Subject: [syzbot] [hams?] KASAN: slab-use-after-free Read in ax25_disconnect

From: syzbot <[email-masked]>
Date: Wed, 11 Mar 2026 11:52:28 -0700

Hello,

syzbot found the following issue on:

HEAD commit:    1f318b96cc84 Linux 7.0-rc3
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=13d41806580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=2a019678b1a3a692
dashboard link: https://syzkaller.appspot.com/bug?extid=41ebf587f04e2bcfa8e5
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/9997eccd7489/disk-1f318b96.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/092609f7d984/vmlinux-1f318b96.xz
kernel image: https://storage.googleapis.com/syzbot-assets/822d2e7e40b1/bzImage-1f318b96.xz

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

==================================================================
BUG: KASAN: slab-use-after-free in ax25_disconnect+0x19c/0x3c0 net/ax25/ax25_subr.c:283
Read of size 8 at addr ffff8880600774c0 by task ktimers/1/29

CPU: 1 UID: 0 PID: 29 Comm: ktimers/1 Tainted: G             L      syzkaller #0 PREEMPT_{RT,(full)} 
Tainted: [L]=SOFTLOCKUP
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
Call Trace:
 <TASK>
 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
 ax25_disconnect+0x19c/0x3c0 net/ax25/ax25_subr.c:283
 call_timer_fn+0x192/0x640 kernel/time/timer.c:1748
 expire_timers kernel/time/timer.c:1799 [inline]
 __run_timers kernel/time/timer.c:2373 [inline]
 __run_timer_base+0x6a3/0x9f0 kernel/time/timer.c:2385
 run_timer_base kernel/time/timer.c:2394 [inline]
 run_timer_softirq+0xb7/0x170 kernel/time/timer.c:2404
 handle_softirqs+0x1de/0x6f0 kernel/softirq.c:622
 __do_softirq kernel/softirq.c:656 [inline]
 run_ktimerd+0x69/0x100 kernel/softirq.c:1138
 smpboot_thread_fn+0x541/0xa50 kernel/smpboot.c:160
 kthread+0x388/0x470 kernel/kthread.c:436
 ret_from_fork+0x51e/0xb90 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>

Allocated by task 20813:
 kasan_save_stack mm/kasan/common.c:57 [inline]
 kasan_save_track+0x3e/0x80 mm/kasan/common.c:78
 poison_kmalloc_redzone mm/kasan/common.c:398 [inline]
 __kasan_kmalloc+0x93/0xb0 mm/kasan/common.c:415
 kasan_kmalloc include/linux/kasan.h:263 [inline]
 __kmalloc_cache_noprof+0x3a6/0x690 mm/slub.c:5383
 kmalloc_noprof include/linux/slab.h:950 [inline]
 kzalloc_noprof include/linux/slab.h:1188 [inline]
 ax25_create_cb+0x50/0x5a0 net/ax25/af_ax25.c:531
 ax25_send_frame+0x445/0x9f0 net/ax25/ax25_out.c:67
 rose_send_frame net/rose/rose_link.c:106 [inline]
 rose_transmit_restart_request net/rose/rose_link.c:198 [inline]
 rose_transmit_link+0x729/0xac0 net/rose/rose_link.c:284
 rose_write_internal+0x1256/0x1b60 net/rose/rose_subr.c:198
 rose_connect+0x93b/0x1110 net/rose/af_rose.c:881
 __sys_connect_file net/socket.c:2089 [inline]
 __sys_connect+0x315/0x450 net/socket.c:2108
 __do_sys_connect net/socket.c:2114 [inline]
 __se_sys_connect net/socket.c:2111 [inline]
 __x64_sys_connect+0x7a/0x90 net/socket.c:2111
 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

