Dear Linux kernel developers and maintainers,
Using a modified version of syzkaller, I identified a new bug and
refined the PoC, and the bug-related information is attached
below.Please let me know if you need any further information.

Summary

A NULL Pointer Dereference occurs because there is no proper check for
whether cl is NULL when a class is found via cops->find() and then
used in cops->qlen_notify().
This description provides an explanation and KASAN report for
qfq_qlen_notify, but the vulnerability can occur in various other
places such as drr_qlen_notify, hfsc_qlen_notify, htb_qlen_notify,
etc.

Keywords
- net/sched

Kernel Info
Version: (Output of /proc/version)
- Linux version 7.0.0-rc1
Commit: (Git hash if applicable)
- 6de23f81a5e08be8fbf5e8d7e9febc72a5b5f27f

Description(Root Cause)

void qdisc_tree_reduce_backlog(struct Qdisc *sch, int n, int len)
{
    const struct Qdisc_class_ops *cops;
    unsigned long cl;
    u32 parentid;
    bool notify;
    int drops;

    drops = max_t(int, n, 0);
    rcu_read_lock();
    while ((parentid = sch->parent)) {
        if (parentid == TC_H_ROOT)
            break;

        if (sch->flags & TCQ_F_NOPARENT)
            break;
        /* Notify parent qdisc only if child qdisc becomes empty. */
        notify = !sch->q.qlen;
        /* TODO: perform the search on a per txq basis */
        sch = qdisc_lookup_rcu(qdisc_dev(sch), TC_H_MAJ(parentid));
        if (sch == NULL) {
            WARN_ON_ONCE(parentid != TC_H_ROOT);
            break;
        }
        cops = sch->ops->cl_ops;
        if (notify && cops->qlen_notify) { // [1]
            /* Note that qlen_notify must be idempotent as it may get called
             * multiple times.
             */
            cl = cops->find(sch, parentid); // [2]
            cops->qlen_notify(sch, cl); // [3]
        }
        sch->q.qlen -= n;
        sch->qstats.backlog -= len;
        __qdisc_qstats_drop(sch, drops);
    }
    rcu_read_unlock();
}

When condition [1] is satisfied, cl is retrieved at [2] and then
passed to [3]. Here, [2] can return NULL, and using this value at [3]
can cause a NULL Pointer Dereference.

static unsigned long qfq_search_class(struct Qdisc *sch, u32 classid)
{
    return (unsigned long)qfq_find_class(sch, classid);
}

static struct qfq_class *qfq_find_class(struct Qdisc *sch, u32 classid)
{
    struct qfq_sched *q = qdisc_priv(sch);
    struct Qdisc_class_common *clc;

    clc = qdisc_class_find(&q->clhash, classid);
    if (clc == NULL)
        return NULL; // [4]
    return container_of(clc, struct qfq_class, common);
}

When QFQ is configured, the above function is executed at [2], and
NULL can be returned at [4].

static void qfq_qlen_notify(struct Qdisc *sch, unsigned long arg)
{
    struct qfq_sched *q = qdisc_priv(sch);
    struct qfq_class *cl = (struct qfq_class *)arg; // [5]

    if (list_empty(&cl->alist)) // [6]
        return;
    qfq_deactivate_class(q, cl);
}

If NULL is returned at [2], that value is passed directly to [3],
which becomes cl through [5]. Subsequently, a NULL Pointer Dereference
occurs at [6].

