Date:   Sun, 20 Nov 2022 09:42:02 +0200
From:   Gal Pressman <gal@...dia.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Maxim Mikityanskiy <maximmi@...dia.com>,
        "David S . Miller" <davem@...emloft.net>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
        Cong Wang <xiyou.wangcong@...il.com>, eric.dumazet@...il.com,
        syzbot <syzkaller@...glegroups.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Tariq Toukan <tariqt@...dia.com>,
        Maxim Mikityanskiy <maxtram95@...il.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net] net: sched: fix race condition in qdisc_graft()

On 10/11/2022 11:08, Gal Pressman wrote:
> On 06/11/2022 10:07, Gal Pressman wrote:
>> On 03/11/2022 18:33, Eric Dumazet wrote:
>>> On Thu, Nov 3, 2022 at 9:31 AM Maxim Mikityanskiy <maxtram95@...il.com> wrote:
>>>> On Thu, 3 Nov 2022 at 17:30, Eric Dumazet <edumazet@...gle.com> wrote:
>>>>> On Thu, Nov 3, 2022 at 8:27 AM Jakub Kicinski <kuba@...nel.org> wrote:
>>>>>> On Thu, 3 Nov 2022 08:17:06 -0700 Eric Dumazet wrote:
>>>>>>> On Thu, Nov 3, 2022 at 6:34 AM Gal Pressman <gal@...dia.com> wrote:
>>>>>>>> On 18/10/2022 23:32, Eric Dumazet wrote:
>>>>>>>>> We had one syzbot report [1] in syzbot queue for a while.
>>>>>>>>> I was waiting for more occurrences and/or a repro but
>>>>>>>>> Dmitry Vyukov spotted the issue right away.
>>>>>>>>>
>>>>>>>>> <quoting Dmitry>
>>>>>>>>> qdisc_graft() drops reference to qdisc in notify_and_destroy
>>>>>>>>> while it's still assigned to dev->qdisc
>>>>>>>>> </quoting>
>>>>>>>>>
>>>>>>>>> Indeed, RCU rules are clear when replacing a data structure.
>>>>>>>>> The visible pointer (dev->qdisc in this case) must be updated
>>>>>>>>> to the new object _before_ RCU grace period is started
>>>>>>>>> (qdisc_put(old) in this case).
>>>>>>>>>
>>>>>>>>> [1]
>>>>>>>>> BUG: KASAN: use-after-free in __tcf_qdisc_find.part.0+0xa3a/0xac0 net/sched/cls_api.c:1066
>>>>>>>>> Read of size 4 at addr ffff88802065e038 by task syz-executor.4/21027
>>>>>>>>>
>>>>>>>>> CPU: 0 PID: 21027 Comm: syz-executor.4 Not tainted 6.0.0-rc3-syzkaller-00363-g7726d4c3e60b #0
>>>>>>>>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/26/2022
>>>>>>>>> Call Trace:
>>>>>>>>> <TASK>
>>>>>>>>> __dump_stack lib/dump_stack.c:88 [inline]
>>>>>>>>> dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
>>>>>>>>> print_address_description mm/kasan/report.c:317 [inline]
>>>>>>>>> print_report.cold+0x2ba/0x719 mm/kasan/report.c:433
>>>>>>>>> kasan_report+0xb1/0x1e0 mm/kasan/report.c:495
>>>>>>>>> __tcf_qdisc_find.part.0+0xa3a/0xac0 net/sched/cls_api.c:1066
>>>>>>>>> __tcf_qdisc_find net/sched/cls_api.c:1051 [inline]
>>>>>>>>> tc_new_tfilter+0x34f/0x2200 net/sched/cls_api.c:2018
>>>>>>>>> rtnetlink_rcv_msg+0x955/0xca0 net/core/rtnetlink.c:6081
>>>>>>>>> netlink_rcv_skb+0x153/0x420 net/netlink/af_netlink.c:2501
>>>>>>>>> netlink_unicast_kernel net/netlink/af_netlink.c:1319 [inline]
>>>>>>>>> netlink_unicast+0x543/0x7f0 net/netlink/af_netlink.c:1345
