[syzbot] [afs?] KASAN: slab-use-after-free Read in afs_dynroot_readdir (2)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 9b0d551bcc05 Merge tag 'pull-misc' of git://git.kernel.org..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=144e8304580000
kernel config: https://syzkaller.appspot.com/x/.config?x=3affc19c35df22d7
dashboard link: https://syzkaller.appspot.com/bug?extid=ae8ad52680705c162471
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=17856a7c580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=15a05214580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/4b8223b2d807/disk-9b0d551b.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/d2ad54c21240/vmlinux-9b0d551b.xz
kernel image: https://storage.googleapis.com/syzbot-assets/702d27fd0804/bzImage-9b0d551b.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+ae8ad5...@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: slab-use-after-free in afs_dynroot_readdir_cells fs/afs/dynroot.c:296 [inline]
BUG: KASAN: slab-use-after-free in afs_dynroot_readdir+0x94e/0xb70 fs/afs/dynroot.c:352
Read of size 4 at addr ffff888076fbe148 by task syz.0.8268/15038

CPU: 1 UID: 0 PID: 15038 Comm: syz.0.8268 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
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
afs_dynroot_readdir_cells fs/afs/dynroot.c:296 [inline]
afs_dynroot_readdir+0x94e/0xb70 fs/afs/dynroot.c:352
iterate_dir+0x296/0xaf0 fs/readdir.c:108
__do_sys_getdents64 fs/readdir.c:410 [inline]
__se_sys_getdents64 fs/readdir.c:396 [inline]
__x64_sys_getdents64+0x13c/0x2c0 fs/readdir.c:396
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xcd/0x4e0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f085598eec9
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:00007fff49e8bd28 EFLAGS: 00000246 ORIG_RAX: 00000000000000d9
RAX: ffffffffffffffda RBX: 00007f0855be5fa0 RCX: 00007f085598eec9
RDX: 0000000000000055 RSI: 00002000000007c0 RDI: 0000000000000003
RBP: 00007f0855a11f91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f0855be5fa0 R14: 00007f0855be5fa0 R15: 0000000000000003
</TASK>

Allocated by task 15036:
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]
kzalloc_noprof include/linux/slab.h:1094 [inline]
afs_alloc_cell fs/afs/cell.c:137 [inline]
afs_lookup_cell+0x61d/0x1680 fs/afs/cell.c:265
afs_dynroot_lookup_cell fs/afs/dynroot.c:111 [inline]
afs_dynroot_lookup+0x2e2/0xa90 fs/afs/dynroot.c:159
__lookup_slow+0x251/0x460 fs/namei.c:1816
lookup_slow fs/namei.c:1833 [inline]
walk_component+0x353/0x5b0 fs/namei.c:2151
lookup_last fs/namei.c:2652 [inline]
path_lookupat+0x142/0x6d0 fs/namei.c:2676
filename_lookup+0x224/0x5f0 fs/namei.c:2705
do_linkat+0x14c/0x5a0 fs/namei.c:4991
__do_sys_link fs/namei.c:5045 [inline]
__se_sys_link fs/namei.c:5043 [inline]
__x64_sys_link+0x7d/0xa0 fs/namei.c:5043
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xcd/0x4e0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

Freed by task 15038:
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:2514 [inline]
slab_free mm/slub.c:6566 [inline]
kfree+0x2b8/0x6d0 mm/slub.c:6773
afs_cell_destroy+0x21c/0x310 fs/afs/cell.c:525
rcu_do_batch kernel/rcu/tree.c:2605 [inline]
rcu_core+0x799/0x1530 kernel/rcu/tree.c:2861
handle_softirqs+0x219/0x8e0 kernel/softirq.c:622
__do_softirq kernel/softirq.c:656 [inline]
invoke_softirq kernel/softirq.c:496 [inline]
__irq_exit_rcu+0x109/0x170 kernel/softirq.c:723
irq_exit_rcu+0x9/0x30 kernel/softirq.c:739
instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1052 [inline]
sysvec_apic_timer_interrupt+0xa4/0xc0 arch/x86/kernel/apic/apic.c:1052
asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:702

