 00007f856a9b05e9
RDX: 0000000000000800 RSI: 0000200000000100 RDI: 0000000000000003
RBP: 00007f856aa23610 R08: 000000000000000a R09: 00007fffa8236fc8
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
R13: 00007fffa8236fb8 R14: 0000000000000001 R15: 0000000000000001
</TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:qdisc_tree_reduce_backlog+0x181/0x360 net/sched/sch_api.c:803
Code: e8 24 b3 b4 fd 4d 89 ef 83 7d d0 00 74 0a e8 c6 4f 70 fd e9 a2 00 00 00 4d 8b 36 49 8d 46 20 48 89 c1 48 c1 e9 03 48 89 4d b8 <80> 3c 19 00 48 89 45 d0 74 0d 48 8b 7d d0 e8 ec b2 b4 fd 48 8b 45
RSP: 0018:ffffc90000e36e68 EFLAGS: 00010202
RAX: 0000000000000020 RBX: dffffc0000000000 RCX: 0000000000000004
RDX: ffff88811218a880 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffffc90000e36eb0 R08: ffff88811218a880 R09: 0000000000000002
R10: 00000000ffffffff R11: 0000000000000000 R12: 00000000000afff2
R13: ffff8881115ef800 R14: 0000000000000000 R15: ffff8881115ef800
FS: 0000555584a72380(0000) GS:ffff8881f7000000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f856a9e8fe0 CR3: 00000001221db000 CR4: 00000000003506b0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
----------------
Code disassembly (best guess):
0: e8 24 b3 b4 fd call 0xfdb4b329
5: 4d 89 ef mov %r13,%r15
8: 83 7d d0 00 cmpl $0x0,-0x30(%rbp)
c: 74 0a je 0x18
e: e8 c6 4f 70 fd call 0xfd704fd9
13: e9 a2 00 00 00 jmp 0xba
18: 4d 8b 36 mov (%r14),%r14
1b: 49 8d 46 20 lea 0x20(%r14),%rax
1f: 48 89 c1 mov %rax,%rcx
22: 48 c1 e9 03 shr $0x3,%rcx
26: 48 89 4d b8 mov %rcx,-0x48(%rbp)
* 2a: 80 3c 19 00 cmpb $0x0,(%rcx,%rbx,1) <-- trapping instruction
2e: 48 89 45 d0 mov %rax,-0x30(%rbp)
32: 74 0d je 0x41
34: 48 8b 7d d0 mov -0x30(%rbp),%rdi
38: e8 ec b2 b4 fd call 0xfdb4b329
3d: 48 rex.W
3e: 8b .byte 0x8b
3f: 45 rex.RB


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