>>>>>>>>> netlink_sendmsg+0x917/0xe10 net/netlink/af_netlink.c:1921
>>>>>>>>> sock_sendmsg_nosec net/socket.c:714 [inline]
>>>>>>>>> sock_sendmsg+0xcf/0x120 net/socket.c:734
>>>>>>>>> ____sys_sendmsg+0x6eb/0x810 net/socket.c:2482
>>>>>>>>> ___sys_sendmsg+0x110/0x1b0 net/socket.c:2536
>>>>>>>>> __sys_sendmsg+0xf3/0x1c0 net/socket.c:2565
>>>>>>>>> do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>>>>>>>>> do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
>>>>>>>>> entry_SYSCALL_64_after_hwframe+0x63/0xcd
>>>>>>>>> RIP: 0033:0x7f5efaa89279
>>>>>>>>> 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 b8 ff ff ff f7 d8 64 89 01 48
>>>>>>>>> RSP: 002b:00007f5efbc31168 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
>>>>>>>>> RAX: ffffffffffffffda RBX: 00007f5efab9bf80 RCX: 00007f5efaa89279
>>>>>>>>> RDX: 0000000000000000 RSI: 0000000020000140 RDI: 0000000000000005
>>>>>>>>> RBP: 00007f5efaae32e9 R08: 0000000000000000 R09: 0000000000000000
>>>>>>>>> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
>>>>>>>>> R13: 00007f5efb0cfb1f R14: 00007f5efbc31300 R15: 0000000000022000
>>>>>>>>> </TASK>
>>>>>>>>>
>>>>>>>>> Allocated by task 21027:
>>>>>>>>> kasan_save_stack+0x1e/0x40 mm/kasan/common.c:38
>>>>>>>>> kasan_set_track mm/kasan/common.c:45 [inline]
>>>>>>>>> set_alloc_info mm/kasan/common.c:437 [inline]
>>>>>>>>> ____kasan_kmalloc mm/kasan/common.c:516 [inline]
>>>>>>>>> ____kasan_kmalloc mm/kasan/common.c:475 [inline]
>>>>>>>>> __kasan_kmalloc+0xa9/0xd0 mm/kasan/common.c:525
>>>>>>>>> kmalloc_node include/linux/slab.h:623 [inline]
>>>>>>>>> kzalloc_node include/linux/slab.h:744 [inline]
>>>>>>>>> qdisc_alloc+0xb0/0xc50 net/sched/sch_generic.c:938
>>>>>>>>> qdisc_create_dflt+0x71/0x4a0 net/sched/sch_generic.c:997
>>>>>>>>> attach_one_default_qdisc net/sched/sch_generic.c:1152 [inline]
>>>>>>>>> netdev_for_each_tx_queue include/linux/netdevice.h:2437 [inline]
>>>>>>>>> attach_default_qdiscs net/sched/sch_generic.c:1170 [inline]
>>>>>>>>> dev_activate+0x760/0xcd0 net/sched/sch_generic.c:1229
>>>>>>>>> __dev_open+0x393/0x4d0 net/core/dev.c:1441
>>>>>>>>> __dev_change_flags+0x583/0x750 net/core/dev.c:8556
>>>>>>>>> rtnl_configure_link+0xee/0x240 net/core/rtnetlink.c:3189
>>>>>>>>> rtnl_newlink_create net/core/rtnetlink.c:3371 [inline]
>>>>>>>>> __rtnl_newlink+0x10b8/0x17e0 net/core/rtnetlink.c:3580
>>>>>>>>> rtnl_newlink+0x64/0xa0 net/core/rtnetlink.c:3593
>>>>>>>>> rtnetlink_rcv_msg+0x43a/0xca0 net/core/rtnetlink.c:6090
>>>>>>>>> netlink_rcv_skb+0x153/0x420 net/netlink/af_netlink.c:2501
>>>>>>>>> netlink_unicast_kernel net/netlink/af_netlink.c:1319 [inline]
>>>>>>>>> netlink_unicast+0x543/0x7f0 net/netlink/af_netlink.c:1345
>>>>>>>>> netlink_sendmsg+0x917/0xe10 net/netlink/af_netlink.c:1921
>>>>>>>>> sock_sendmsg_nosec net/socket.c:714 [inline]
>>>>>>>>> sock_sendmsg+0xcf/0x120 net/socket.c:734
>>>>>>>>> ____sys_sendmsg+0x6eb/0x810 net/socket.c:2482
>>>>>>>>> ___sys_sendmsg+0x110/0x1b0 net/socket.c:2536
>>>>>>>>> __sys_sendmsg+0xf3/0x1c0 net/socket.c:2565
