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

[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+ae8ad52680705c162471@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
==================================================================


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want syzbot to run the reproducer, reply with:
#syz test: git://repo/address.git branch-or-commit-hash
If you attach or paste a git patch, syzbot will apply it before testing.

If you want to overwrite report's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup

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

[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;
 }
--

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

[syzbot]

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
KASAN: slab-use-after-free Read in afs_dynroot_readdir

==================================================================
BUG: KASAN: slab-use-after-free in afs_dynroot_readdir_cells fs/afs/dynroot.c:299 [inline]
BUG: KASAN: slab-use-after-free in afs_dynroot_readdir+0x106f/0x12e0 fs/afs/dynroot.c:363
Read of size 4 at addr ffff8880283cf948 by task syz.0.1618/9809

CPU: 0 UID: 0 PID: 9809 Comm: syz.0.1618 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:299 [inline]
 afs_dynroot_readdir+0x106f/0x12e0 fs/afs/dynroot.c:363
 iterate_dir+0x293/0xaf0 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: 0033:0x7fe27498eec9
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:00007fe27587a038 EFLAGS: 00000246 ORIG_RAX: 00000000000000d9
RAX: ffffffffffffffda RBX: 00007fe274be5fa0 RCX: 00007fe27498eec9
RDX: 0000000000000055 RSI: 00002000000007c0 RDI: 0000000000000003
RBP: 00007fe274a11f91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fe274be6038 R14: 00007fe274be5fa0 R15: 00007ffd3fb02548
 </TASK>

Allocated by task 9807:
 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+0x3d8/0xd60 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 15:
 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
 run_ksoftirqd kernel/softirq.c:1063 [inline]
 run_ksoftirqd+0x3a/0x60 kernel/softirq.c:1055
 smpboot_thread_fn+0x3f7/0xae0 kernel/smpboot.c:160
 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

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 ffff8880283cf800
 which belongs to the cache kmalloc-1k of size 1024
The buggy address is located 328 bytes inside of
 freed 1024-byte region [ffff8880283cf800, ffff8880283cfc00)

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x283c8
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 dead000000000100 dead000000000122
raw: 0000000000000000 0000000080100010 00000000f5000000 0000000000000000
head: 00fff00000000040 ffff88801b026dc0 dead000000000100 dead000000000122
head: 0000000000000000 0000000080100010 00000000f5000000 0000000000000000
head: 00fff00000000003 ffffea0000a0f201 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 0x252800(GFP_NOWAIT|__GFP_NORETRY|__GFP_COMP|__GFP_THISNODE), pid 5894, tgid 5894 (syz-executor), ts 75218089779, free_ts 73045805122
 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_slab_page mm/slub.c:3032 [inline]
 allocate_slab mm/slub.c:3203 [inline]
 new_slab+0xa5/0x360 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_node_noprof+0x4dd/0x8a0 mm/slub.c:5609
 kmalloc_array_node_noprof include/linux/slab.h:1075 [inline]
 alloc_slab_obj_exts+0x3a/0xd0 mm/slub.c:2115
 account_slab mm/slub.c:3157 [inline]
 allocate_slab mm/slub.c:3222 [inline]
 new_slab+0x283/0x360 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]
 __kmalloc_cache_noprof+0x477/0x780 mm/slub.c:5719
 kmalloc_noprof include/linux/slab.h:957 [inline]
 kzalloc_noprof include/linux/slab.h:1094 [inline]
 alloc_netdev_mqs+0xed3/0x1550 net/core/dev.c:11976
 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
page last free pid 5852 tgid 5852 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
 discard_slab mm/slub.c:3301 [inline]
 __put_partials+0x130/0x170 mm/slub.c:3848
 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]
 __kmalloc_cache_noprof+0x274/0x780 mm/slub.c:5719
 kmalloc_noprof include/linux/slab.h:957 [inline]
 kzalloc_noprof include/linux/slab.h:1094 [inline]
 tomoyo_environ security/tomoyo/domain.c:633 [inline]
 tomoyo_find_next_domain+0xbd2/0x20b0 security/tomoyo/domain.c:888
 tomoyo_bprm_check_security security/tomoyo/tomoyo.c:102 [inline]
 tomoyo_bprm_check_security+0x12e/0x1d0 security/tomoyo/tomoyo.c:92
 security_bprm_check+0x1b9/0x1e0 security/security.c:1341
 search_binary_handler fs/exec.c:1660 [inline]
 exec_binprm fs/exec.c:1702 [inline]
 bprm_execve fs/exec.c:1754 [inline]
 bprm_execve+0x81a/0x1640 fs/exec.c:1730
 do_execveat_common.isra.0+0x4a5/0x610 fs/exec.c:1860
 do_execve fs/exec.c:1934 [inline]
 __do_sys_execve fs/exec.c:2010 [inline]
 __se_sys_execve fs/exec.c:2005 [inline]
 __x64_sys_execve+0x8e/0xb0 fs/exec.c:2005
 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:
 ffff8880283cf800: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8880283cf880: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff8880283cf900: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                                              ^
 ffff8880283cf980: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8880283cfa00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================