Last potentially related work creation:
kasan_save_stack+0x33/0x60 mm/kasan/common.c:56
kasan_record_aux_stack+0xa7/0xc0 mm/kasan/generic.c:559
__call_rcu_common.constprop.0+0xa5/0xa10 kernel/rcu/tree.c:3123
process_one_work+0x9cf/0x1b70 kernel/workqueue.c:3263
process_scheduled_works kernel/workqueue.c:3346 [inline]
worker_thread+0x6c8/0xf10 kernel/workqueue.c:3427
kthread+0x3c2/0x780 kernel/kthread.c:463
ret_from_fork+0x56a/0x730 arch/x86/kernel/process.c:148
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

Second to last potentially related work creation:
kasan_save_stack+0x33/0x60 mm/kasan/common.c:56
kasan_record_aux_stack+0xa7/0xc0 mm/kasan/generic.c:559
insert_work+0x36/0x230 kernel/workqueue.c:2186
__queue_work+0x97e/0x1160 kernel/workqueue.c:2341
queue_work_on+0x1a4/0x1f0 kernel/workqueue.c:2392
queue_work include/linux/workqueue.h:669 [inline]
afs_put_cell.part.0+0x1ab/0x240 fs/afs/cell.c:570
afs_put_cell fs/afs/cell.c:558 [inline]
afs_manage_cell_work+0x6de/0x1940 fs/afs/cell.c:860
process_one_work+0x9cf/0x1b70 kernel/workqueue.c:3263
process_scheduled_works kernel/workqueue.c:3346 [inline]
worker_thread+0x6c8/0xf10 kernel/workqueue.c:3427
kthread+0x3c2/0x780 kernel/kthread.c:463
ret_from_fork+0x56a/0x730 arch/x86/kernel/process.c:148
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

The buggy address belongs to the object at ffff888076fbe000
which belongs to the cache kmalloc-1k of size 1024
The buggy address is located 328 bytes inside of
freed 1024-byte region [ffff888076fbe000, ffff888076fbe400)

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x76fb8
head: order:3 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
flags: 0xfff00000000040(head|node=0|zone=1|lastcpupid=0x7ff)
page_type: f5(slab)
raw: 00fff00000000040 ffff88801b026dc0 ffffea0000b9c200 dead000000000002
raw: 0000000000000000 0000000000100010 00000000f5000000 0000000000000000
head: 00fff00000000040 ffff88801b026dc0 ffffea0000b9c200 dead000000000002
head: 0000000000000000 0000000000100010 00000000f5000000 0000000000000000
head: 00fff00000000003 ffffea0001dbee01 00000000ffffffff 00000000ffffffff
head: ffffffffffffffff 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 0x52820(GFP_ATOMIC|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP), pid 49, tgid 49 (kworker/u8:3), ts 178491208774, free_ts 178478998089
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:3030 [inline]
allocate_slab mm/slub.c:3203 [inline]
new_slab+0x24a/0x360 mm/slub.c:3257
___slab_alloc+0xdc4/0x1ae0 mm/slub.c:4627
__slab_alloc.constprop.0+0x63/0x110 mm/slub.c:4746
__slab_alloc_node mm/slub.c:4822 [inline]
slab_alloc_node mm/slub.c:5233 [inline]
__do_kmalloc_node mm/slub.c:5602 [inline]
__kmalloc_noprof+0x501/0x880 mm/slub.c:5615
kmalloc_noprof include/linux/slab.h:961 [inline]
kzalloc_noprof include/linux/slab.h:1094 [inline]
neigh_alloc net/core/neighbour.c:522 [inline]
___neigh_create+0x14e6/0x28c0 net/core/neighbour.c:656
ip6_finish_output2+0x11aa/0x1cf0 net/ipv6/ip6_output.c:128
__ip6_finish_output+0x3cd/0x1010 net/ipv6/ip6_output.c:209
ip6_finish_output net/ipv6/ip6_output.c:220 [inline]
NF_HOOK_COND include/linux/netfilter.h:307 [inline]
ip6_output+0x253/0x710 net/ipv6/ip6_output.c:247
dst_output include/net/dst.h:464 [inline]
NF_HOOK include/linux/netfilter.h:318 [inline]
ndisc_send_skb+0xa85/0x1f50 net/ipv6/ndisc.c:512
ndisc_send_ns+0xc6/0x140 net/ipv6/ndisc.c:670
addrconf_dad_work+0xbc9/0x14e0 net/ipv6/addrconf.c:4282
process_one_work+0x9cf/0x1b70 kernel/workqueue.c:3263
page last free pid 5848 tgid 5848 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
qlink_free mm/kasan/quarantine.c:163 [inline]
qlist_free_all+0x4d/0x120 mm/kasan/quarantine.c:179
kasan_quarantine_reduce+0x195/0x1e0 mm/kasan/quarantine.c:286
__kasan_slab_alloc+0x69/0x90 mm/kasan/common.c:352
kasan_slab_alloc include/linux/kasan.h:252 [inline]
slab_post_alloc_hook mm/slub.c:4946 [inline]
slab_alloc_node mm/slub.c:5245 [inline]
kmem_cache_alloc_noprof+0x250/0x6e0 mm/slub.c:5252
getname_flags.part.0+0x4c/0x550 fs/namei.c:146
getname_flags+0x93/0xf0 include/linux/audit.h:345
getname include/linux/fs.h:2916 [inline]
do_sys_openat2+0xb8/0x1d0 fs/open.c:1431
do_sys_open fs/open.c:1452 [inline]
__do_sys_openat fs/open.c:1468 [inline]
__se_sys_openat fs/open.c:1463 [inline]
__x64_sys_openat+0x174/0x210 fs/open.c:1463
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xcd/0x4e0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