>>>>>>>>> do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>>>>>>>>> do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
>>>>>>>>> entry_SYSCALL_64_after_hwframe+0x63/0xcd
>>>>>>>>>
>>>>>>>>> Freed by task 21020:
>>>>>>>>> kasan_save_stack+0x1e/0x40 mm/kasan/common.c:38
>>>>>>>>> kasan_set_track+0x21/0x30 mm/kasan/common.c:45
>>>>>>>>> kasan_set_free_info+0x20/0x30 mm/kasan/generic.c:370
>>>>>>>>> ____kasan_slab_free mm/kasan/common.c:367 [inline]
>>>>>>>>> ____kasan_slab_free+0x166/0x1c0 mm/kasan/common.c:329
>>>>>>>>> kasan_slab_free include/linux/kasan.h:200 [inline]
>>>>>>>>> slab_free_hook mm/slub.c:1754 [inline]
>>>>>>>>> slab_free_freelist_hook+0x8b/0x1c0 mm/slub.c:1780
>>>>>>>>> slab_free mm/slub.c:3534 [inline]
>>>>>>>>> kfree+0xe2/0x580 mm/slub.c:4562
>>>>>>>>> rcu_do_batch kernel/rcu/tree.c:2245 [inline]
>>>>>>>>> rcu_core+0x7b5/0x1890 kernel/rcu/tree.c:2505
>>>>>>>>> __do_softirq+0x1d3/0x9c6 kernel/softirq.c:571
>>>>>>>>>
>>>>>>>>> Last potentially related work creation:
>>>>>>>>> kasan_save_stack+0x1e/0x40 mm/kasan/common.c:38
>>>>>>>>> __kasan_record_aux_stack+0xbe/0xd0 mm/kasan/generic.c:348
>>>>>>>>> call_rcu+0x99/0x790 kernel/rcu/tree.c:2793
>>>>>>>>> qdisc_put+0xcd/0xe0 net/sched/sch_generic.c:1083
>>>>>>>>> notify_and_destroy net/sched/sch_api.c:1012 [inline]
>>>>>>>>> qdisc_graft+0xeb1/0x1270 net/sched/sch_api.c:1084
>>>>>>>>> tc_modify_qdisc+0xbb7/0x1a00 net/sched/sch_api.c:1671
>>>>>>>>> rtnetlink_rcv_msg+0x43a/0xca0 net/core/rtnetlink.c:6090
>>>>>>>>> netlink_rcv_skb+0x153/0x420 net/netlink/af_netlink.c:2501
>>>>>>>>> netlink_unicast_kernel net/netlink/af_netlink.c:1319 [inline]
>>>>>>>>> netlink_unicast+0x543/0x7f0 net/netlink/af_netlink.c:1345
>>>>>>>>> netlink_sendmsg+0x917/0xe10 net/netlink/af_netlink.c:1921
>>>>>>>>> sock_sendmsg_nosec net/socket.c:714 [inline]
>>>>>>>>> sock_sendmsg+0xcf/0x120 net/socket.c:734
>>>>>>>>> ____sys_sendmsg+0x6eb/0x810 net/socket.c:2482
>>>>>>>>> ___sys_sendmsg+0x110/0x1b0 net/socket.c:2536
>>>>>>>>> __sys_sendmsg+0xf3/0x1c0 net/socket.c:2565
>>>>>>>>> do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>>>>>>>>> do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
>>>>>>>>> entry_SYSCALL_64_after_hwframe+0x63/0xcd
>>>>>>>>>
>>>>>>>>> Second to last potentially related work creation:
>>>>>>>>> kasan_save_stack+0x1e/0x40 mm/kasan/common.c:38
>>>>>>>>> __kasan_record_aux_stack+0xbe/0xd0 mm/kasan/generic.c:348
>>>>>>>>> kvfree_call_rcu+0x74/0x940 kernel/rcu/tree.c:3322
>>>>>>>>> neigh_destroy+0x431/0x630 net/core/neighbour.c:912
>>>>>>>>> neigh_release include/net/neighbour.h:454 [inline]
>>>>>>>>> neigh_cleanup_and_release+0x1f8/0x330 net/core/neighbour.c:103
>>>>>>>>> neigh_del net/core/neighbour.c:225 [inline]
>>>>>>>>> neigh_remove_one+0x37d/0x460 net/core/neighbour.c:246
>>>>>>>>> neigh_forced_gc net/core/neighbour.c:276 [inline]
>>>>>>>>> neigh_alloc net/core/neighbour.c:447 [inline]
>>>>>>>>> ___neigh_create+0x18b5/0x29a0 net/core/neighbour.c:642
