[syzbot ci] Re: net: optimize TX throughput and efficiency

[syzbot ci]

syzbot ci has tested the following series

[v1] net: optimize TX throughput and efficiency
https://lore.kernel.org/all/20251006193103.2...@google.com
* [PATCH RFC net-next 1/5] net: add add indirect call wrapper in skb_release_head_state()
* [PATCH RFC net-next 2/5] net/sched: act_mirred: add loop detection
* [PATCH RFC net-next 3/5] Revert "net/sched: Fix mirred deadlock on device recursion"
* [PATCH RFC net-next 4/5] net: sched: claim one cache line in Qdisc
* [PATCH RFC net-next 5/5] net: dev_queue_xmit() llist adoption

and found the following issue:
KASAN: slab-use-after-free Read in netem_dequeue

Full report is available here:
https://ci.syzbot.org/series/e8660f67-35a0-406e-96ee-a401d3f30ff9

***

KASAN: slab-use-after-free Read in netem_dequeue

tree: net-next
URL: https://kernel.googlesource.com/pub/scm/linux/kernel/git/netdev/net-next.git
base: f1455695d2d99894b65db233877acac9a0e120b9
arch: amd64
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
config: https://ci.syzbot.org/builds/2db7ec92-610b-4887-bf33-d0b4c04760c8/config
syz repro: https://ci.syzbot.org/findings/3ca47f46-1b94-48b6-bab9-5996b7162c30/syz_repro

==================================================================
BUG: KASAN: slab-use-after-free in netem_dequeue+0x4e7/0x1430 net/sched/sch_netem.c:720
Read of size 8 at addr ffff888020b65b30 by task ksoftirqd/1/23

CPU: 1 UID: 0 PID: 23 Comm: ksoftirqd/1 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
Call Trace:
<TASK>
dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
print_address_description mm/kasan/report.c:378 [inline]
print_report+0xca/0x240 mm/kasan/report.c:482
kasan_report+0x118/0x150 mm/kasan/report.c:595
netem_dequeue+0x4e7/0x1430 net/sched/sch_netem.c:720
dequeue_skb net/sched/sch_generic.c:294 [inline]
qdisc_restart net/sched/sch_generic.c:399 [inline]
__qdisc_run+0x23c/0x15f0 net/sched/sch_generic.c:417
qdisc_run+0xc5/0x290 include/net/pkt_sched.h:126
net_tx_action+0x7c9/0x980 net/core/dev.c:5731
handle_softirqs+0x286/0x870 kernel/softirq.c:579
run_ksoftirqd+0x9b/0x100 kernel/softirq.c:968
smpboot_thread_fn+0x542/0xa60 kernel/smpboot.c:160
kthread+0x711/0x8a0 kernel/kthread.c:463
ret_from_fork+0x439/0x7d0 arch/x86/kernel/process.c:148
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
</TASK>

Allocated by task 5913:
kasan_save_stack mm/kasan/common.c:47 [inline]
kasan_save_track+0x3e/0x80 mm/kasan/common.c:68
unpoison_slab_object mm/kasan/common.c:330 [inline]
__kasan_slab_alloc+0x6c/0x80 mm/kasan/common.c:356
kasan_slab_alloc include/linux/kasan.h:250 [inline]
slab_post_alloc_hook mm/slub.c:4191 [inline]
slab_alloc_node mm/slub.c:4240 [inline]
kmem_cache_alloc_node_noprof+0x1bb/0x3c0 mm/slub.c:4292
__alloc_skb+0x112/0x2d0 net/core/skbuff.c:660
alloc_skb include/linux/skbuff.h:1383 [inline]
mld_newpack+0x13c/0xc40 net/ipv6/mcast.c:1775
add_grhead+0x5a/0x2a0 net/ipv6/mcast.c:1886
add_grec+0x1452/0x1740 net/ipv6/mcast.c:2025
mld_send_cr net/ipv6/mcast.c:2148 [inline]
mld_ifc_work+0x6ed/0xd60 net/ipv6/mcast.c:2693
process_one_work kernel/workqueue.c:3236 [inline]
process_scheduled_works+0xae1/0x17b0 kernel/workqueue.c:3319
worker_thread+0x8a0/0xda0 kernel/workqueue.c:3400
kthread+0x711/0x8a0 kernel/kthread.c:463
ret_from_fork+0x439/0x7d0 arch/x86/kernel/process.c:148
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