Memory state around the buggy address:
ffff888076fbe000: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff888076fbe080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff888076fbe100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
^
ffff888076fbe180: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff888076fbe200: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================


---
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If the report is already addressed, let syzbot know by replying with:
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#syz undup

[Hillf Danton]

> Date: Fri, 03 Oct 2025 22:30:28 -0700 [thread overview]

> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit: 9b0d551bcc05 Merge tag 'pull-misc' of git://git.kernel.org..
> git tree: upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=144e8304580000
> kernel config: https://syzkaller.appspot.com/x/.config?x=3affc19c35df22d7
> dashboard link: https://syzkaller.appspot.com/bug?extid=ae8ad52680705c162471
> 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=17856a7c580000
> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=15a05214580000

#syz test

--- x/fs/afs/dynroot.c
+++ y/fs/afs/dynroot.c
@@ -290,15 +290,21 @@ static int afs_dynroot_readdir_cells(str
for (;;) {
unsigned int ix = ctx->pos >> 1;

+ rcu_read_lock();
cell = idr_get_next(&net->cells_dyn_ino, &ix);
- if (!cell)
+ if (!cell) {
+ rcu_read_unlock();
return 0;
+ }
if (READ_ONCE(cell->state) == AFS_CELL_REMOVING ||
- READ_ONCE(cell->state) == AFS_CELL_DEAD) {
+ READ_ONCE(cell->state) == AFS_CELL_DEAD ||
+ !refcount_inc_not_zero(&cell->ref)) {
+ rcu_read_unlock();
ctx->pos += 2;
ctx->pos &= ~1;
continue;
}
+ rcu_read_unlock();

newpos = ix << 1;
if (newpos > ctx->pos)
@@ -308,16 +314,21 @@ static int afs_dynroot_readdir_cells(str

if ((ctx->pos & 1) == 0) {
if (!dir_emit(ctx, cell->name, cell->name_len,
- cell->dynroot_ino, DT_DIR))
+ cell->dynroot_ino, DT_DIR)) {
+ afs_put_cell(cell, afs_cell_trace_put_atcell);
return 0;
+ }
ctx->pos++;
}
if ((ctx->pos & 1) == 1) {
if (!dir_emit(ctx, cell->name - 1, cell->name_len + 1,
- cell->dynroot_ino + 1, DT_DIR))
+ cell->dynroot_ino + 1, DT_DIR)) {
+ afs_put_cell(cell, afs_cell_trace_put_atcell);
return 0;
+ }
ctx->pos++;
}
+ afs_put_cell(cell, afs_cell_trace_put_atcell);
}
return 0;
}
--