[moderation] [net?] KASAN: slab-use-after-free Read in ipv6_chk_acast_addr

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 550604d6c9b9 Add linux-next specific files for 20260521
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=1726e47e580000
kernel config: https://syzkaller.appspot.com/x/.config?x=17ab782ed835fc0e
dashboard link: https://syzkaller.appspot.com/bug?extid=a01df04303c131efbf3a
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
CC: [da...@davemloft.net dsa...@kernel.org edum...@google.com ho...@kernel.org ido...@nvidia.com ku...@kernel.org linux-...@vger.kernel.org net...@vger.kernel.org pab...@redhat.com]

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/ea4e4e647faa/disk-550604d6.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/5ea9373b711c/vmlinux-550604d6.xz
kernel image: https://storage.googleapis.com/syzbot-assets/a2f381baa460/bzImage-550604d6.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+a01df0...@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: slab-use-after-free in ipv6_chk_acast_addr+0x2c9/0xb20 net/ipv6/anycast.c:504
Read of size 8 at addr ffff88805f5cd510 by task ktimers/0/16

CPU: 0 UID: 0 PID: 16 Comm: ktimers/0 Tainted: G L syzkaller #0 PREEMPT_{RT,(full)}
Tainted: [L]=SOFTLOCKUP
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
Call Trace:
<TASK>
dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
print_address_description+0x55/0x1e0 mm/kasan/report.c:378
print_report+0x58/0x70 mm/kasan/report.c:482
kasan_report+0x117/0x150 mm/kasan/report.c:595
ipv6_chk_acast_addr+0x2c9/0xb20 net/ipv6/anycast.c:504
icmp6_send+0x4c3/0x1b50 net/ipv6/icmp.c:679
icmpv6_send include/linux/icmpv6.h:24 [inline]
ip6_link_failure+0x3b/0x4c0 net/ipv6/route.c:2862
dst_link_failure include/net/dst.h:456 [inline]
ipip6_tunnel_xmit net/ipv6/sit.c:1035 [inline]
sit_tunnel_xmit+0xa44/0x1b80 net/ipv6/sit.c:1072
__netdev_start_xmit include/linux/netdevice.h:5394 [inline]
netdev_start_xmit include/linux/netdevice.h:5403 [inline]
xmit_one net/core/dev.c:3889 [inline]
dev_hard_start_xmit+0x2df/0x860 net/core/dev.c:3905
__dev_queue_xmit+0x1428/0x3900 net/core/dev.c:4872
NF_HOOK_COND include/linux/netfilter.h:307 [inline]
ip6_output+0x340/0x550 net/ipv6/ip6_output.c:246
ip6tunnel_xmit+0x226/0x6c0 include/net/ip6_tunnel.h:174
ip6_tnl_xmit+0x216d/0x2930 net/ipv6/ip6_tunnel.c:1292
__gre6_xmit+0xa4d/0xd50 net/ipv6/ip6_gre.c:787
ip6gre_xmit_ipv6 net/ipv6/ip6_gre.c:845 [inline]
ip6gre_tunnel_xmit+0xc3b/0xf70 net/ipv6/ip6_gre.c:897
__netdev_start_xmit include/linux/netdevice.h:5394 [inline]
netdev_start_xmit include/linux/netdevice.h:5403 [inline]
xmit_one net/core/dev.c:3889 [inline]
dev_hard_start_xmit+0x2df/0x860 net/core/dev.c:3905
sch_direct_xmit+0x251/0x4c0 net/sched/sch_generic.c:372
__dev_xmit_skb net/core/dev.c:4211 [inline]
__dev_queue_xmit+0x171e/0x3900 net/core/dev.c:4833
NF_HOOK_COND include/linux/netfilter.h:307 [inline]
ip6_output+0x340/0x550 net/ipv6/ip6_output.c:246
dst_output include/net/dst.h:488 [inline]
NF_HOOK include/linux/netfilter.h:318 [inline]
ndisc_send_skb+0xd0b/0x1670 net/ipv6/ndisc.c:512
addrconf_rs_timer+0x395/0x6d0 net/ipv6/addrconf.c:4056
call_timer_fn+0x192/0x5e0 kernel/time/timer.c:1748
expire_timers kernel/time/timer.c:1799 [inline]
__run_timers kernel/time/timer.c:2374 [inline]
__run_timer_base+0x6a3/0x9f0 kernel/time/timer.c:2386
run_timer_base kernel/time/timer.c:2395 [inline]
run_timer_softirq+0xb7/0x170 kernel/time/timer.c:2405
handle_softirqs+0x1de/0x6d0 kernel/softirq.c:626
__do_softirq kernel/softirq.c:660 [inline]
run_ktimerd+0x69/0x100 kernel/softirq.c:1155
smpboot_thread_fn+0x541/0xa50 kernel/smpboot.c:160
kthread+0x389/0x470 kernel/kthread.c:436
ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
</TASK>