Freed by task 23:
kasan_save_stack mm/kasan/common.c:47 [inline]
kasan_save_track+0x3e/0x80 mm/kasan/common.c:68
kasan_save_free_info+0x46/0x50 mm/kasan/generic.c:576
poison_slab_object mm/kasan/common.c:243 [inline]
__kasan_slab_free+0x5b/0x80 mm/kasan/common.c:275
kasan_slab_free include/linux/kasan.h:233 [inline]
slab_free_hook mm/slub.c:2422 [inline]
slab_free mm/slub.c:4695 [inline]
kmem_cache_free+0x18f/0x400 mm/slub.c:4797
br_dev_xmit+0x11b3/0x1840 net/bridge/br_device.c:108
__netdev_start_xmit include/linux/netdevice.h:5248 [inline]
netdev_start_xmit include/linux/netdevice.h:5257 [inline]
xmit_one net/core/dev.c:3845 [inline]
dev_hard_start_xmit+0x2d7/0x830 net/core/dev.c:3861
sch_direct_xmit+0x241/0x4b0 net/sched/sch_generic.c:344
qdisc_restart net/sched/sch_generic.c:409 [inline]
__qdisc_run+0xb16/0x15f0 net/sched/sch_generic.c:417
qdisc_run+0xc5/0x290 include/net/pkt_sched.h:126
net_tx_action+0x7c9/0x980 net/core/dev.c:5731
handle_softirqs+0x286/0x870 kernel/softirq.c:579
run_ksoftirqd+0x9b/0x100 kernel/softirq.c:968
smpboot_thread_fn+0x542/0xa60 kernel/smpboot.c:160
kthread+0x711/0x8a0 kernel/kthread.c:463
ret_from_fork+0x439/0x7d0 arch/x86/kernel/process.c:148
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

The buggy address belongs to the object at ffff888020b65b00
which belongs to the cache skbuff_head_cache of size 240
The buggy address is located 48 bytes inside of
freed 240-byte region [ffff888020b65b00, ffff888020b65bf0)

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x20b64
head: order:1 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
flags: 0xfff00000000040(head|node=0|zone=1|lastcpupid=0x7ff)
page_type: f5(slab)
raw: 00fff00000000040 ffff88801cedf8c0 dead000000000122 0000000000000000
raw: 0000000000000000 0000000000150015 00000000f5000000 0000000000000000
head: 00fff00000000040 ffff88801cedf8c0 dead000000000122 0000000000000000
head: 0000000000000000 0000000000150015 00000000f5000000 0000000000000000
head: 00fff00000000001 ffffea000082d901 00000000ffffffff 00000000ffffffff
head: ffffffffffffffff 0000000000000000 00000000ffffffff 0000000000000002
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 1, migratetype Unmovable, gfp_mask 0x72820(GFP_ATOMIC|__GFP_NOWARN|__GFP_NORETRY|__GFP_MEMALLOC|__GFP_COMP), pid 0, tgid 0 (swapper/0), ts 96149950019, free_ts 42914869228
set_page_owner include/linux/page_owner.h:32 [inline]
post_alloc_hook+0x240/0x2a0 mm/page_alloc.c:1851
prep_new_page mm/page_alloc.c:1859 [inline]
get_page_from_freelist+0x21e4/0x22c0 mm/page_alloc.c:3858
__alloc_frozen_pages_noprof+0x181/0x370 mm/page_alloc.c:5148
alloc_pages_mpol+0x232/0x4a0 mm/mempolicy.c:2416
alloc_slab_page mm/slub.c:2492 [inline]
allocate_slab+0x8a/0x370 mm/slub.c:2660
new_slab mm/slub.c:2714 [inline]
___slab_alloc+0xbeb/0x1420 mm/slub.c:3901
__slab_alloc mm/slub.c:3992 [inline]
__slab_alloc_node mm/slub.c:4067 [inline]
slab_alloc_node mm/slub.c:4228 [inline]
kmem_cache_alloc_node_noprof+0x280/0x3c0 mm/slub.c:4292
__alloc_skb+0x112/0x2d0 net/core/skbuff.c:660
__netdev_alloc_skb+0x108/0x970 net/core/skbuff.c:734
netdev_alloc_skb include/linux/skbuff.h:3484 [inline]
dev_alloc_skb include/linux/skbuff.h:3497 [inline]
__ieee80211_beacon_get+0xc06/0x1880 net/mac80211/tx.c:5652
ieee80211_beacon_get_tim+0xb4/0x2b0 net/mac80211/tx.c:5774
ieee80211_beacon_get include/net/mac80211.h:5667 [inline]
mac80211_hwsim_beacon_tx+0x3ce/0x860 drivers/net/wireless/virtual/mac80211_hwsim.c:2355
__iterate_interfaces+0x2ab/0x590 net/mac80211/util.c:761
ieee80211_iterate_active_interfaces_atomic+0xdb/0x180 net/mac80211/util.c:797
mac80211_hwsim_beacon+0xbb/0x1c0 drivers/net/wireless/virtual/mac80211_hwsim.c:2389
__run_hrtimer kernel/time/hrtimer.c:1761 [inline]
__hrtimer_run_queues+0x52c/0xc60 kernel/time/hrtimer.c:1825
page last free pid 0 tgid 0 stack trace:
reset_page_owner include/linux/page_owner.h:25 [inline]
free_pages_prepare mm/page_alloc.c:1395 [inline]
__free_frozen_pages+0xbc4/0xd30 mm/page_alloc.c:2895
rcu_do_batch kernel/rcu/tree.c:2605 [inline]
rcu_core+0xcab/0x1770 kernel/rcu/tree.c:2861
handle_softirqs+0x286/0x870 kernel/softirq.c:579
__do_softirq kernel/softirq.c:613 [inline]
invoke_softirq kernel/softirq.c:453 [inline]
__irq_exit_rcu+0xca/0x1f0 kernel/softirq.c:680
irq_exit_rcu+0x9/0x30 kernel/softirq.c:696
instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1050 [inline]
sysvec_apic_timer_interrupt+0xa6/0xc0 arch/x86/kernel/apic/apic.c:1050
asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:702

