[syzbot] [hams?] KASAN: slab-use-after-free Write in nr_neigh_put

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 6ad42b244136 Merge branch 'bng_en-enhancements-for-rx-and-..
git tree: net-next
console output: https://syzkaller.appspot.com/x/log.txt?x=1421a72a580000
kernel config: https://syzkaller.appspot.com/x/.config?x=2c36acc86fd56a9d
dashboard link: https://syzkaller.appspot.com/bug?extid=08ca620c862d89afe6d6
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/2b03def814aa/disk-6ad42b24.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/e677d64cb110/vmlinux-6ad42b24.xz
kernel image: https://storage.googleapis.com/syzbot-assets/e89660ed5369/bzImage-6ad42b24.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+08ca62...@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: slab-use-after-free in instrument_atomic_read_write include/linux/instrumented.h:96 [inline]
BUG: KASAN: slab-use-after-free in atomic_fetch_sub_release include/linux/atomic/atomic-instrumented.h:400 [inline]
BUG: KASAN: slab-use-after-free in __refcount_sub_and_test include/linux/refcount.h:389 [inline]
BUG: KASAN: slab-use-after-free in __refcount_dec_and_test include/linux/refcount.h:432 [inline]
BUG: KASAN: slab-use-after-free in refcount_dec_and_test include/linux/refcount.h:450 [inline]
BUG: KASAN: slab-use-after-free in nr_neigh_put+0x21/0x170 include/net/netrom.h:139
Write of size 4 at addr ffff888026ddaf3c by task syz.1.8003/5600

CPU: 0 UID: 0 PID: 5600 Comm: syz.1.8003 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/24/2026
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
check_region_inline mm/kasan/generic.c:-1 [inline]
kasan_check_range+0x264/0x2c0 mm/kasan/generic.c:200
instrument_atomic_read_write include/linux/instrumented.h:96 [inline]
atomic_fetch_sub_release include/linux/atomic/atomic-instrumented.h:400 [inline]
__refcount_sub_and_test include/linux/refcount.h:389 [inline]
__refcount_dec_and_test include/linux/refcount.h:432 [inline]
refcount_dec_and_test include/linux/refcount.h:450 [inline]
nr_neigh_put+0x21/0x170 include/net/netrom.h:139
nr_del_neigh+0x25a/0x2d0 net/netrom/nr_route.c:440
nr_rt_ioctl+0xd62/0xf90 net/netrom/nr_route.c:682
sock_do_ioctl+0x101/0x320 net/socket.c:1254
sock_ioctl+0x5c6/0x7f0 net/socket.c:1375
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xe2/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f3bd0f9aeb9
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:00007f3bd1d7a028 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007f3bd1216090 RCX: 00007f3bd0f9aeb9
RDX: 0000200000000680 RSI: 000000000000890c RDI: 0000000000000006
RBP: 00007f3bd1008c1f R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f3bd1216128 R14: 00007f3bd1216090 R15: 00007fffa738a9e8
</TASK>

Allocated by task 5599:
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+0x3d1/0x6e0 mm/slub.c:5780
kmalloc_noprof include/linux/slab.h:957 [inline]
nr_add_neigh+0x104/0x520 net/netrom/nr_route.c:392
nr_rt_ioctl+0xdf9/0xf90 net/netrom/nr_route.c:660
sock_do_ioctl+0x101/0x320 net/socket.c:1254
sock_ioctl+0x5c6/0x7f0 net/socket.c:1375
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xe2/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

Freed by task 5600:
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:2540 [inline]
slab_free mm/slub.c:6674 [inline]
kfree+0x1be/0x650 mm/slub.c:6882
__nr_remove_neigh net/netrom/nr_route.c:299 [inline]
nr_remove_neigh net/netrom/nr_route.c:308 [inline]
nr_del_neigh+0x246/0x2d0 net/netrom/nr_route.c:439
nr_rt_ioctl+0xd62/0xf90 net/netrom/nr_route.c:682
sock_do_ioctl+0x101/0x320 net/socket.c:1254
sock_ioctl+0x5c6/0x7f0 net/socket.c:1375
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xe2/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