Tested on:

commit:         cbf33b8e Merge tag 'bpf-fixes' of git://git.kernel.org..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=16533942580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=2e82a64dcfd006b7
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
patch:          https://syzkaller.appspot.com/x/patch.diff?x=113b9ee2580000

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

[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;
 }
--- x/fs/afs/cell.c
+++ y/fs/afs/cell.c
@@ -520,7 +520,6 @@ static void afs_cell_destroy(struct rcu_
 	afs_put_vlserverlist(net, rcu_access_pointer(cell->vl_servers));
 	afs_unuse_cell(cell->alias_of, afs_cell_trace_unuse_alias);
 	key_put(cell->anonymous_key);
-	idr_remove(&net->cells_dyn_ino, cell->dynroot_ino);
 	kfree(cell->name - 1);
 	kfree(cell);
 
@@ -565,6 +564,9 @@ void afs_put_cell(struct afs_cell *cell,
 		zero = __refcount_dec_and_test(&cell->ref, &r);
 		trace_afs_cell(debug_id, r - 1, a, reason);
 		if (zero) {
+			struct afs_net *net = cell->net;
+
+			idr_remove(&net->cells_dyn_ino, cell->dynroot_ino);
 			a = atomic_read(&cell->active);
 			WARN(a != 0, "Cell active count %u > 0\n", a);
 			WARN_ON(!queue_work(afs_wq, &cell->destroyer));
--

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

[syzbot]

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
KASAN: slab-use-after-free Read in afs_dynroot_readdir

==================================================================
BUG: KASAN: slab-use-after-free in afs_dynroot_readdir_cells fs/afs/dynroot.c:299 [inline]
BUG: KASAN: slab-use-after-free in afs_dynroot_readdir+0x106f/0x12e0 fs/afs/dynroot.c:363
Read of size 4 at addr ffff88803504a148 by task syz.0.404/7214

CPU: 1 UID: 0 PID: 7214 Comm: syz.0.404 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:299 [inline]
 afs_dynroot_readdir+0x106f/0x12e0 fs/afs/dynroot.c:363
 iterate_dir+0x293/0xaf0 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: 0033:0x7f820038eec9
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:00007f820115a038 EFLAGS: 00000246 ORIG_RAX: 00000000000000d9
RAX: ffffffffffffffda RBX: 00007f82005e5fa0 RCX: 00007f820038eec9
RDX: 0000000000000055 RSI: 00002000000007c0 RDI: 0000000000000003
RBP: 00007f8200411f91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f82005e6038 R14: 00007f82005e5fa0 R15: 00007ffd74cb1208
 </TASK>

Allocated by task 7208:
 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+0x3d8/0xd60 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 0:
 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+0x1e2/0x2c0 fs/afs/cell.c:524
 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+0x9cc/0x1b70 kernel/workqueue.c:3263
 process_scheduled_works kernel/workqueue.c:3346 [inline]
 worker_thread+0x6c8/0xf10 kernel/workqueue.c:3427
 kthread+0x3c5/0x780 kernel/kthread.c:463
 ret_from_fork+0x56d/0x730 arch/x86/kernel/process.c:158
 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_unuse_cell+0x259/0x2e0 fs/afs/cell.c:621
 __dentry_kill+0x23e/0x600 fs/dcache.c:674
 dput.part.0+0x4b1/0x9b0 fs/dcache.c:911
 dput+0x1f/0x30 fs/dcache.c:901
 path_put fs/namei.c:626 [inline]
 do_linkat+0x320/0x5a0 fs/namei.c:5028
 __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

The buggy address belongs to the object at ffff88803504a000
 which belongs to the cache kmalloc-1k of size 1024
The buggy address is located 328 bytes inside of
 freed 1024-byte region [ffff88803504a000, ffff88803504a400)

