==================================================================
BUG: KASAN: use-after-free in l2tp_tunnel_get_session+0xd2/0x170 net/l2tp/l2tp_core.c:270
Read of size 4 at addr ffff888116722018 by task syz.0.37/434

CPU: 1 PID: 434 Comm: syz.0.37 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/16/2026
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
 __asan_report_load4_noabort+0x14/0x20 mm/kasan/report_generic.c:350
 l2tp_tunnel_get_session+0xd2/0x170 net/l2tp/l2tp_core.c:270
 pppol2tp_connect+0xb3c/0x1640 net/l2tp/l2tp_ppp.c:746
 __sys_connect_file net/socket.c:2000 [inline]
 __sys_connect+0x3da/0x460 net/socket.c:2017
 __do_sys_connect net/socket.c:2027 [inline]
 __se_sys_connect net/socket.c:2024 [inline]
 __x64_sys_connect+0x7a/0x90 net/socket.c:2024
 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
RIP: 0033:0x7f61f5d9de99
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:00007f61f6cab028 EFLAGS: 00000246 ORIG_RAX: 000000000000002a
RAX: ffffffffffffffda RBX: 00007f61f6025fa0 RCX: 00007f61f5d9de99
RDX: 0000000000000032 RSI: 0000200000000240 RDI: 0000000000000003
RBP: 00007f61f5e33eaf R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f61f6026038 R14: 00007f61f6025fa0 R15: 00007fff404e6dc8
 </TASK>

Allocated by task 431:
 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:1609
 pppol2tp_connect+0xbf5/0x1640 net/l2tp/l2tp_ppp.c:771
 __sys_connect_file net/socket.c:2000 [inline]
 __sys_connect+0x3da/0x460 net/socket.c:2017
 __do_sys_connect net/socket.c:2027 [inline]
 __se_sys_connect net/socket.c:2024 [inline]
 __x64_sys_connect+0x7a/0x90 net/socket.c:2024
 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 431:
 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:1191
 sock_ioctl+0x475/0x700 net/socket.c:1310
 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 ffff888116722000
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 24 bytes inside of
 512-byte region [ffff888116722000, ffff888116722200)

The buggy address belongs to the physical page:
page:ffffea000459c800 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x116720
head:ffffea000459c800 order:2 compound_mapcount:0 compound_pincount:0
flags: 0x4000000000010200(slab|head|zone=1)
raw: 4000000000010200 ffffea0004562800 dead000000000002 ffff888100042f00
raw: 0000000000000000 0000000000100010 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 118, tgid 118 (udevd), ts 5794939265, free_ts 0
 set_page_owner include/linux/page_owner.h:33 [inline]
 post_alloc_hook+0x1f5/0x210 mm/page_alloc.c:2672
 prep_new_page+0x1c/0x110 mm/page_alloc.c:2679
 get_page_from_freelist+0x2ca9/0x2d20 mm/page_alloc.c:4586
 __alloc_pages+0x1fa/0x610 mm/page_alloc.c:5930
 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
 kmalloc_trace+0x29/0xb0 mm/slab_common.c:1028
 kmalloc include/linux/slab.h:563 [inline]
 kzalloc include/linux/slab.h:699 [inline]
 alloc_bprm+0x59/0x750 fs/exec.c:1543
 do_execveat_common+0x1a7/0xaa0 fs/exec.c:1934
 do_execve fs/exec.c:2057 [inline]
 __do_sys_execve fs/exec.c:2133 [inline]
 __se_sys_execve fs/exec.c:2128 [inline]
 __x64_sys_execve+0x92/0xb0 fs/exec.c:2128
 x64_sys_call+0x98/0x9a0 arch/x86/include/generated/asm/syscalls_64.h:60
 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
page_owner free stack trace missing

