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

From: syzbot <[email-masked]>
Date: Thu, 15 Jan 2026 12:27:21 -0800

Hello,

syzbot found the following issue on:

HEAD commit:    be548645527a Merge tag 'for-linus' of git://git.kernel.org..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=13a78a18580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=7ac4cd61d548c1ef
dashboard link: https://syzkaller.appspot.com/bug?extid=df52f4216bf7b4d768e7
compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1064ebc4580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1751b1df980000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/91e0bf046854/disk-be548645.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/ff709482aab5/vmlinux-be548645.xz
kernel image: https://storage.googleapis.com/syzbot-assets/2fe3e1268671/bzImage-be548645.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 nr_dec_obs net/netrom/nr_route.c:471 [inline]
BUG: KASAN: slab-use-after-free in nr_rt_ioctl+0x599/0xfb0 net/netrom/nr_route.c:692
Read of size 2 at addr ffff88807ee561b2 by task syz-executor425/6336

CPU: 1 UID: 0 PID: 6336 Comm: syz-executor425 Not tainted 6.13.0-rc6-syzkaller-00290-gbe548645527a #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120
 print_address_description mm/kasan/report.c:378 [inline]
 print_report+0x169/0x550 mm/kasan/report.c:489
 kasan_report+0x143/0x180 mm/kasan/report.c:602
 nr_dec_obs net/netrom/nr_route.c:471 [inline]
 nr_rt_ioctl+0x599/0xfb0 net/netrom/nr_route.c:692
 sock_do_ioctl+0x158/0x460 net/socket.c:1209
 sock_ioctl+0x626/0x8e0 net/socket.c:1328
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:906 [inline]
 __se_sys_ioctl+0xf5/0x170 fs/ioctl.c:892
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7ff6b4589869
Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 31 1a 00 00 90 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 b8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffd2b36d5d8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007ff6b4589869
RDX: 0000000000000000 RSI: 00000000000089e2 RDI: 0000000000000005
RBP: 0000000000000000 R08: 00007ff6b45d7214 R09: 00007ff6b45d7214
R10: 00007ff6b45d7214 R11: 0000000000000246 R12: 00007ffd2b36d5fc
R13: 00007ffd2b36d630 R14: 00007ffd2b36d610 R15: 0000000000000067
 </TASK>

Allocated by task 6336:
 kasan_save_stack mm/kasan/common.c:47 [inline]
 kasan_save_track+0x3f/0x80 mm/kasan/common.c:68
 poison_kmalloc_redzone mm/kasan/common.c:377 [inline]
 __kasan_kmalloc+0x98/0xb0 mm/kasan/common.c:394
 kasan_kmalloc include/linux/kasan.h:260 [inline]
 __kmalloc_cache_noprof+0x243/0x390 mm/slub.c:4329
 kmalloc_noprof include/linux/slab.h:901 [inline]
 nr_add_node+0x856/0x2230 net/netrom/nr_route.c:146
 nr_rt_ioctl+0xd43/0xfb0 net/netrom/nr_route.c:651
 sock_do_ioctl+0x158/0x460 net/socket.c:1209
 sock_ioctl+0x626/0x8e0 net/socket.c:1328
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:906 [inline]
 __se_sys_ioctl+0xf5/0x170 fs/ioctl.c:892
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Freed by task 6336:
 kasan_save_stack mm/kasan/common.c:47 [inline]
 kasan_save_track+0x3f/0x80 mm/kasan/common.c:68
 kasan_save_free_info+0x40/0x50 mm/kasan/generic.c:582
 poison_slab_object mm/kasan/common.c:247 [inline]
 __kasan_slab_free+0x59/0x70 mm/kasan/common.c:264
 kasan_slab_free include/linux/kasan.h:233 [inline]
 slab_free_hook mm/slub.c:2353 [inline]
 slab_free mm/slub.c:4613 [inline]
 kfree+0x196/0x430 mm/slub.c:4761
 nr_dec_obs net/netrom/nr_route.c:469 [inline]
 nr_rt_ioctl+0x2e5/0xfb0 net/netrom/nr_route.c:692
 sock_do_ioctl+0x158/0x460 net/socket.c:1209
 sock_ioctl+0x626/0x8e0 net/socket.c:1328
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:906 [inline]
 __se_sys_ioctl+0xf5/0x170 fs/ioctl.c:892
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

The buggy address belongs to the object at ffff88807ee56180
 which belongs to the cache kmalloc-64 of size 64
The buggy address is located 50 bytes inside of
 freed 64-byte region [ffff88807ee56180, ffff88807ee561c0)

