Subject: [syzbot] [bridge?] KASAN: use-after-free Read in qdisc_pkt_len_segs_init

From: syzbot <[email-masked]>
Date: Tue, 14 Apr 2026 04:58:38 -0700

Hello,

syzbot found the following issue on:

HEAD commit:    17ad4759a082 Merge branch 'wangxun-improvement'
git tree:       net-next
console output: https://syzkaller.appspot.com/x/log.txt?x=1505dcd2580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=229411a0a13ccb7d
dashboard link: https://syzkaller.appspot.com/bug?extid=83181a31faf9455499c5
compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/b67be09d914c/disk-17ad4759.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/20a2548795c3/vmlinux-17ad4759.xz
kernel image: https://storage.googleapis.com/syzbot-assets/29e723395cef/bzImage-17ad4759.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: [email-masked]

==================================================================
BUG: KASAN: use-after-free in __tcp_hdrlen include/linux/tcp.h:31 [inline]
BUG: KASAN: use-after-free in qdisc_pkt_len_segs_init+0x7f8/0xa30 net/core/dev.c:4146
Read of size 2 at addr ffff88815ace2434 by task syz.2.24/6033

CPU: 0 UID: 0 PID: 6033 Comm: syz.2.24 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/18/2026
Call Trace:
 <IRQ>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 print_address_description mm/kasan/report.c:378 [inline]
 print_report+0xba/0x230 mm/kasan/report.c:482
 kasan_report+0x117/0x150 mm/kasan/report.c:595
 __tcp_hdrlen include/linux/tcp.h:31 [inline]
 qdisc_pkt_len_segs_init+0x7f8/0xa30 net/core/dev.c:4146
 sch_handle_ingress net/core/dev.c:4483 [inline]
 __netif_receive_skb_core+0x13bd/0x31a0 net/core/dev.c:6065
 __netif_receive_skb_list_core+0x24d/0x810 net/core/dev.c:6289
 __netif_receive_skb_list net/core/dev.c:6356 [inline]
 netif_receive_skb_list_internal+0x995/0xcf0 net/core/dev.c:6447
 gro_normal_list include/net/gro.h:523 [inline]
 gro_flush_normal include/net/gro.h:531 [inline]
 napi_complete_done+0x299/0x730 net/core/dev.c:6815
 gro_cell_poll+0x5a9/0x5d0 net/core/gro_cells.c:74
 __napi_poll+0xae/0x340 net/core/dev.c:7742
 napi_poll net/core/dev.c:7805 [inline]
 net_rx_action+0x627/0xf70 net/core/dev.c:7962
 handle_softirqs+0x22a/0x870 kernel/softirq.c:622
 do_softirq+0x76/0xd0 kernel/softirq.c:523
 </IRQ>
 <TASK>
 __local_bh_enable_ip+0xf8/0x130 kernel/softirq.c:450
 local_bh_enable include/linux/bottom_half.h:33 [inline]
 tun_rx_batched+0x617/0x790 drivers/net/tun.c:-1
 tun_get_user+0x2aeb/0x3ed0 drivers/net/tun.c:1953
 tun_chr_write_iter+0x113/0x200 drivers/net/tun.c:1999
 new_sync_write fs/read_write.c:595 [inline]
 vfs_write+0x61d/0xb90 fs/read_write.c:688
 ksys_write+0x150/0x270 fs/read_write.c:740
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f177e39c819
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 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 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f177f1e3028 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007f177e616180 RCX: 00007f177e39c819
RDX: 000000000000fdef RSI: 00002000000002c0 RDI: 0000000000000003
RBP: 00007f177e432c91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f177e616218 R14: 00007f177e616180 R15: 00007ffee7d40588
 </TASK>

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x15ace2
flags: 0x57ff00000000000(node=1|zone=2|lastcpupid=0x7ff)
raw: 057ff00000000000 ffffea00056b3888 ffffea00056b3888 0000000000000000
raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
page dumped because: kasan: bad access detected
page_owner info is not present (never set?)

Memory state around the buggy address:
 ffff88815ace2300: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff88815ace2380: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>ffff88815ace2400: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
                                     ^
 ffff88815ace2480: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff88815ace2500: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
==================================================================

---
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