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

From: syzbot <[email-masked]>
Date: Sat, 18 Oct 2025 13:37:30 -0700

syzbot has found a reproducer for the following issue on:

HEAD commit:    f406055cb18c Merge tag 'arm64-fixes' of git://git.kernel.o..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=11cf767c580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=f3e7b5a3627a90dd
dashboard link: https://syzkaller.appspot.com/bug?extid=2860e75836a08b172755
compiler:       gcc (Debian 12.2.0-14+deb12u1) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=155f1de2580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=12b6b52f980000

Downloadable assets:
disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-f406055c.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/a4db2a99bfb1/vmlinux-f406055c.xz
kernel image: https://storage.googleapis.com/syzbot-assets/91d1ca420bac/bzImage-f406055c.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_add_node+0x25db/0x2c00 net/netrom/nr_route.c:248
Read of size 4 at addr ffff888054b8cc30 by task syz.3.7839/22393

CPU: 3 UID: 0 PID: 22393 Comm: syz.3.7839 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0x116/0x1f0 lib/dump_stack.c:120
 print_address_description mm/kasan/report.c:378 [inline]
 print_report+0xcd/0x630 mm/kasan/report.c:482
 kasan_report+0xe0/0x110 mm/kasan/report.c:595
 nr_add_node+0x25db/0x2c00 net/netrom/nr_route.c:248
 nr_rt_ioctl+0x11b7/0x29b0 net/netrom/nr_route.c:651
 nr_ioctl+0x19a/0x2d0 net/netrom/af_netrom.c:1254
 sock_do_ioctl+0x118/0x280 net/socket.c:1254
 sock_ioctl+0x227/0x6b0 net/socket.c:1375
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:597 [inline]
 __se_sys_ioctl fs/ioctl.c:583 [inline]
 __x64_sys_ioctl+0x18e/0x210 fs/ioctl.c:583
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xcd/0xfa0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f7f4fb8efc9
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 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 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f7f509c5038 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007f7f4fde5fa0 RCX: 00007f7f4fb8efc9
RDX: 0000200000000280 RSI: 000000000000890b RDI: 0000000000000004
RBP: 00007f7f4fc11f91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f7f4fde6038 R14: 00007f7f4fde5fa0 R15: 00007ffd1bd56f18
 </TASK>

Allocated by task 22380:
 kasan_save_stack+0x33/0x60 mm/kasan/common.c:56
 kasan_save_track+0x14/0x30 mm/kasan/common.c:77
 poison_kmalloc_redzone mm/kasan/common.c:400 [inline]
 __kasan_kmalloc+0xaa/0xb0 mm/kasan/common.c:417
 kmalloc_noprof include/linux/slab.h:957 [inline]
 nr_add_node+0xe4e/0x2c00 net/netrom/nr_route.c:146
 nr_rt_ioctl+0x11b7/0x29b0 net/netrom/nr_route.c:651
 nr_ioctl+0x19a/0x2d0 net/netrom/af_netrom.c:1254
 sock_do_ioctl+0x118/0x280 net/socket.c:1254
 sock_ioctl+0x227/0x6b0 net/socket.c:1375
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:597 [inline]
 __se_sys_ioctl fs/ioctl.c:583 [inline]
 __x64_sys_ioctl+0x18e/0x210 fs/ioctl.c:583
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xcd/0xfa0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Freed by task 22393:
 kasan_save_stack+0x33/0x60 mm/kasan/common.c:56
 kasan_save_track+0x14/0x30 mm/kasan/common.c:77
 __kasan_save_free_info+0x3b/0x60 mm/kasan/generic.c:587
 kasan_save_free_info mm/kasan/kasan.h:406 [inline]
 poison_slab_object mm/kasan/common.c:252 [inline]
 __kasan_slab_free+0x5f/0x80 mm/kasan/common.c:284
 kasan_slab_free include/linux/kasan.h:234 [inline]
 slab_free_hook mm/slub.c:2523 [inline]
 slab_free mm/slub.c:6611 [inline]
 kfree+0x2b8/0x6d0 mm/slub.c:6818
 nr_neigh_put include/net/netrom.h:143 [inline]
 nr_neigh_put include/net/netrom.h:137 [inline]
 nr_add_node+0x23b9/0x2c00 net/netrom/nr_route.c:246
 nr_rt_ioctl+0x11b7/0x29b0 net/netrom/nr_route.c:651
 nr_ioctl+0x19a/0x2d0 net/netrom/af_netrom.c:1254
 sock_do_ioctl+0x118/0x280 net/socket.c:1254
 sock_ioctl+0x227/0x6b0 net/socket.c:1375
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:597 [inline]
 __se_sys_ioctl fs/ioctl.c:583 [inline]
 __x64_sys_ioctl+0x18e/0x210 fs/ioctl.c:583
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xcd/0xfa0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

The buggy address belongs to the object at ffff888054b8cc00
 which belongs to the cache kmalloc-64 of size 64
The buggy address is located 48 bytes inside of
 freed 64-byte region [ffff888054b8cc00, ffff888054b8cc40)

