KASAN: use-after-free Write in l2tp_session_get_nth

Status: premoderation: reported on 2026/09/12 06:57
Reported-by: syzbot+56c653020226751f1a09@syzkaller.appspotmail.com
First crash: 10d, last: 10d

Sample crash report:
==================================================================
BUG: KASAN: use-after-free in instrument_atomic_read_write include/linux/instrumented.h:102 [inline]
BUG: KASAN: use-after-free in atomic_fetch_add_relaxed include/linux/atomic/atomic-instrumented.h:116 [inline]
BUG: KASAN: use-after-free in __refcount_add include/linux/refcount.h:193 [inline]
BUG: KASAN: use-after-free in __refcount_inc include/linux/refcount.h:250 [inline]
BUG: KASAN: use-after-free in refcount_inc include/linux/refcount.h:267 [inline]
BUG: KASAN: use-after-free in l2tp_session_inc_refcount net/l2tp/l2tp_core.c:211 [inline]
BUG: KASAN: use-after-free in l2tp_session_get_nth+0xe8/0x180 net/l2tp/l2tp_core.c:313
Write of size 4 at addr ffff8881104a7c88 by task syz.2.345/1536

CPU: 1 PID: 1536 Comm: syz.2.345 Tainted: G        W          syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
Call Trace:
 <TASK>
 __dump_stack+0x21/0x24 lib/dump_stack.c:88
 dump_stack_lvl+0x110/0x170 lib/dump_stack.c:106
 print_address_description+0x71/0x200 mm/kasan/report.c:316
 print_report+0x4a/0x60 mm/kasan/report.c:420
 kasan_report+0x122/0x150 mm/kasan/report.c:524
 check_region_inline mm/kasan/generic.c:-1 [inline]
 kasan_check_range+0x249/0x2a0 mm/kasan/generic.c:189
 __kasan_check_write+0x14/0x20 mm/kasan/shadow.c:37
 instrument_atomic_read_write include/linux/instrumented.h:102 [inline]
 atomic_fetch_add_relaxed include/linux/atomic/atomic-instrumented.h:116 [inline]
 __refcount_add include/linux/refcount.h:193 [inline]
 __refcount_inc include/linux/refcount.h:250 [inline]
 refcount_inc include/linux/refcount.h:267 [inline]
 l2tp_session_inc_refcount net/l2tp/l2tp_core.c:211 [inline]
 l2tp_session_get_nth+0xe8/0x180 net/l2tp/l2tp_core.c:313
 pppol2tp_next_session net/l2tp/l2tp_ppp.c:1451 [inline]
 pppol2tp_seq_start+0x2a4/0x620 net/l2tp/l2tp_ppp.c:1480
 seq_read_iter+0x3c4/0xd80 fs/seq_file.c:225
 seq_read+0x171/0x210 fs/seq_file.c:162
 pde_read fs/proc/inode.c:316 [inline]
 proc_reg_read+0x1e4/0x2d0 fs/proc/inode.c:328
 vfs_read+0x286/0x930 fs/read_write.c:468
 ksys_read+0x14a/0x260 fs/read_write.c:613
 __do_sys_read fs/read_write.c:623 [inline]
 __se_sys_read fs/read_write.c:621 [inline]
 __x64_sys_read+0x7b/0x90 fs/read_write.c:621
 x64_sys_call+0x2f/0x9a0 arch/x86/include/generated/asm/syscalls_64.h:1
 do_syscall_x64 arch/x86/entry/common.c:46 [inline]
 do_syscall_64+0x4c/0xa0 arch/x86/entry/common.c:76
 entry_SYSCALL_64_after_hwframe+0x68/0xd2
RIP: 0033:0x7fa40c99e159
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:00007fa40d91b028 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
RAX: ffffffffffffffda RBX: 00007fa40cc25fa0 RCX: 00007fa40c99e159
RDX: 0000000000001000 RSI: 0000200000000500 RDI: 000000000000000b
RBP: 00007fa40ca3503b R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fa40cc26038 R14: 00007fa40cc25fa0 R15: 00007ffe96d8b838
 </TASK>