Memory state around the buggy address:
 ffff888116721f00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff888116721f80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff888116722000: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                            ^
 ffff888116722080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff888116722100: 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: 434 at lib/refcount.c:25 refcount_warn_saturate+0x104/0x1a0 lib/refcount.c:25
Modules linked in:
CPU: 1 PID: 434 Comm: syz.0.37 Tainted: G    B              syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/16/2026
RIP: 0010:refcount_warn_saturate+0x104/0x1a0 lib/refcount.c:25
Code: 05 01 48 c7 c7 40 0d ca 85 e8 d8 28 d9 fe 0f 0b eb df e8 2f 9a 08 ff c6 05 0f ca 26 05 01 48 c7 c7 80 0c ca 85 e8 bc 28 d9 fe <0f> 0b eb c3 e8 13 9a 08 ff c6 05 f4 c9 26 05 01 48 c7 c7 e0 0c ca
RSP: 0018:ffffc9000f73fba8 EFLAGS: 00010246
RAX: 2e813a4a297ff700 RBX: 0000000000000002 RCX: ffff888116b08000
RDX: 0000000000000003 RSI: 0000000000000000 RDI: 0000000000000002
RBP: ffffc9000f73fbb8 R08: 000000000001ffff R09: 0000000000400000
R10: 0000000000400008 R11: fffff52001ee7ef1 R12: 0000000000000000
R13: dffffc0000000000 R14: 0000000000000002 R15: ffff888116722088
FS:  00007f61f6cab6c0(0000) GS:ffff8881f6f00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000200000000240 CR3: 0000000136ede000 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_tunnel_get_session+0x16b/0x170 net/l2tp/l2tp_core.c:271
 pppol2tp_connect+0xb3c/0x1640 net/l2tp/l2tp_ppp.c:746
 __sys_connect_file net/socket.c:2000 [inline]
 __sys_connect+0x3da/0x460 net/socket.c:2017
 __do_sys_connect net/socket.c:2027 [inline]
 __se_sys_connect net/socket.c:2024 [inline]
 __x64_sys_connect+0x7a/0x90 net/socket.c:2024
 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
RIP: 0033:0x7f61f5d9de99
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:00007f61f6cab028 EFLAGS: 00000246 ORIG_RAX: 000000000000002a
RAX: ffffffffffffffda RBX: 00007f61f6025fa0 RCX: 00007f61f5d9de99
RDX: 0000000000000032 RSI: 0000200000000240 RDI: 0000000000000003
RBP: 00007f61f5e33eaf R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f61f6026038 R14: 00007f61f6025fa0 R15: 00007fff404e6dc8
 </TASK>
---[ end trace 0000000000000000 ]---
------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: CPU: 1 PID: 434 at lib/refcount.c:28 refcount_warn_saturate+0x120/0x1a0 lib/refcount.c:28
Modules linked in:

CPU: 1 PID: 434 Comm: syz.0.37 Tainted: G    B   W          syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/16/2026
RIP: 0010:refcount_warn_saturate+0x120/0x1a0 lib/refcount.c:28
Code: 05 01 48 c7 c7 80 0c ca 85 e8 bc 28 d9 fe 0f 0b eb c3 e8 13 9a 08 ff c6 05 f4 c9 26 05 01 48 c7 c7 e0 0c ca 85 e8 a0 28 d9 fe <0f> 0b eb a7 e8 f7 99 08 ff c6 05 d5 c9 26 05 01 48 c7 c7 20 0c ca
RSP: 0018:ffffc9000f73fbb8 EFLAGS: 00010246

RAX: 2e813a4a297ff700 RBX: 0000000000000003 RCX: ffff888116b08000
RDX: 0000000000000003 RSI: 0000000000000001 RDI: 0000000000000002
RBP: ffffc9000f73fbc8 R08: 000000000001ffff R09: 0000000000400000
R10: 0000000000400008 R11: fffff52001ee7f28 R12: ffff888116720800
R13: 1ffff92001ee7f8c R14: 0000000000000003 R15: 00000000c0000000
FS:  00007f61f6cab6c0(0000) GS:ffff8881f6f00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f61f6c89ff8 CR3: 0000000136ede000 CR4: 00000000003526a0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 __refcount_sub_and_test include/linux/refcount.h:283 [inline]
 __refcount_dec_and_test include/linux/refcount.h:315 [inline]
 refcount_dec_and_test include/linux/refcount.h:333 [inline]
 l2tp_session_put+0xc8/0x1a0 net/l2tp/l2tp_core.c:192
 pppol2tp_connect+0xa33/0x1640 net/l2tp/l2tp_ppp.c:842
 __sys_connect_file net/socket.c:2000 [inline]
 __sys_connect+0x3da/0x460 net/socket.c:2017
 __do_sys_connect net/socket.c:2027 [inline]
 __se_sys_connect net/socket.c:2024 [inline]
 __x64_sys_connect+0x7a/0x90 net/socket.c:2024
 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
RIP: 0033:0x7f61f5d9de99
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:00007f61f6cab028 EFLAGS: 00000246 ORIG_RAX: 000000000000002a
RAX: ffffffffffffffda RBX: 00007f61f6025fa0 RCX: 00007f61f5d9de99
RDX: 0000000000000032 RSI: 0000200000000240 RDI: 0000000000000003
RBP: 00007f61f5e33eaf R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f61f6026038 R14: 00007f61f6025fa0 R15: 00007fff404e6dc8
 </TASK>
---[ end trace 0000000000000000 ]---