The buggy address belongs to the physical page:
page: refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x7ee56
ksm flags: 0xfff00000000000(node=0|zone=1|lastcpupid=0x7ff)
page_type: f5(slab)
raw: 00fff00000000000 ffff88801ac418c0 ffffea0000cf4700 0000000000000007
raw: 0000000000000000 0000000080200020 00000001f5000000 0000000000000000
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 0, migratetype Unmovable, gfp_mask 0x52cc0(GFP_KERNEL|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP), pid 5824, tgid 5824 (syz-executor425), ts 77902070664, free_ts 77575093775
 set_page_owner include/linux/page_owner.h:32 [inline]
 post_alloc_hook+0x1f3/0x230 mm/page_alloc.c:1558
 prep_new_page mm/page_alloc.c:1566 [inline]
 get_page_from_freelist+0x3651/0x37a0 mm/page_alloc.c:3476
 __alloc_pages_noprof+0x292/0x710 mm/page_alloc.c:4753
 alloc_pages_mpol_noprof+0x3e8/0x680 mm/mempolicy.c:2269
 alloc_slab_page+0x6a/0x110 mm/slub.c:2423
 allocate_slab+0x5a/0x2b0 mm/slub.c:2589
 new_slab mm/slub.c:2642 [inline]
 ___slab_alloc+0xc27/0x14a0 mm/slub.c:3830
 __slab_alloc+0x58/0xa0 mm/slub.c:3920
 __slab_alloc_node mm/slub.c:3995 [inline]
 slab_alloc_node mm/slub.c:4156 [inline]
 __kmalloc_cache_noprof+0x27b/0x390 mm/slub.c:4324
 kmalloc_noprof include/linux/slab.h:901 [inline]
 __netdev_adjacent_dev_insert+0x157/0x8d0 net/core/dev.c:7889
 __netdev_adjacent_dev_link_lists net/core/dev.c:7981 [inline]
 __netdev_adjacent_dev_link_neighbour net/core/dev.c:8010 [inline]
 __netdev_upper_dev_link+0x404/0x670 net/core/dev.c:8070
 netdev_upper_dev_link+0x9e/0xf0 net/core/dev.c:8116
 ipvlan_link_new+0x5cb/0xa30 drivers/net/ipvlan/ipvlan_main.c:621
 rtnl_newlink_create+0x2ee/0xa40 net/core/rtnetlink.c:3786
 __rtnl_newlink net/core/rtnetlink.c:3897 [inline]
 rtnl_newlink+0x1c7e/0x2210 net/core/rtnetlink.c:4012
 rtnetlink_rcv_msg+0x791/0xcf0 net/core/rtnetlink.c:6922
page last free pid 5829 tgid 5829 stack trace:
 reset_page_owner include/linux/page_owner.h:25 [inline]
 free_pages_prepare mm/page_alloc.c:1127 [inline]
 free_unref_page+0xd2c/0x1000 mm/page_alloc.c:2659
 __slab_free+0x2c2/0x380 mm/slub.c:4524
 qlink_free mm/kasan/quarantine.c:163 [inline]
 qlist_free_all+0x9a/0x140 mm/kasan/quarantine.c:179
 kasan_quarantine_reduce+0x14f/0x170 mm/kasan/quarantine.c:286
 __kasan_slab_alloc+0x23/0x80 mm/kasan/common.c:329
 kasan_slab_alloc include/linux/kasan.h:250 [inline]
 slab_post_alloc_hook mm/slub.c:4119 [inline]
 slab_alloc_node mm/slub.c:4168 [inline]
 __kmalloc_cache_noprof+0x1d9/0x390 mm/slub.c:4324
 kmalloc_noprof include/linux/slab.h:901 [inline]
 kzalloc_noprof include/linux/slab.h:1037 [inline]
 kset_create lib/kobject.c:965 [inline]
 kset_create_and_add+0x5b/0x170 lib/kobject.c:1008
 register_queue_kobjects net/core/net-sysfs.c:1890 [inline]
 netdev_register_kobject+0x181/0x2e0 net/core/net-sysfs.c:2143
 register_netdevice+0x12c5/0x1b00 net/core/dev.c:10618
 veth_newlink+0x3fd/0xb00 drivers/net/veth.c:1815
 rtnl_newlink_create+0x2ee/0xa40 net/core/rtnetlink.c:3786
 __rtnl_newlink net/core/rtnetlink.c:3897 [inline]
 rtnl_newlink+0x1c7e/0x2210 net/core/rtnetlink.c:4012
 rtnetlink_rcv_msg+0x791/0xcf0 net/core/rtnetlink.c:6922
 netlink_rcv_skb+0x1e3/0x430 net/netlink/af_netlink.c:2542
 netlink_unicast_kernel net/netlink/af_netlink.c:1321 [inline]
 netlink_unicast+0x7f6/0x990 net/netlink/af_netlink.c:1347
 netlink_sendmsg+0x8e4/0xcb0 net/netlink/af_netlink.c:1891

Memory state around the buggy address:
 ffff88807ee56080: 00 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc
 ffff88807ee56100: 00 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc
>ffff88807ee56180: fa fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
                                     ^
 ffff88807ee56200: fa fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
 ffff88807ee56280: 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc fc
==================================================================

---
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