>>>>>>>>> ip6_finish_output2+0xfb8/0x1520 net/ipv6/ip6_output.c:125
>>>>>>>>> __ip6_finish_output net/ipv6/ip6_output.c:195 [inline]
>>>>>>>>> ip6_finish_output+0x690/0x1160 net/ipv6/ip6_output.c:206
>>>>>>>>> NF_HOOK_COND include/linux/netfilter.h:296 [inline]
>>>>>>>>> ip6_output+0x1ed/0x540 net/ipv6/ip6_output.c:227
>>>>>>>>> dst_output include/net/dst.h:451 [inline]
>>>>>>>>> NF_HOOK include/linux/netfilter.h:307 [inline]
>>>>>>>>> NF_HOOK include/linux/netfilter.h:301 [inline]
>>>>>>>>> mld_sendpack+0xa09/0xe70 net/ipv6/mcast.c:1820
>>>>>>>>> mld_send_cr net/ipv6/mcast.c:2121 [inline]
>>>>>>>>> mld_ifc_work+0x71c/0xdc0 net/ipv6/mcast.c:2653
>>>>>>>>> process_one_work+0x991/0x1610 kernel/workqueue.c:2289
>>>>>>>>> worker_thread+0x665/0x1080 kernel/workqueue.c:2436
>>>>>>>>> kthread+0x2e4/0x3a0 kernel/kthread.c:376
>>>>>>>>> ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:306
>>>>>>>>>
>>>>>>>>> The buggy address belongs to the object at ffff88802065e000
>>>>>>>>> which belongs to the cache kmalloc-1k of size 1024
>>>>>>>>> The buggy address is located 56 bytes inside of
>>>>>>>>> 1024-byte region [ffff88802065e000, ffff88802065e400)
>>>>>>>>>
>>>>>>>>> The buggy address belongs to the physical page:
>>>>>>>>> page:ffffea0000819600 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x20658
>>>>>>>>> head:ffffea0000819600 order:3 compound_mapcount:0 compound_pincount:0
>>>>>>>>> flags: 0xfff00000010200(slab|head|node=0|zone=1|lastcpupid=0x7ff)
>>>>>>>>> raw: 00fff00000010200 0000000000000000 dead000000000001 ffff888011841dc0
>>>>>>>>> raw: 0000000000000000 0000000000100010 00000001ffffffff 0000000000000000
>>>>>>>>> page dumped because: kasan: bad access detected
>>>>>>>>> page_owner tracks the page as allocated
>>>>>>>>> page last allocated via order 3, migratetype Unmovable, gfp_mask 0xd20c0(__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 3523, tgid 3523 (sshd), ts 41495190986, free_ts 41417713212
>>>>>>>>> prep_new_page mm/page_alloc.c:2532 [inline]
>>>>>>>>> get_page_from_freelist+0x109b/0x2ce0 mm/page_alloc.c:4283
>>>>>>>>> __alloc_pages+0x1c7/0x510 mm/page_alloc.c:5515
>>>>>>>>> alloc_pages+0x1a6/0x270 mm/mempolicy.c:2270
>>>>>>>>> alloc_slab_page mm/slub.c:1824 [inline]
>>>>>>>>> allocate_slab+0x27e/0x3d0 mm/slub.c:1969
>>>>>>>>> new_slab mm/slub.c:2029 [inline]
>>>>>>>>> ___slab_alloc+0x7f1/0xe10 mm/slub.c:3031
>>>>>>>>> __slab_alloc.constprop.0+0x4d/0xa0 mm/slub.c:3118
>>>>>>>>> slab_alloc_node mm/slub.c:3209 [inline]
>>>>>>>>> __kmalloc_node_track_caller+0x2f2/0x380 mm/slub.c:4955
>>>>>>>>> kmalloc_reserve net/core/skbuff.c:358 [inline]
>>>>>>>>> __alloc_skb+0xd9/0x2f0 net/core/skbuff.c:430
>>>>>>>>> alloc_skb_fclone include/linux/skbuff.h:1307 [inline]
>>>>>>>>> tcp_stream_alloc_skb+0x38/0x580 net/ipv4/tcp.c:861
>>>>>>>>> tcp_sendmsg_locked+0xc36/0x2f80 net/ipv4/tcp.c:1325
>>>>>>>>> tcp_sendmsg+0x2b/0x40 net/ipv4/tcp.c:1483
>>>>>>>>> inet_sendmsg+0x99/0xe0 net/ipv4/af_inet.c:819
>>>>>>>>> sock_sendmsg_nosec net/socket.c:714 [inline]