Allocated by task 10052:
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_node_noprof+0x27d/0x6c0 mm/slub.c:5437
_kmalloc_node_noprof include/linux/slab.h:1168 [inline]
__get_vm_area_node+0x13f/0x300 mm/vmalloc.c:3215
__vmalloc_node_range_noprof+0x36a/0x1750 mm/vmalloc.c:4022
__vmalloc_node_noprof mm/vmalloc.c:4123 [inline]
vmalloc_noprof+0xb2/0xe0 mm/vmalloc.c:4158
bpf_prog_calc_tag+0x4c/0x370 kernel/bpf/core.c:307
check_and_resolve_insns+0xbd/0x2510 kernel/bpf/verifier.c:18084
bpf_check+0x12bf/0x51d0 kernel/bpf/verifier.c:19694
bpf_prog_load+0x1406/0x1a10 kernel/bpf/syscall.c:3128
__sys_bpf+0xd0b/0xd90 kernel/bpf/syscall.c:6350
__do_sys_bpf kernel/bpf/syscall.c:6469 [inline]
__se_sys_bpf kernel/bpf/syscall.c:6466 [inline]
__x64_sys_bpf+0xba/0xd0 kernel/bpf/syscall.c:6466
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x15f/0x560 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

Freed by task 10052:
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:2700 [inline]
slab_free mm/slub.c:6284 [inline]
kfree+0x1c5/0x6c0 mm/slub.c:6599
bpf_prog_calc_tag+0x315/0x370 kernel/bpf/core.c:334
check_and_resolve_insns+0xbd/0x2510 kernel/bpf/verifier.c:18084
bpf_check+0x12bf/0x51d0 kernel/bpf/verifier.c:19694
bpf_prog_load+0x1406/0x1a10 kernel/bpf/syscall.c:3128
__sys_bpf+0xd0b/0xd90 kernel/bpf/syscall.c:6350
__do_sys_bpf kernel/bpf/syscall.c:6469 [inline]
__se_sys_bpf kernel/bpf/syscall.c:6466 [inline]
__x64_sys_bpf+0xba/0xd0 kernel/bpf/syscall.c:6466
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x15f/0x560 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

The buggy address belongs to the object at ffff88805f5cd500
which belongs to the cache kmalloc-96 of size 96
The buggy address is located 16 bytes inside of
freed 96-byte region [ffff88805f5cd500, ffff88805f5cd560)

