Re: Private message regarding: [syzbot] [mm?] KASAN: slab-use-after-free Read in lookup_object_or_alloc (3)

[syzbot]

> From: Arnav Kapoor <kapoor...@gmail.com>
> Date: Sun, 12 Jan 2026 17:50:00 +0000
> Subject: [PATCH] timer: add virt_addr_valid check in timer_is_static_object
>
> Syzbot reported a KASAN slab-use-after-free in lookup_object_or_alloc
> when timer_is_static_object() accesses freed timer memory.
>
> #syz test
>
> The issue occurs when debug_object_activate() is called on a timer address
> that points to freed memory. The timer_is_static_object() function tries
> to check timer->entry.pprev and timer->entry.next to determine if the
> timer is static, but these accesses are invalid if the memory has been
> freed and potentially reused.
>
> Add a virt_addr_valid() check before accessing the timer structure to
> prevent use-after-free accesses. If the address is not valid, return
> false to indicate it's not a static timer.
>
> Reported-by: syzbot+62360d...@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=62360d4b8e8c4b8b8f6a
> Signed-off-by: Arnav Kapoor <kapoor...@gmail.com>
> ---
> diff --git a/kernel/time/timer.c b/kernel/time/timer.c
> index 1f2364126..e139e413a 100644
> --- a/kernel/time/timer.c
> +++ b/kernel/time/timer.c
> @@ -688,6 +688,9 @@ static bool timer_is_static_object(void *addr)
> {
> struct timer_list *timer = addr;
>
> + if (!virt_addr_valid(timer))
> + return false;
> +
> return (timer->entry.pprev == NULL &&
> timer->entry.next == TIMER_ENTRY_STATIC);
> }
>
> On Friday, 16 January 2026 at 02:18:22 UTC+5:30 syzbot wrote:
>
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit: 79b95d74470d Merge tag 'hid-for-linus-2026010801' of git:/..
> git tree: upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=11895f92580000
> kernel config: https://syzkaller.appspot.com/x/.config?x=a11e0f726bfb6765
> dashboard link: https://syzkaller.appspot.com/bug?extid=62360d745376b40120b5
> compiler: gcc (Debian 12.2.0-14+deb12u1) 12.2.0, GNU ld (GNU Binutils for
> Debian) 2.40
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> Downloadable assets:
> disk image:
> https://storage.googleapis.com/syzbot-assets/4d234fb6afe1/disk-79b95d74.raw.xz
> vmlinux:
> https://storage.googleapis.com/syzbot-assets/bb336424f007/vmlinux-79b95d74.xz
> kernel image:
> https://storage.googleapis.com/syzbot-assets/e8dee85fdda1/bzImage-79b95d74.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the
> commit:
> Reported-by: syzbot+62360d...@syzkaller.appspotmail.com
>
> ==================================================================
> BUG: KASAN: slab-use-after-free in timer_is_static_object+0x80/0x90
> kernel/time/timer.c:691
> Read of size 8 at addr ffff88807e5e8498 by task syz.4.6813/32052
>
> CPU: 1 UID: 0 PID: 32052 Comm: syz.4.6813 Tainted: G L syzkaller #0
> PREEMPT(full)
> Tainted: [L]=SOFTLOCKUP
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> Google 10/25/2025
> Call Trace:
> <IRQ>
> __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
> timer_is_static_object+0x80/0x90 kernel/time/timer.c:691
> lookup_object_or_alloc.part.0+0x420/0x590 lib/debugobjects.c:679