The buggy address belongs to the object at ffff888026ddaf00
which belongs to the cache kmalloc-64 of size 64
The buggy address is located 60 bytes inside of
freed 64-byte region [ffff888026ddaf00, ffff888026ddaf40)

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x26dda
ksm flags: 0xfff00000000000(node=0|zone=1|lastcpupid=0x7ff)
page_type: f5(slab)
raw: 00fff00000000000 ffff88813fe268c0 ffffea0000a56ec0 dead000000000003
raw: 0000000000000000 0000000080200020 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 5835, tgid 5835 (syz-executor), ts 88717173954, free_ts 88621883212
set_page_owner include/linux/page_owner.h:32 [inline]
post_alloc_hook+0x228/0x280 mm/page_alloc.c:1884
prep_new_page mm/page_alloc.c:1892 [inline]
get_page_from_freelist+0x24dc/0x2580 mm/page_alloc.c:3945
__alloc_frozen_pages_noprof+0x18d/0x380 mm/page_alloc.c:5240
alloc_pages_mpol+0x232/0x4a0 mm/mempolicy.c:2486
alloc_slab_page mm/slub.c:3075 [inline]
allocate_slab+0x86/0x3a0 mm/slub.c:3248
new_slab mm/slub.c:3302 [inline]
___slab_alloc+0xd82/0x1760 mm/slub.c:4656
__slab_alloc+0x65/0x100 mm/slub.c:4779
__slab_alloc_node mm/slub.c:4855 [inline]
slab_alloc_node mm/slub.c:5251 [inline]
__do_kmalloc_node mm/slub.c:5656 [inline]
__kmalloc_noprof+0x46c/0x7e0 mm/slub.c:5669
kmalloc_noprof include/linux/slab.h:961 [inline]
kzalloc_noprof include/linux/slab.h:1094 [inline]
tnode_alloc net/ipv4/fib_trie.c:327 [inline]
tnode_new+0xc3/0x3b0 net/ipv4/fib_trie.c:380
fib_insert_node net/ipv4/fib_trie.c:1125 [inline]
fib_insert_alias+0x9f7/0x1100 net/ipv4/fib_trie.c:1159
fib_table_insert+0x695/0x1b50 net/ipv4/fib_trie.c:1354
fib_magic+0x434/0x510 net/ipv4/fib_frontend.c:1134
fib_add_ifaddr+0x144/0x5f0 net/ipv4/fib_frontend.c:1156
fib_inetaddr_event+0x12e/0x190 net/ipv4/fib_frontend.c:1470
notifier_call_chain+0x19d/0x3a0 kernel/notifier.c:85
blocking_notifier_call_chain+0x6a/0x90 kernel/notifier.c:380
page last free pid 5832 tgid 5832 stack trace:
reset_page_owner include/linux/page_owner.h:25 [inline]
free_pages_prepare mm/page_alloc.c:1433 [inline]
__free_frozen_pages+0xbf8/0xd70 mm/page_alloc.c:2973
rcu_do_batch kernel/rcu/tree.c:2605 [inline]
rcu_core+0xc9e/0x1750 kernel/rcu/tree.c:2857
handle_softirqs+0x22a/0x7c0 kernel/softirq.c:622
__do_softirq kernel/softirq.c:656 [inline]
invoke_softirq kernel/softirq.c:496 [inline]
__irq_exit_rcu+0x5f/0x150 kernel/softirq.c:723
irq_exit_rcu+0x9/0x30 kernel/softirq.c:739
instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1056 [inline]
sysvec_apic_timer_interrupt+0xa6/0xc0 arch/x86/kernel/apic/apic.c:1056
asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:697

Memory state around the buggy address:
ffff888026ddae00: 00 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc
ffff888026ddae80: 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc fc
>ffff888026ddaf00: fa fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
^
ffff888026ddaf80: fa fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
ffff888026ddb000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
==================================================================


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