Allocated by task 1536:
 kasan_save_stack mm/kasan/common.c:46 [inline]
 kasan_set_track+0x4b/0x70 mm/kasan/common.c:53
 kasan_save_alloc_info+0x1f/0x30 mm/kasan/generic.c:505
 ____kasan_kmalloc mm/kasan/common.c:380 [inline]
 __kasan_kmalloc+0x95/0xb0 mm/kasan/common.c:389
 kasan_kmalloc include/linux/kasan.h:212 [inline]
 __do_kmalloc_node mm/slab_common.c:938 [inline]
 __kmalloc+0xb4/0x1e0 mm/slab_common.c:951
 kmalloc include/linux/slab.h:568 [inline]
 kzalloc include/linux/slab.h:699 [inline]
 l2tp_session_create+0x38/0xbd0 net/l2tp/l2tp_core.c:1614
 pppol2tp_connect+0xbf5/0x1640 net/l2tp/l2tp_ppp.c:771
 __sys_connect_file net/socket.c:2001 [inline]
 __sys_connect+0x3da/0x460 net/socket.c:2018
 __do_sys_connect net/socket.c:2028 [inline]
 __se_sys_connect net/socket.c:2025 [inline]
 __x64_sys_connect+0x7a/0x90 net/socket.c:2025
 x64_sys_call+0x88d/0x9a0 arch/x86/include/generated/asm/syscalls_64.h:43
 do_syscall_x64 arch/x86/entry/common.c:46 [inline]
 do_syscall_64+0x4c/0xa0 arch/x86/entry/common.c:76
 entry_SYSCALL_64_after_hwframe+0x68/0xd2

Freed by task 1538:
 kasan_save_stack mm/kasan/common.c:46 [inline]
 kasan_set_track+0x4b/0x70 mm/kasan/common.c:53
 kasan_save_free_info+0x2b/0x40 mm/kasan/generic.c:516
 ____kasan_slab_free+0x132/0x180 mm/kasan/common.c:242
 __kasan_slab_free+0x11/0x20 mm/kasan/common.c:250
 kasan_slab_free include/linux/kasan.h:178 [inline]
 slab_free_hook mm/slub.c:1750 [inline]
 slab_free_freelist_hook+0xc2/0x190 mm/slub.c:1776
 slab_free mm/slub.c:3712 [inline]
 __kmem_cache_free+0xb7/0x1b0 mm/slub.c:3728
 kfree+0x6f/0xf0 mm/slab_common.c:990
 l2tp_session_free net/l2tp/l2tp_core.c:168 [inline]
 l2tp_session_put+0xaf/0x1a0 net/l2tp/l2tp_core.c:193
 pppol2tp_ioctl+0x748/0x960 net/l2tp/l2tp_ppp.c:-1
 pppox_ioctl+0x1f6/0x330 drivers/net/ppp/pppox.c:93
 sock_do_ioctl+0x114/0x330 net/socket.c:1192
 sock_ioctl+0x475/0x700 net/socket.c:1311
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:870 [inline]
 __se_sys_ioctl+0x12f/0x1b0 fs/ioctl.c:856
 __x64_sys_ioctl+0x7b/0x90 fs/ioctl.c:856
 x64_sys_call+0x58b/0x9a0 arch/x86/include/generated/asm/syscalls_64.h:17
 do_syscall_x64 arch/x86/entry/common.c:46 [inline]
 do_syscall_64+0x4c/0xa0 arch/x86/entry/common.c:76
 entry_SYSCALL_64_after_hwframe+0x68/0xd2

The buggy address belongs to the object at ffff8881104a7c00
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 136 bytes inside of
 512-byte region [ffff8881104a7c00, ffff8881104a7e00)

The buggy address belongs to the physical page:
page:ffffea0004412900 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x1104a4
head:ffffea0004412900 order:2 compound_mapcount:0 compound_pincount:0
flags: 0x4000000000010200(slab|head|zone=1)
raw: 4000000000010200 dead000000000100 dead000000000122 ffff888100042f00
raw: 0000000000000000 0000000080100010 00000001ffffffff 0000000000000000
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 92, tgid 92 (klogd), ts 5284205981, free_ts 0
 set_page_owner include/linux/page_owner.h:33 [inline]
 post_alloc_hook+0x1f5/0x210 mm/page_alloc.c:2673
 prep_new_page+0x1c/0x110 mm/page_alloc.c:2680
 get_page_from_freelist+0x2ca9/0x2d20 mm/page_alloc.c:4587
 __alloc_pages+0x1fa/0x610 mm/page_alloc.c:5931
 alloc_slab_page+0x6e/0xf0 include/linux/gfp.h:-1
 allocate_slab mm/slub.c:1967 [inline]
 new_slab+0x98/0x3e0 mm/slub.c:2020
 ___slab_alloc+0x70f/0xb70 mm/slub.c:3177
 __slab_alloc+0x5e/0xa0 mm/slub.c:3263
 slab_alloc_node mm/slub.c:3348 [inline]
 __kmem_cache_alloc_node+0x204/0x2d0 mm/slub.c:3423
 __do_kmalloc_node mm/slab_common.c:937 [inline]
 __kmalloc_node_track_caller+0xa1/0x1e0 mm/slab_common.c:958
 kmalloc_reserve net/core/skbuff.c:449 [inline]
 __alloc_skb+0x226/0x4a0 net/core/skbuff.c:518
 alloc_skb include/linux/skbuff.h:1322 [inline]
 alloc_skb_with_frags+0xa8/0x620 net/core/skbuff.c:6176
 sock_alloc_send_pskb+0x87f/0x9a0 net/core/sock.c:2770
 unix_dgram_sendmsg+0x5c1/0x1710 net/unix/af_unix.c:1949
 sock_sendmsg_nosec net/socket.c:716 [inline]
 __sock_sendmsg net/socket.c:728 [inline]
 __sys_sendto+0x4ac/0x660 net/socket.c:2143
 __do_sys_sendto net/socket.c:2155 [inline]
 __se_sys_sendto net/socket.c:2151 [inline]
 __x64_sys_sendto+0xe5/0x100 net/socket.c:2151
page_owner free stack trace missing