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x35048
head: order:3 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
anon flags: 0xfff00000000040(head|node=0|zone=1|lastcpupid=0x7ff)
page_type: f5(slab)
raw: 00fff00000000040 ffff88801b026dc0 0000000000000000 dead000000000001
raw: 0000000000000000 0000000080100010 00000000f5000000 0000000000000000
head: 00fff00000000040 ffff88801b026dc0 0000000000000000 dead000000000001
head: 0000000000000000 0000000080100010 00000000f5000000 0000000000000000
head: 00fff00000000003 ffffea0000d41201 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 3527, tgid 3527 (kworker/u8:9), ts 87582678068, free_ts 87538643162
 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+0x9cc/0x1b70 kernel/workqueue.c:3263
page last free pid 5935 tgid 5935 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
 discard_slab mm/slub.c:3301 [inline]
 __put_partials+0x130/0x170 mm/slub.c:3848
 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_node_noprof+0x28a/0x770 mm/slub.c:5297
 __alloc_skb+0x2b2/0x380 net/core/skbuff.c:660
 alloc_skb include/linux/skbuff.h:1383 [inline]
 netlink_alloc_large_skb+0x69/0x140 net/netlink/af_netlink.c:1186
 netlink_sendmsg+0x698/0xdd0 net/netlink/af_netlink.c:1871
 sock_sendmsg_nosec net/socket.c:727 [inline]
 __sock_sendmsg net/socket.c:742 [inline]
 __sys_sendto+0x4a0/0x520 net/socket.c:2244
 __do_sys_sendto net/socket.c:2251 [inline]
 __se_sys_sendto net/socket.c:2247 [inline]
 __x64_sys_sendto+0xe0/0x1c0 net/socket.c:2247
 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:
 ffff88803504a000: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff88803504a080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff88803504a100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                                              ^
 ffff88803504a180: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff88803504a200: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================


Tested on:

commit:         6093a688 Merge tag 'char-misc-6.18-rc1' of git://git.k..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=171a15cd980000
kernel config:  https://syzkaller.appspot.com/x/.config?x=8f142ebe84501b0b
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
patch:          https://syzkaller.appspot.com/x/patch.diff?x=1485c458580000

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

[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;
 }
--- x/fs/afs/cell.c
+++ y/fs/afs/cell.c
@@ -520,7 +520,6 @@ static void afs_cell_destroy(struct rcu_
 	afs_put_vlserverlist(net, rcu_access_pointer(cell->vl_servers));
 	afs_unuse_cell(cell->alias_of, afs_cell_trace_unuse_alias);
 	key_put(cell->anonymous_key);
-	idr_remove(&net->cells_dyn_ino, cell->dynroot_ino);
 	kfree(cell->name - 1);
 	kfree(cell);
 
@@ -565,6 +564,9 @@ void afs_put_cell(struct afs_cell *cell,
 		zero = __refcount_dec_and_test(&cell->ref, &r);
 		trace_afs_cell(debug_id, r - 1, a, reason);
 		if (zero) {
+			struct afs_net *net = cell->net;
+
+			idr_remove(&net->cells_dyn_ino, cell->dynroot_ino);
 			a = atomic_read(&cell->active);
 			WARN(a != 0, "Cell active count %u > 0\n", a);
 			WARN_ON(!queue_work(afs_wq, &cell->destroyer));
@@ -615,8 +617,12 @@ void afs_unuse_cell(struct afs_cell *cel
 
 	zero = __refcount_dec_and_test(&cell->ref, &r);
 	trace_afs_cell(debug_id, r - 1, a, reason);
-	if (zero)
+	if (zero) {
+		struct afs_net *net = cell->net;
+
+		idr_remove(&net->cells_dyn_ino, cell->dynroot_ino);
 		WARN_ON(!queue_work(afs_wq, &cell->destroyer));
+	}
 }
 
 /*
--

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

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-by: syzbot+ae8ad52680705c162471@syzkaller.appspotmail.com
Tested-by: syzbot+ae8ad52680705c162471@syzkaller.appspotmail.com

Tested on:

commit:         6093a688 Merge tag 'char-misc-6.18-rc1' of git://git.k..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=14bd0ee2580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=8f142ebe84501b0b
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
patch:          https://syzkaller.appspot.com/x/patch.diff?x=164d5092580000

Note: testing is done by a robot and is best-effort only.