> lookup_object_or_alloc lib/debugobjects.c:665 [inline]
> debug_object_activate+0x326/0x4c0 lib/debugobjects.c:820
> debug_timer_activate kernel/time/timer.c:793 [inline]
> __mod_timer+0x814/0xd30 kernel/time/timer.c:1124
> add_timer+0x62/0x90 kernel/time/timer.c:1249
> call_timer_fn+0x19a/0x5a0 kernel/time/timer.c:1748
> expire_timers kernel/time/timer.c:1799 [inline]
> __run_timers+0x74a/0xae0 kernel/time/timer.c:2373
> __run_timer_base kernel/time/timer.c:2385 [inline]
> __run_timer_base kernel/time/timer.c:2377 [inline]
> run_timer_base+0x114/0x190 kernel/time/timer.c:2394
> run_timer_softirq+0x1a/0x40 kernel/time/timer.c:2404
> handle_softirqs+0x219/0x950 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:1056 [inline]
> sysvec_apic_timer_interrupt+0xa4/0xc0 arch/x86/kernel/apic/apic.c:1056
> </IRQ>
> <TASK>
> asm_sysvec_apic_timer_interrupt+0x1a/0x20
> arch/x86/include/asm/idtentry.h:697
> RIP: 0010:__raw_spin_unlock_irqrestore include/linux/spinlock_api_smp.h:152
> [inline]
> RIP: 0010:_raw_spin_unlock_irqrestore+0x31/0x80
> kernel/locking/spinlock.c:194
> Code: f5 53 48 8b 74 24 10 48 89 fb 48 83 c7 18 e8 16 fe 1e f6 48 89 df e8
> ce 50 1f f6 f7 c5 00 02 00 00 75 23 9c 58 f6 c4 02 75 37 <bf> 01 00 00 00
> e8 d5 53 0f f6 65 8b 05 2e b9 37 08 85 c0 74 16 5b
> RSP: 0018:ffffc90005137760 EFLAGS: 00000246
> RAX: 0000000000000006 RBX: ffff88803400d880 RCX: 0000000000000006
> RDX: 0000000000000000 RSI: ffffffff8dace187 RDI: ffffffff8bf2b380
> RBP: 0000000000000246 R08: 0000000000000001 R09: 0000000000000001
> R10: ffffffff9088b8d7 R11: ffff888046ec54b0 R12: ffff888022b76000
> R13: 0000000000000246 R14: ffff88803400d868 R15: ffffc900051378e8
> spin_unlock_irqrestore include/linux/spinlock.h:406 [inline]
> __skb_try_recv_datagram+0x172/0x4f0 net/core/datagram.c:267
> __unix_dgram_recvmsg+0x1bc/0xc30 net/unix/af_unix.c:2576
> unix_dgram_recvmsg+0xd0/0x110 net/unix/af_unix.c:2675
> sock_recvmsg_nosec net/socket.c:1078 [inline]
> ____sys_recvmsg+0x5f9/0x6b0 net/socket.c:2810
> ___sys_recvmsg+0x114/0x1a0 net/socket.c:2854
> do_recvmmsg+0x2fe/0x750 net/socket.c:2949
> __sys_recvmmsg net/socket.c:3023 [inline]
> __do_sys_recvmmsg net/socket.c:3046 [inline]
> __se_sys_recvmmsg net/socket.c:3039 [inline]
> __x64_sys_recvmmsg+0x22a/0x280 net/socket.c:3039
> do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> do_syscall_64+0xcd/0xf80 arch/x86/entry/syscall_64.c:94
> entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7fdc3eb8f749
> 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:00007fdc3f959038 EFLAGS: 00000246 ORIG_RAX: 000000000000012b
> RAX: ffffffffffffffda RBX: 00007fdc3ede6090 RCX: 00007fdc3eb8f749
> RDX: 0000000000010106 RSI: 00002000000000c0 RDI: 0000000000000005
> RBP: 00007fdc3ec13f91 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000002 R11: 0000000000000246 R12: 0000000000000000
> R13: 00007fdc3ede6128 R14: 00007fdc3ede6090 R15: 00007ffd6a8a5748
> </TASK>
>
> Allocated by task 29852:
> kasan_save_stack+0x33/0x60 mm/kasan/common.c:57
> kasan_save_track+0x14/0x30 mm/kasan/common.c:78