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x54b8c
flags: 0xfff00000000000(node=0|zone=1|lastcpupid=0x7ff)
page_type: f5(slab)
raw: 00fff00000000000 ffff88801b4428c0 dead000000000100 dead000000000122
raw: 0000000000000000 0000000000200020 00000000f5000000 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 6074, tgid 6074 (syz-executor), ts 126099390276, free_ts 126098882568
 set_page_owner include/linux/page_owner.h:32 [inline]
 post_alloc_hook+0x1c0/0x230 mm/page_alloc.c:1850
 prep_new_page mm/page_alloc.c:1858 [inline]
 get_page_from_freelist+0x10a3/0x3a30 mm/page_alloc.c:3884
 __alloc_frozen_pages_noprof+0x25f/0x2470 mm/page_alloc.c:5183
 alloc_pages_mpol+0x1fb/0x550 mm/mempolicy.c:2416
 alloc_slab_page mm/slub.c:3039 [inline]
 allocate_slab mm/slub.c:3212 [inline]
 new_slab+0x24a/0x360 mm/slub.c:3266
 ___slab_alloc+0xdc4/0x1ae0 mm/slub.c:4636
 __slab_alloc.constprop.0+0x63/0x110 mm/slub.c:4755
 __slab_alloc_node mm/slub.c:4831 [inline]
 slab_alloc_node mm/slub.c:5253 [inline]
 __kmalloc_cache_noprof+0x477/0x780 mm/slub.c:5743
 kmalloc_noprof include/linux/slab.h:957 [inline]
 __team_option_inst_add+0xbf/0x330 drivers/net/team/team_core.c:160
 __team_option_inst_add_option drivers/net/team/team_core.c:182 [inline]
 __team_options_register+0x355/0x720 drivers/net/team/team_core.c:276
 team_options_register drivers/net/team/team_core.c:341 [inline]
 team_init+0x5c8/0xce0 drivers/net/team/team_core.c:1658
 register_netdevice+0x653/0x2270 net/core/dev.c:11221
 team_newlink+0xb4/0x190 drivers/net/team/team_core.c:2213
 rtnl_newlink_create net/core/rtnetlink.c:3833 [inline]
 __rtnl_newlink net/core/rtnetlink.c:3950 [inline]
 rtnl_newlink+0xc45/0x2000 net/core/rtnetlink.c:4065
 rtnetlink_rcv_msg+0x95e/0xe90 net/core/rtnetlink.c:6954
 netlink_rcv_skb+0x158/0x420 net/netlink/af_netlink.c:2552
page last free pid 6074 tgid 6074 stack trace:
 reset_page_owner include/linux/page_owner.h:25 [inline]
 free_pages_prepare mm/page_alloc.c:1394 [inline]
 __free_frozen_pages+0x7df/0x1160 mm/page_alloc.c:2906
 selinux_genfs_get_sid security/selinux/hooks.c:1357 [inline]
 inode_doinit_with_dentry+0xacb/0x12e0 security/selinux/hooks.c:1555
 selinux_d_instantiate+0x26/0x30 security/selinux/hooks.c:6523
 security_d_instantiate+0x142/0x1a0 security/security.c:4148
 d_instantiate+0x5c/0x90 fs/dcache.c:1961
 __debugfs_create_file+0x286/0x6b0 fs/debugfs/inode.c:459
 debugfs_create_file_full+0x41/0x60 fs/debugfs/inode.c:469
 ref_tracker_dir_debugfs+0x19d/0x290 lib/ref_tracker.c:441
 ref_tracker_dir_init include/linux/ref_tracker.h:70 [inline]
 alloc_netdev_mqs+0x314/0x1550 net/core/dev.c:11907
 rtnl_create_link+0xc08/0xf90 net/core/rtnetlink.c:3641
 rtnl_newlink_create net/core/rtnetlink.c:3823 [inline]
 __rtnl_newlink net/core/rtnetlink.c:3950 [inline]
 rtnl_newlink+0xb69/0x2000 net/core/rtnetlink.c:4065
 rtnetlink_rcv_msg+0x95e/0xe90 net/core/rtnetlink.c:6954
 netlink_rcv_skb+0x158/0x420 net/netlink/af_netlink.c:2552
 netlink_unicast_kernel net/netlink/af_netlink.c:1320 [inline]
 netlink_unicast+0x5aa/0x870 net/netlink/af_netlink.c:1346
 netlink_sendmsg+0x8c8/0xdd0 net/netlink/af_netlink.c:1896
 sock_sendmsg_nosec net/socket.c:727 [inline]
 __sock_sendmsg net/socket.c:742 [inline]
 __sys_sendto+0x4a3/0x520 net/socket.c:2244

Memory state around the buggy address:
 ffff888054b8cb00: 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc fc
 ffff888054b8cb80: fa fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
>ffff888054b8cc00: fa fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
                                     ^
 ffff888054b8cc80: 00 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc
 ffff888054b8cd00: 00 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc
==================================================================

---
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#syz test: git://repo/address.git branch-or-commit-hash
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