Memory state around the buggy address:
 ffff8881104a7b80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff8881104a7c00: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff8881104a7c80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                      ^
 ffff8881104a7d00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8881104a7d80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================
------------[ cut here ]------------
refcount_t: addition on 0; use-after-free.
WARNING: CPU: 1 PID: 1536 at lib/refcount.c:25 refcount_warn_saturate+0x104/0x1a0 lib/refcount.c:25
Modules linked in:
CPU: 1 PID: 1536 Comm: syz.2.345 Tainted: G    B   W          syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
RIP: 0010:refcount_warn_saturate+0x104/0x1a0 lib/refcount.c:25
Code: 05 01 48 c7 c7 40 15 ca 85 e8 38 c9 d8 fe 0f 0b eb df e8 ef 42 08 ff c6 05 7f 82 26 05 01 48 c7 c7 80 14 ca 85 e8 1c c9 d8 fe <0f> 0b eb c3 e8 d3 42 08 ff c6 05 64 82 26 05 01 48 c7 c7 e0 14 ca
RSP: 0018:ffffc90000af79a0 EFLAGS: 00010246
RAX: 5f300ab31ff9e800 RBX: 0000000000000002 RCX: ffff888109781440
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000002
RBP: ffffc90000af79b0 R08: 0000000000000003 R09: 0000000000000004
R10: dffffc0000000000 R11: fffff5200015ee84 R12: 0000000000000000
R13: 0000000000000001 R14: 0000000000000002 R15: ffff8881104a7c88
FS:  00007fa40d91b6c0(0000) GS:ffff8881f6f00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000200000001180 CR3: 00000001214d7000 CR4: 00000000003506a0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 __refcount_add include/linux/refcount.h:-1 [inline]
 __refcount_inc include/linux/refcount.h:250 [inline]
 refcount_inc include/linux/refcount.h:267 [inline]
 l2tp_session_inc_refcount net/l2tp/l2tp_core.c:211 [inline]
 l2tp_session_get_nth+0x174/0x180 net/l2tp/l2tp_core.c:313
 pppol2tp_next_session net/l2tp/l2tp_ppp.c:1451 [inline]
 pppol2tp_seq_start+0x2a4/0x620 net/l2tp/l2tp_ppp.c:1480
 seq_read_iter+0x3c4/0xd80 fs/seq_file.c:225
 seq_read+0x171/0x210 fs/seq_file.c:162
 pde_read fs/proc/inode.c:316 [inline]
 proc_reg_read+0x1e4/0x2d0 fs/proc/inode.c:328
 vfs_read+0x286/0x930 fs/read_write.c:468
 ksys_read+0x14a/0x260 fs/read_write.c:613
 __do_sys_read fs/read_write.c:623 [inline]
 __se_sys_read fs/read_write.c:621 [inline]
 __x64_sys_read+0x7b/0x90 fs/read_write.c:621
 x64_sys_call+0x2f/0x9a0 arch/x86/include/generated/asm/syscalls_64.h:1
 do_syscall_x64 arch/x86/entry/common.c:46 [inline]
 do_syscall_64+0x4c/0xa0 arch/x86/entry/common.c:76
 entry_SYSCALL_64_after_hwframe+0x68/0xd2
RIP: 0033:0x7fa40c99e159
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:00007fa40d91b028 EFLAGS: 00000246
 ORIG_RAX: 0000000000000000
RAX: ffffffffffffffda RBX: 00007fa40cc25fa0 RCX: 00007fa40c99e159
RDX: 0000000000001000 RSI: 0000200000000500 RDI: 000000000000000b
RBP: 00007fa40ca3503b R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fa40cc26038 R14: 00007fa40cc25fa0 R15: 00007ffe96d8b838
 </TASK>
---[ end trace 0000000000000000 ]---

Crashes (1):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2026/09/12 06:56 android14-6.1 6308d6d3f1b1 70a60e12 .config console log report info [disk image] [vmlinux] [kernel image] ci2-android-6-1 KASAN: use-after-free Write in l2tp_session_get_nth
* Struck through repros no longer work on HEAD.