> poison_kmalloc_redzone mm/kasan/common.c:398 [inline]
> __kasan_kmalloc+0xaa/0xb0 mm/kasan/common.c:415
> kmalloc_noprof include/linux/slab.h:957 [inline]
> rose_add_node net/rose/rose_route.c:85 [inline]
> rose_rt_ioctl+0x880/0x2580 net/rose/rose_route.c:748
> rose_ioctl+0x64d/0x7c0 net/rose/af_rose.c:1382
> 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/0xf80 arch/x86/entry/syscall_64.c:94
> entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> Freed by task 32051:
> kasan_save_stack+0x33/0x60 mm/kasan/common.c:57
> kasan_save_track+0x14/0x30 mm/kasan/common.c:78
> kasan_save_free_info+0x3b/0x60 mm/kasan/generic.c:584
> poison_slab_object mm/kasan/common.c:253 [inline]
> __kasan_slab_free+0x5f/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:6670 [inline]
> kfree+0x2f8/0x6e0 mm/slub.c:6878
> rose_neigh_put include/net/rose.h:166 [inline]
> rose_timer_expiry+0x53f/0x630 net/rose/rose_timer.c:183
> call_timer_fn+0x19a/0x5a0 kernel/time/timer.c:1748
> expire_timers kernel/time/timer.c:1799 [inline]
> __run_timers+0x74a/0xae0 kernel/time/timer.c:2373
> __run_timer_base kernel/time/timer.c:2385 [inline]
> __run_timer_base kernel/time/timer.c:2377 [inline]
> run_timer_base+0x114/0x190 kernel/time/timer.c:2394
> run_timer_softirq+0x1a/0x40 kernel/time/timer.c:2404
> handle_softirqs+0x219/0x950 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:1056 [inline]
> sysvec_apic_timer_interrupt+0xa4/0xc0 arch/x86/kernel/apic/apic.c:1056
> asm_sysvec_apic_timer_interrupt+0x1a/0x20
> arch/x86/include/asm/idtentry.h:697
>
> The buggy address belongs to the object at ffff88807e5e8400
> which belongs to the cache kmalloc-512 of size 512
> The buggy address is located 152 bytes inside of
> freed 512-byte region [ffff88807e5e8400, ffff88807e5e8600)
>
> The buggy address belongs to the physical page:
> page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x7e5e8
> head: order:2 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
> ksm flags: 0xfff00000000040(head|node=0|zone=1|lastcpupid=0x7ff)
> page_type: f5(slab)
> raw: 00fff00000000040 ffff88813ff26c80 ffffea0000a54f00 dead000000000003
> raw: 0000000000000000 0000000000100010 00000000f5000000 0000000000000000
> head: 00fff00000000040 ffff88813ff26c80 ffffea0000a54f00 dead000000000003
> head: 0000000000000000 0000000000100010 00000000f5000000 0000000000000000
> head: 00fff00000000002 ffffea0001f97a01 00000000ffffffff 00000000ffffffff
> head: ffffffffffffffff 0000000000000000 00000000ffffffff 0000000000000004
> page dumped because: kasan: bad access detected
> page_owner tracks the page as allocated
> page last allocated via order 2, migratetype Unmovable, gfp_mask
> 0xd20c0(__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC),
> pid 29165, tgid 29165 (syz-executor), ts 2066518998187, free_ts
> 1956315063990
> set_page_owner include/linux/page_owner.h:32 [inline]
> post_alloc_hook+0x1af/0x220 mm/page_alloc.c:1857
> prep_new_page mm/page_alloc.c:1865 [inline]
> get_page_from_freelist+0xd0b/0x31a0 mm/page_alloc.c:3915
> __alloc_frozen_pages_noprof+0x25f/0x2430 mm/page_alloc.c:5210
> alloc_pages_mpol+0x1fb/0x550 mm/mempolicy.c:2486