Freed by task 16:
 kasan_save_stack mm/kasan/common.c:57 [inline]
 kasan_save_track+0x3e/0x80 mm/kasan/common.c:78
 kasan_save_free_info+0x46/0x50 mm/kasan/generic.c:584
 poison_slab_object mm/kasan/common.c:253 [inline]
 __kasan_slab_free+0x5c/0x80 mm/kasan/common.c:285
 kasan_slab_free include/linux/kasan.h:235 [inline]
 slab_free_hook mm/slub.c:2692 [inline]
 slab_free mm/slub.c:6168 [inline]
 kfree+0x1c1/0x6c0 mm/slub.c:6486
 call_timer_fn+0x192/0x640 kernel/time/timer.c:1748
 expire_timers kernel/time/timer.c:1799 [inline]
 __run_timers kernel/time/timer.c:2373 [inline]
 __run_timer_base+0x6a3/0x9f0 kernel/time/timer.c:2385
 run_timer_base kernel/time/timer.c:2394 [inline]
 run_timer_softirq+0xb7/0x170 kernel/time/timer.c:2404
 handle_softirqs+0x1de/0x6f0 kernel/softirq.c:622
 __do_softirq kernel/softirq.c:656 [inline]
 run_ktimerd+0x69/0x100 kernel/softirq.c:1138
 smpboot_thread_fn+0x541/0xa50 kernel/smpboot.c:160
 kthread+0x388/0x470 kernel/kthread.c:436
 ret_from_fork+0x51e/0xb90 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

The buggy address belongs to the object at ffff888060077000
 which belongs to the cache kmalloc-2k of size 2048
The buggy address is located 1216 bytes inside of
 freed 2048-byte region [ffff888060077000, ffff888060077800)

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0xffff888060070000 pfn:0x60070
head: order:3 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
flags: 0x80000000000240(workingset|head|node=0|zone=1)
page_type: f5(slab)
raw: 0080000000000240 ffff88813fe1d000 ffff88813fe17c88 ffffea0001d8b810
raw: ffff888060070000 0000000800080006 00000000f5000000 0000000000000000
head: 0080000000000240 ffff88813fe1d000 ffff88813fe17c88 ffffea0001d8b810
head: ffff888060070000 0000000800080006 00000000f5000000 0000000000000000
head: 0080000000000003 ffffea0001801c01 00000000ffffffff 00000000ffffffff
head: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000008
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 3, migratetype Unmovable, gfp_mask 0xd2820(GFP_ATOMIC|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 5801, tgid 5801 (syz-executor), ts 82913456262, free_ts 0
 set_page_owner include/linux/page_owner.h:32 [inline]
 post_alloc_hook+0x231/0x280 mm/page_alloc.c:1889
 prep_new_page mm/page_alloc.c:1897 [inline]
 get_page_from_freelist+0x28bb/0x2950 mm/page_alloc.c:3962
 __alloc_frozen_pages_noprof+0x18d/0x380 mm/page_alloc.c:5250
 alloc_slab_page mm/slub.c:3296 [inline]
 allocate_slab+0x77/0x660 mm/slub.c:3485
 new_slab mm/slub.c:3543 [inline]
 refill_objects+0x334/0x3c0 mm/slub.c:7178
 __pcs_replace_empty_main+0x371/0x5c0 mm/slub.c:-1
 alloc_from_pcs mm/slub.c:4720 [inline]
 slab_alloc_node mm/slub.c:4854 [inline]
 __do_kmalloc_node mm/slub.c:5262 [inline]
 __kmalloc_node_track_caller_noprof+0x60b/0x7e0 mm/slub.c:5371
 kmalloc_reserve net/core/skbuff.c:635 [inline]
 pskb_expand_head+0x228/0x1320 net/core/skbuff.c:2315
 netlink_trim+0x1b3/0x2c0 net/netlink/af_netlink.c:1299
 netlink_broadcast_filtered+0xd6/0x1010 net/netlink/af_netlink.c:1512
 nlmsg_multicast_filtered include/net/netlink.h:1165 [inline]
 nlmsg_multicast include/net/netlink.h:1184 [inline]
 nlmsg_notify+0xf0/0x1a0 net/netlink/af_netlink.c:2593
 __dev_notify_flags+0xf2/0x310 net/core/dev.c:9799
 netif_change_flags+0xe8/0x1a0 net/core/dev.c:9832
 do_setlink+0xf82/0x4590 net/core/rtnetlink.c:3158
 rtnl_changelink net/core/rtnetlink.c:3776 [inline]
 __rtnl_newlink net/core/rtnetlink.c:3935 [inline]
 rtnl_newlink+0x15a9/0x1be0 net/core/rtnetlink.c:4072
 rtnetlink_rcv_msg+0x7d5/0xbe0 net/core/rtnetlink.c:6958
page_owner free stack trace missing

Memory state around the buggy address:
 ffff888060077380: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff888060077400: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff888060077480: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                                           ^
 ffff888060077500: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff888060077580: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================

---
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