Kasan Report
Oops: general protection fault, probably for non-canonical address
0xdffffc000000000b: 0000 [#1] SMP KASAN NOPTI
KASAN: null-ptr-deref in range [0x0000000000000058-0x000000000000005f]
CPU: 0 UID: 0 PID: 377 Comm: test Not tainted 7.0.0-rc1 #54 PREEMPT(full)
Hardware name: QEMU Ubuntu 24.04 PC v2 (i440FX + PIIX, arch_caps fix,
1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
RIP: 0010:qfq_qlen_notify+0x2e/0x180 net/sched/sch_qfq.c:1434
Code: 41 57 41 56 41 55 41 54 55 48 89 fd 53 48 89 f3 4c 8d 7b 58 e8
e3 b3 8a fd 4c 89 fa 48 b8 00 00 00 00 00 fc ff df 48 c1 ea 03 <80> 3c
02 00 0f 85 f8 00 00 00 48 8b 43 58 49 39 c7 0f 84 b9 00 00
RSP: 0018:ffff88811a447258 EFLAGS: 00010212
RAX: dffffc0000000000 RBX: 0000000000000000 RCX: ffffffff83e2af6d
RDX: 000000000000000b RSI: 0000000000000000 RDI: ffff88811b524000
RBP: ffff88811b524000 R08: 0000000000000000 R09: fffffbfff0d3595c
R10: ffffffff869acae7 R11: 00000000ac3123a9 R12: ffffffff859b0b60
R13: ffffffff859b0b40 R14: 0000000000000000 R15: 0000000000000058
FS:  00007ff24e5da700(0000) GS:ffff88834ad0d000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007ff24e6e95f0 CR3: 000000011a5ff000 CR4: 0000000000750ef0
PKRU: 55555554
Call Trace:
 <TASK>
 __qdisc_qstats_drop include/net/sch_generic.h:958 [inline]
 qdisc_tree_reduce_backlog+0x128/0x400 net/sched/sch_api.c:812
 tbf_offload_graft net/sched/sch_tbf.c:191 [inline]
 tbf_graft+0x1cc/0x590 net/sched/sch_tbf.c:573
 qdisc_refcount_dec_if_one include/net/sch_generic.h:152 [inline]
 qdisc_graft+0x2ef/0x1460 net/sched/sch_api.c:1117
 __tc_modify_qdisc net/sched/sch_api.c:1631 [inline]
 tc_modify_qdisc+0xf4f/0x1e40 net/sched/sch_api.c:1819
 rcu_read_unlock include/linux/rcupdate.h:883 [inline]
 rtnetlink_rcv_msg+0x3b9/0xa90 net/core/rtnetlink.c:6913
 netlink_rcv_skb+0x12e/0x390 net/netlink/af_netlink.c:2539
 kfree_skb_reason include/linux/skbuff.h:1322 [inline]
 kfree_skb include/linux/skbuff.h:1331 [inline]
 netlink_unicast_kernel net/netlink/af_netlink.c:1321 [inline]
 netlink_unicast+0x6c1/0x970 net/netlink/af_netlink.c:1344
 netlink_sendmsg+0x79c/0xc50 net/netlink/af_netlink.c:2465
 __sock_release net/socket.c:673 [inline]
 ____sys_sendmsg+0x8b2/0xa50 net/socket.c:690
 sendmsg_copy_msghdr net/socket.c:2621 [inline]
 ___sys_sendmsg+0x120/0x1c0 net/socket.c:2642
 __sys_sendmsg+0x147/0x1f0 net/socket.c:2681
 arch_atomic64_read arch/x86/include/asm/atomic64_64.h:15 [inline]
 raw_atomic64_read include/linux/atomic/atomic-arch-fallback.h:2583 [inline]
 raw_atomic_long_read include/linux/atomic/atomic-long.h:38 [inline]
 atomic_long_read include/linux/atomic/atomic-instrumented.h:3189 [inline]
 unwind_reset_info include/linux/unwind_deferred.h:37 [inline]
 exit_to_user_mode include/linux/irq-entry-common.h:296 [inline]
 syscall_exit_to_user_mode include/linux/entry-common.h:327 [inline]
 do_syscall_64+0xf1/0x530 arch/x86/entry/syscall_64.c:100
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7ff24e7c4e4d
Code: 28 89 54 24 1c 48 89 74 24 10 89 7c 24 08 e8 ca ee ff ff 8b 54
24 1c 48 8b 74 24 10 41 89 c0 8b 7c 24 08 b8 2e 00 00 00 0f 05 <48> 3d
00 f0 ff ff 77 33 44 89 c7 48 89 44 24 08 e8 fe ee ff ff 48
RSP: 002b:00007ff24e5d9d90 EFLAGS: 00000293 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007ff24e7c4e4d
RDX: 0000000000000000 RSI: 00007ff24e5d9dd0 RDI: 0000000000000003
RBP: 00007ff24e5d9e30 R08: 0000000000000000 R09: 0000000000050000
R10: 0000000000000000 R11: 0000000000000293 R12: 00007ffcdc50ddde
R13: 00007ffcdc50dddf R14: 00007ff24e5d9fc0 R15: 0000000000022000
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:qfq_qlen_notify+0x2e/0x180 net/sched/sch_qfq.c:1434
Code: 41 57 41 56 41 55 41 54 55 48 89 fd 53 48 89 f3 4c 8d 7b 58 e8
e3 b3 8a fd 4c 89 fa 48 b8 00 00 00 00 00 fc ff df 48 c1 ea 03 <80> 3c
02 00 0f 85 f8 00 00 00 48 8b 43 58 49 39 c7 0f 84 b9 00 00
RSP: 0018:ffff88811a447258 EFLAGS: 00010212
RAX: dffffc0000000000 RBX: 0000000000000000 RCX: ffffffff83e2af6d
RDX: 000000000000000b RSI: 0000000000000000 RDI: ffff88811b524000
RBP: ffff88811b524000 R08: 0000000000000000 R09: fffffbfff0d3595c
R10: ffffffff869acae7 R11: 00000000ac3123a9 R12: ffffffff859b0b60
R13: ffffffff859b0b40 R14: 0000000000000000 R15: 0000000000000058
FS:  00007ff24e5da700(0000) GS:ffff88834ad0d000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007ff24e6e95f0 CR3: 000000011a5ff000 CR4: 0000000000750ef0
PKRU: 55555554
----------------
Code disassembly (best guess):
   0:   41 57                   push   %r15
   2:   41 56                   push   %r14
   4:   41 55                   push   %r13
   6:   41 54                   push   %r12
   8:   55                      push   %rbp
   9:   48 89 fd                mov    %rdi,%rbp
   c:   53                      push   %rbx
   d:   48 89 f3                mov    %rsi,%rbx
  10:   4c 8d 7b 58             lea    0x58(%rbx),%r15
  14:   e8 e3 b3 8a fd          call   0xfd8ab3fc
  19:   4c 89 fa                mov    %r15,%rdx
  1c:   48 b8 00 00 00 00 00    movabs $0xdffffc0000000000,%rax
  23:   fc ff df
  26:   48 c1 ea 03             shr    $0x3,%rdx
* 2a:   80 3c 02 00             cmpb   $0x0,(%rdx,%rax,1) <-- trapping
instruction
  2e:   0f 85 f8 00 00 00       jne    0x12c
  34:   48 8b 43 58             mov    0x58(%rbx),%rax
  38:   49 39 c7                cmp    %rax,%r15
  3b:   0f                      .byte 0xf
  3c:   84                      .byte 0x84
  3d:   b9                      .byte 0xb9
R13: ffffffff859b0b40 R14: 0000000000000000 R15: 0000000000000058
FS:  00007ff24e5da700(0000) GS:ffff88834ad0d000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007ff24e6e95f0 CR3: 000000011a5ff000 CR4: 0000000000750ef0
PKRU: 55555554
Kernel panic - not syncing: Fatal exception in interrupt
Kernel Offset: disabled
---[ end Kernel panic - not syncing: Fatal exception in interrupt ]---