>>>>>>>>> sock_sendmsg+0xcf/0x120 net/socket.c:734
>>>>>>>>> sock_write_iter+0x291/0x3d0 net/socket.c:1108
>>>>>>>>> call_write_iter include/linux/fs.h:2187 [inline]
>>>>>>>>> new_sync_write fs/read_write.c:491 [inline]
>>>>>>>>> vfs_write+0x9e9/0xdd0 fs/read_write.c:578
>>>>>>>>> ksys_write+0x1e8/0x250 fs/read_write.c:631
>>>>>>>>> page last free stack trace:
>>>>>>>>> reset_page_owner include/linux/page_owner.h:24 [inline]
>>>>>>>>> free_pages_prepare mm/page_alloc.c:1449 [inline]
>>>>>>>>> free_pcp_prepare+0x5e4/0xd20 mm/page_alloc.c:1499
>>>>>>>>> free_unref_page_prepare mm/page_alloc.c:3380 [inline]
>>>>>>>>> free_unref_page+0x19/0x4d0 mm/page_alloc.c:3476
>>>>>>>>> __unfreeze_partials+0x17c/0x1a0 mm/slub.c:2548
>>>>>>>>> qlink_free mm/kasan/quarantine.c:168 [inline]
>>>>>>>>> qlist_free_all+0x6a/0x170 mm/kasan/quarantine.c:187
>>>>>>>>> kasan_quarantine_reduce+0x180/0x200 mm/kasan/quarantine.c:294
>>>>>>>>> __kasan_slab_alloc+0xa2/0xc0 mm/kasan/common.c:447
>>>>>>>>> kasan_slab_alloc include/linux/kasan.h:224 [inline]
>>>>>>>>> slab_post_alloc_hook mm/slab.h:727 [inline]
>>>>>>>>> slab_alloc_node mm/slub.c:3243 [inline]
>>>>>>>>> slab_alloc mm/slub.c:3251 [inline]
>>>>>>>>> __kmem_cache_alloc_lru mm/slub.c:3258 [inline]
>>>>>>>>> kmem_cache_alloc+0x267/0x3b0 mm/slub.c:3268
>>>>>>>>> kmem_cache_zalloc include/linux/slab.h:723 [inline]
>>>>>>>>> alloc_buffer_head+0x20/0x140 fs/buffer.c:2974
>>>>>>>>> alloc_page_buffers+0x280/0x790 fs/buffer.c:829
>>>>>>>>> create_empty_buffers+0x2c/0xee0 fs/buffer.c:1558
>>>>>>>>> ext4_block_write_begin+0x1004/0x1530 fs/ext4/inode.c:1074
>>>>>>>>> ext4_da_write_begin+0x422/0xae0 fs/ext4/inode.c:2996
>>>>>>>>> generic_perform_write+0x246/0x560 mm/filemap.c:3738
>>>>>>>>> ext4_buffered_write_iter+0x15b/0x460 fs/ext4/file.c:270
>>>>>>>>> ext4_file_write_iter+0x44a/0x1660 fs/ext4/file.c:679
>>>>>>>>> call_write_iter include/linux/fs.h:2187 [inline]
>>>>>>>>> new_sync_write fs/read_write.c:491 [inline]
>>>>>>>>> vfs_write+0x9e9/0xdd0 fs/read_write.c:578
>>>>>>>>>
>>>>>>>>> Fixes: af356afa010f ("net_sched: reintroduce dev->qdisc for use by sch_api")
>>>>>>>>> Reported-by: syzbot <syzkaller@...glegroups.com>
>>>>>>>>> Diagnosed-by: Dmitry Vyukov <dvyukov@...gle.com>
>>>>>>>>> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
>>>>>>>>> ---
>>>>>>>>>  net/sched/sch_api.c | 5 +++--
>>>>>>>>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>>>>>>>>
>>>>>>>>> diff --git a/net/sched/sch_api.c b/net/sched/sch_api.c
>>>>>>>>> index c98af0ada706efee202a20a6bfb6f2b984106f45..4a27dfb1ba0faab3692a82969fb8b78768742779 100644
>>>>>>>>> --- a/net/sched/sch_api.c
>>>>>>>>> +++ b/net/sched/sch_api.c
>>>>>>>>> @@ -1099,12 +1099,13 @@ static int qdisc_graft(struct net_device *dev, struct Qdisc *parent,
>>>>>>>>>
>>>>>>>>>  skip:
>>>>>>>>>               if (!ingress) {
>>>>>>>>> -                     notify_and_destroy(net, skb, n, classid,
>>>>>>>>> -                                        rtnl_dereference(dev->qdisc), new);