Memory state around the buggy address:
ffff888020b65a00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fc fc
ffff888020b65a80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff888020b65b00: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
^
ffff888020b65b80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fc fc
ffff888020b65c00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================


***

If these findings have caused you to resend the series or submit a
separate fix, please add the following tag to your commit message:
Tested-by: syz...@syzkaller.appspotmail.com

---
This report is generated by a bot. It may contain errors.
syzbot ci engineers can be reached at syzk...@googlegroups.com.

[Eric Dumazet]

On Mon, Oct 6, 2025 at 10:24 PM syzbot ci
<syzbot+ciad4...@syzkaller.appspotmail.com> wrote:
>
> syzbot ci has tested the following series
>
> [v1] net: optimize TX throughput and efficiency
> https://lore.kernel.org/all/20251006193103.2...@google.com
> * [PATCH RFC net-next 1/5] net: add add indirect call wrapper in skb_release_head_state()
> * [PATCH RFC net-next 2/5] net/sched: act_mirred: add loop detection
> * [PATCH RFC net-next 3/5] Revert "net/sched: Fix mirred deadlock on device recursion"
> * [PATCH RFC net-next 4/5] net: sched: claim one cache line in Qdisc
> * [PATCH RFC net-next 5/5] net: dev_queue_xmit() llist adoption
>
> and found the following issue:
> KASAN: slab-use-after-free Read in netem_dequeue
>
> Full report is available here:
> https://ci.syzbot.org/series/e8660f67-35a0-406e-96ee-a401d3f30ff9
>

I was unsure if I needed to clear skb->next or not before calling
dev_qdisc_enqueue()

We could either do this in netem, or generically in dev_qdisc_enqueue()

diff --git a/net/core/dev.c b/net/core/dev.c
index 6094768bf3c028f0ad1e52b9b12b7258fa0ecff6..547efbfb63adb4a093ce4b4ea0934256c15e263b
100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4115,6 +4115,7 @@ static int dev_qdisc_enqueue(struct sk_buff
*skb, struct Qdisc *q,
{
int rc;

+ skb_mark_not_on_list(skb);
rc = q->enqueue(skb, q, to_free) & NET_XMIT_MASK;
if (rc == NET_XMIT_SUCCESS)
trace_qdisc_enqueue(q, txq, skb);