CVE-2026-22976: net/sched: sch_qfq: Fix NULL deref when deactivating inactive aggregate in qfq_reset

In the Linux kernel, the following vulnerability has been resolved:

net/sched: schqfq: Fix NULL deref when deactivating inactive aggregate in qfqreset

qfqclass->leafqdisc->q.qlen > 0 does not imply that the class itself is active.

Two qfqclass objects may point to the same leafqdisc. This happens when:

1. one QFQ qdisc is attached to the dev as the root qdisc, and

2. another QFQ qdisc is temporarily referenced (e.g., via qdiscget() / qdiscput()) and is pending to be destroyed, as in function tcnewtfilter.

When packets are enqueued through the root QFQ qdisc, the shared leafqdisc->q.qlen increases. At the same time, the second QFQ qdisc triggers qdiscput and qdiscdestroy: the qdisc enters qfqreset() with its own q->q.qlen == 0, but its class's leaf qdisc->q.qlen > 0. Therefore, the qfqreset would wrongly deactivate an inactive aggregate and trigger a null-deref in qfqdeactivateagg:

[ 0.903172] BUG: kernel NULL pointer dereference, address: 0000000000000000 [ 0.903571] #PF: supervisor write access in kernel mode [ 0.903860] #PF: errorcode(0x0002) - not-present page [ 0.904177] PGD 10299b067 P4D 10299b067 PUD 10299c067 PMD 0 [ 0.904502] Oops: Oops: 0002 [#1] SMP NOPTI [ 0.904737] CPU: 0 UID: 0 PID: 135 Comm: exploit Not tainted 6.19.0-rc3+ #2 NONE [ 0.905157] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.17.0-0-gb52ca86e094d-prebuilt.qemu.org 04/01/2014 [ 0.905754] RIP: 0010:qfqdeactivateagg (include/linux/list.h:992 (discriminator 2) include/linux/list.h:1006 (discriminator 2) net/sched/schqfq.c:1367 (discriminator 2) net/sched/schqfq.c:1393 (discriminator 2)) [ 0.906046] Code: 0f 84 4d 01 00 00 48 89 70 18 8b 4b 10 48 c7 c2 ff ff ff ff 48 8b 78 08 48 d3 e2 48 21 f2 48 2b 13 48 8b 30 48 d3 ea 8b 4b 18 0

Code starting with the faulting instruction =========================================== 0: 0f 84 4d 01 00 00 je 0x153 6: 48 89 70 18 mov %rsi,0x18(%rax) a: 8b 4b 10 mov 0x10(%rbx),%ecx d: 48 c7 c2 ff ff ff ff mov $0xffffffffffffffff,%rdx 14: 48 8b 78 08 mov 0x8(%rax),%rdi 18: 48 d3 e2 shl %cl,%rdx 1b: 48 21 f2 and %rsi,%rdx 1e: 48 2b 13 sub (%rbx),%rdx 21: 48 8b 30 mov (%rax),%rsi 24: 48 d3 ea shr %cl,%rdx 27: 8b 4b 18 mov 0x18(%rbx),%ecx ... [ 0.907095] RSP: 0018:ffffc900004a39a0 EFLAGS: 00010246 [ 0.907368] RAX: ffff8881043a0880 RBX: ffff888102953340 RCX: 0000000000000000 [ 0.907723] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000 [ 0.908100] RBP: ffff888102952180 R08: 0000000000000000 R09: 0000000000000000 [ 0.908451] R10: ffff8881043a0000 R11: 0000000000000000 R12: ffff888102952000 [ 0.908804] R13: ffff888102952180 R14: ffff8881043a0ad8 R15: ffff8881043a0880 [ 0.909179] FS: 000000002a1a0380(0000) GS:ffff888196d8d000(0000) knlGS:0000000000000000 [ 0.909572] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 0.909857] CR2: 0000000000000000 CR3: 0000000102993002 CR4: 0000000000772ef0 [ 0.910247] PKRU: 55555554 [ 0.910391] Call Trace: [ 0.910527] <TASK> [ 0.910638] qfqresetqdisc (net/sched/schqfq.c:357 net/sched/schqfq.c:1485) [ 0.910826] qdiscreset (include/linux/skbuff.h:2195 include/linux/skbuff.h:2501 include/linux/skbuff.h:3424 include/linux/skbuff.h:3430 net/sched/schgeneric.c:1036) [ 0.911040] qdiscdestroy (net/sched/schgeneric.c:1076) [ 0.911236] tcnewtfilter (net/sched/clsapi.c:2447) [ 0.911447] rtnetlinkrcvmsg (net/core/rtnetlink.c:6958) [ 0.911663] ? pfxrtnetlinkrcvmsg (net/core/rtnetlink.c:6861) [ 0.911894] netlinkrcvskb (net/netlink/afnetlink.c:2550) [ 0.912100] netlinkunicast (net/netlink/afnetlink.c:1319 net/netlink/afnetlink.c:1344) [ 0.912296] ? allocskb (net/core/skbuff.c:706) [ 0.912484] netlinksendmsg (net/netlink/af ---truncated---