>>>>>>>>> +                     old = rtnl_dereference(dev->qdisc);
>>>>>>>>>                       if (new && !new->ops->attach)
>>>>>>>>>                               qdisc_refcount_inc(new);
>>>>>>>>>                       rcu_assign_pointer(dev->qdisc, new ? : &noop_qdisc);
>>>>>>>>>
>>>>>>>>> +                     notify_and_destroy(net, skb, n, classid, old, new);
>>>>>>>>> +
>>>>>>>>>                       if (new && new->ops->attach)
>>>>>>>>>                               new->ops->attach(new);
>>>>>>>>>               } else {
>>>>>>>> Hi Eric,
>>>>>>>> We started seeing the following WARN_ON happening on htb destroy
>>>>>>>> following your patch:
>>>>>>>> https://elixir.bootlin.com/linux/v6.1-rc3/source/net/sched/sch_htb.c#L1561
>>>>>>>>
>>>>>>>> Anything comes to mind?
>>>>>>> Not really. Let's ask Maxim Mikityanskiy <maximmi@...dia.com>
>>>>>> CC: Maxim on non-nvidia address
>>>>>>
>>>>> Wild guess is that we need this fix:
>>>>>
>>>>> diff --git a/net/sched/sch_htb.c b/net/sched/sch_htb.c
>>>>> index e5b4bbf3ce3d5f36edb512d4017ebd97209bb377..f82c07bd3d60e26e5a1fe2b335dbec29aebb602e
>>>>> 100644
>>>>> --- a/net/sched/sch_htb.c
>>>>> +++ b/net/sched/sch_htb.c
>>>>> @@ -1558,7 +1558,7 @@ static int htb_destroy_class_offload(struct
>>>>> Qdisc *sch, struct htb_class *cl,
>>>>>                 /* Before HTB is destroyed, the kernel grafts noop_qdisc to
>>>>>                  * all queues.
>>>>>                  */
>>>>> -               WARN_ON(!(old->flags & TCQ_F_BUILTIN));
>>>>> +               WARN_ON(old && !(old->flags & TCQ_F_BUILTIN));
>>>> I don't think it's the right fix. If the WARN_ON happens and doesn't
>>>> complain about old == NULL, it's not NULL, so this extra check won't
>>>> help. In any case, old comes from dev_graft_qdisc, which should never
>>>> return NULL.
>>>>
>>>> Maybe something changed after Eric's patch, so that the statement
>>>> above is no longer true? I.e. I expect a noop_qdisc here, but there is
>>>> some other qdisc attached to the queue. I need to recall what was the
>>>> destroy flow and what code assigned noop_qdisc there...
>>> Gal please provide a repro. It is not clear if you get a WARN or a NULL deref.
>> The WARN_ON is triggering, old is not NULL and old->flags is equal to 0x174.
>>
>> It reproduces consistently:
>> ip link set dev eth2 up
>> ip addr add 194.237.173.123/16 dev eth2
>> tc qdisc add dev eth2 clsact
>> tc qdisc add dev eth2 root handle 1: htb default 1 offload
>> tc class add dev eth2 classid 1: parent root htb rate 18000mbit ceil
>> 22500.0mbit burst 450000kbit cburst 450000kbit
>> tc class add dev eth2 classid 1:3 parent 1: htb rate 3596mbit burst
>> 89900kbit cburst 89900kbit
>> tc qdisc delete dev eth2 clsact
>> tc qdisc delete dev eth2 root handle 1: htb default 1
>>
>> Please let me know if there's anything else you want me to check.
> Hi Eric, did you get a chance to take a look?

No response for quite a long time, Jakub, should I submit a revert?

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