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0xffff88805f5cdd00 pfn:0x5f5cd
flags: 0x80000000000200(workingset|node=0|zone=1)
page_type: f5(slab)
raw: 0080000000000200 ffff88813fe20280 ffffea00009c7810 ffffea000130be50
raw: ffff88805f5cdd00 0000000800200004 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 0xd2cc0(GFP_KERNEL|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 10, tgid 10 (kworker/0:1), ts 109626662632, free_ts 109611667027
set_page_owner include/linux/page_owner.h:32 [inline]
post_alloc_hook+0x1f9/0x250 mm/page_alloc.c:1866
prep_new_page mm/page_alloc.c:1874 [inline]
get_page_from_freelist+0x265c/0x26e0 mm/page_alloc.c:3953
__alloc_frozen_pages_noprof+0x18d/0x380 mm/page_alloc.c:5296
alloc_slab_page mm/slub.c:3289 [inline]
allocate_slab+0x74/0x5e0 mm/slub.c:3404
new_slab mm/slub.c:3447 [inline]
refill_objects+0x33c/0x3d0 mm/slub.c:7226
refill_sheaf mm/slub.c:2827 [inline]
__pcs_replace_empty_main+0x373/0x720 mm/slub.c:4665
alloc_from_pcs mm/slub.c:4763 [inline]
slab_alloc_node mm/slub.c:4897 [inline]
__kmalloc_cache_noprof+0x44e/0x690 mm/slub.c:5420
_kmalloc_noprof include/linux/slab.h:967 [inline]
_kzalloc_noprof include/linux/slab.h:1284 [inline]
nsim_fib6_rt_create drivers/net/netdevsim/fib.c:547 [inline]
nsim_fib6_rt_insert drivers/net/netdevsim/fib.c:752 [inline]
nsim_fib6_event drivers/net/netdevsim/fib.c:856 [inline]
nsim_fib_event+0xf36/0xa390 drivers/net/netdevsim/fib.c:889
nsim_fib_event_work+0x26e/0x3e0 drivers/net/netdevsim/fib.c:1493
process_one_work+0x98b/0x1630 kernel/workqueue.c:3318
process_scheduled_works kernel/workqueue.c:3401 [inline]
worker_thread+0xb49/0x1140 kernel/workqueue.c:3482
kthread+0x389/0x470 kernel/kthread.c:436
ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
page last free pid 4981 tgid 4981 stack trace:
reset_page_owner include/linux/page_owner.h:25 [inline]
__free_pages_prepare mm/page_alloc.c:1413 [inline]
__free_frozen_pages+0x10af/0x1190 mm/page_alloc.c:2957
__slab_free+0x252/0x2a0 mm/slub.c:5646
qlink_free mm/kasan/quarantine.c:163 [inline]
qlist_free_all+0x99/0x100 mm/kasan/quarantine.c:179
kasan_quarantine_reduce+0x148/0x160 mm/kasan/quarantine.c:286
__kasan_slab_alloc+0x22/0x80 mm/kasan/common.c:350
kasan_slab_alloc include/linux/kasan.h:253 [inline]
slab_post_alloc_hook mm/slub.c:4583 [inline]
slab_alloc_node mm/slub.c:4912 [inline]
__kmalloc_cache_noprof+0x338/0x690 mm/slub.c:5420
_kmalloc_noprof include/linux/slab.h:967 [inline]
_kzalloc_noprof include/linux/slab.h:1284 [inline]
kernfs_fop_open+0x39a/0xcb0 fs/kernfs/file.c:641
do_dentry_open+0x83d/0x13e0 fs/open.c:947
vfs_open+0x3b/0x350 fs/open.c:1052
do_open fs/namei.c:4688 [inline]
path_openat+0x2eea/0x3960 fs/namei.c:4847
do_file_open+0x23e/0x4a0 fs/namei.c:4876
do_sys_openat2+0x115/0x200 fs/open.c:1344
do_sys_open fs/open.c:1350 [inline]
__do_sys_openat fs/open.c:1366 [inline]
__se_sys_openat fs/open.c:1361 [inline]
__x64_sys_openat+0x138/0x170 fs/open.c:1361
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x15f/0x560 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

Memory state around the buggy address:
ffff88805f5cd400: fa fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
ffff88805f5cd480: fa fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
>ffff88805f5cd500: fa fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
^
ffff88805f5cd580: 00 00 00 00 00 00 00 00 00 00 fc fc fc fc fc fc
ffff88805f5cd600: fa fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
==================================================================


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