> alloc_slab_page mm/slub.c:3075 [inline]
> allocate_slab mm/slub.c:3248 [inline]
> new_slab+0x2c3/0x430 mm/slub.c:3302
> ___slab_alloc+0xe18/0x1c90 mm/slub.c:4656
> __slab_alloc.constprop.0+0x63/0x110 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+0x4fc/0x910 mm/slub.c:5669
> kmalloc_noprof include/linux/slab.h:961 [inline]
> kzalloc_noprof include/linux/slab.h:1094 [inline]
> fib6_info_alloc+0x40/0x160 net/ipv6/ip6_fib.c:155
> ip6_route_info_create+0x14c/0xaa0 net/ipv6/route.c:3820
> ip6_route_add.part.0+0x22/0x1d0 net/ipv6/route.c:3949
> ip6_route_add+0x45/0x60 net/ipv6/route.c:3946
> addrconf_add_mroute+0x1dd/0x350 net/ipv6/addrconf.c:2552
> addrconf_add_dev+0x14e/0x1a0 net/ipv6/addrconf.c:2570
> inet6_addr_add+0xfe/0x9b0 net/ipv6/addrconf.c:3032
> inet6_rtm_newaddr+0x1619/0x1c50 net/ipv6/addrconf.c:5059
> page last free pid 27669 tgid 27669 stack trace:
> reset_page_owner include/linux/page_owner.h:25 [inline]
> free_pages_prepare mm/page_alloc.c:1406 [inline]
> __free_frozen_pages+0x7df/0x1170 mm/page_alloc.c:2943
> __pagetable_free include/linux/mm.h:3200 [inline]
> pagetable_free include/linux/mm.h:3224 [inline]
> pagetable_dtor_free include/linux/mm.h:3328 [inline]
> __tlb_remove_table include/asm-generic/tlb.h:220 [inline]
> __tlb_remove_table_free mm/mmu_gather.c:227 [inline]
> tlb_remove_table_rcu+0x2b2/0x390 mm/mmu_gather.c:290
> rcu_do_batch kernel/rcu/tree.c:2605 [inline]
> rcu_core+0x79c/0x15f0 kernel/rcu/tree.c:2857
> handle_softirqs+0x219/0x950 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:1056 [inline]
> sysvec_apic_timer_interrupt+0xa4/0xc0 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:
> ffff88807e5e8380: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> ffff88807e5e8400: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>>ffff88807e5e8480: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ^
> ffff88807e5e8500: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ffff88807e5e8580: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ==================================================================
> ----------------
> Code disassembly (best guess):
> 0: f5 cmc
> 1: 53 push %rbx
> 2: 48 8b 74 24 10 mov 0x10(%rsp),%rsi
> 7: 48 89 fb mov %rdi,%rbx
> a: 48 83 c7 18 add $0x18,%rdi
> e: e8 16 fe 1e f6 call 0xf61efe29
> 13: 48 89 df mov %rbx,%rdi
> 16: e8 ce 50 1f f6 call 0xf61f50e9
> 1b: f7 c5 00 02 00 00 test $0x200,%ebp
> 21: 75 23 jne 0x46
> 23: 9c pushf
> 24: 58 pop %rax
> 25: f6 c4 02 test $0x2,%ah
> 28: 75 37 jne 0x61
> * 2a: bf 01 00 00 00 mov $0x1,%edi <-- trapping instruction
> 2f: e8 d5 53 0f f6 call 0xf60f5409
> 34: 65 8b 05 2e b9 37 08 mov %gs:0x837b92e(%rip),%eax # 0x837b969
> 3b: 85 c0 test %eax,%eax
> 3d: 74 16 je 0x55
> 3f: 5b pop %rbx
>
>
> ---
> 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 syzk...@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:
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>
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>
> 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
>

Command #1:
This crash does not have a reproducer. I cannot test it.