KASAN: use-after-free Read in l2tp_tunnel_get_session

Status: premoderation: reported on 2026/07/05 05:21
Reported-by: syzbot+525f2200000ed7667792@syzkaller.appspotmail.com
First crash: 99d, last: 15d
▶ ▼ Similar bugs (2)
Kernel Title Rank 🛈 Repro Cause bisect Fix bisect Count Last Reported Patched Status
android-6-1 KASAN: use-after-free Read in l2tp_tunnel_get_session 19 6 6h57m 80d 0/2 premoderation: reported on 2026/07/23 21:14
android-5-15 KASAN: use-after-free Read in l2tp_tunnel_get_session 19 4 79d 107d 0/2 premoderation: reported on 2026/06/26 07:57

Sample crash report:
netlink: 4 bytes leftover after parsing attributes in process `syz.4.61'.
==================================================================
BUG: KASAN: use-after-free in l2tp_tunnel_get_session+0xcd/0x170 net/l2tp/l2tp_core.c:270
Read of size 4 at addr ffff8881184fd818 by task syz.4.61/615

CPU: 1 PID: 615 Comm: syz.4.61 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/16/2026
Call Trace:
 __dump_stack+0x21/0x24 lib/dump_stack.c:77
 dump_stack_lvl+0x1a7/0x208 lib/dump_stack.c:118
 print_address_description+0x7f/0x2c0 mm/kasan/report.c:248
 __kasan_report mm/kasan/report.c:435 [inline]
 kasan_report+0x100/0x140 mm/kasan/report.c:452
 __asan_report_load4_noabort+0x14/0x20 mm/kasan/report_generic.c:308
 l2tp_tunnel_get_session+0xcd/0x170 net/l2tp/l2tp_core.c:270
 l2tp_udp_recv_core net/l2tp/l2tp_core.c:875 [inline]
 l2tp_udp_encap_recv+0x318/0x7e0 net/l2tp/l2tp_core.c:929
 udpv6_queue_rcv_one_skb+0x11ea/0x1350 net/ipv6/udp.c:712
 udpv6_queue_rcv_skb+0x470/0x680 net/ipv6/udp.c:770
 udp6_unicast_rcv_skb+0x1e0/0x310 net/ipv6/udp.c:912
 __udp6_lib_rcv+0xa45/0xe10 net/ipv6/udp.c:996
 udpv6_rcv+0x21/0x30 net/ipv6/udp.c:1098
 ip6_protocol_deliver_rcu+0x8ad/0x1380 net/ipv6/ip6_input.c:427
 ip6_input_finish net/ipv6/ip6_input.c:468 [inline]
 NF_HOOK include/linux/netfilter.h:299 [inline]
 ip6_input+0xda/0x210 net/ipv6/ip6_input.c:477
 dst_input include/net/dst.h:461 [inline]
 ip6_rcv_finish+0x18c/0x360 net/ipv6/ip6_input.c:80
 NF_HOOK include/linux/netfilter.h:299 [inline]
 ipv6_rcv+0xf0/0x230 net/ipv6/ip6_input.c:301
 __netif_receive_skb_one_core net/core/dev.c:5413 [inline]
 __netif_receive_skb+0xc8/0x280 net/core/dev.c:5527
 netif_receive_skb_internal net/core/dev.c:5632 [inline]
 netif_receive_skb+0x9b/0x3e0 net/core/dev.c:5691
 tun_rx_batched+0x600/0x750 drivers/net/tun.c:-1
 tun_get_user+0x270e/0x31c0 drivers/net/tun.c:2026
 tun_chr_write_iter+0x1c1/0x270 drivers/net/tun.c:2059
 call_write_iter include/linux/fs.h:2072 [inline]
 new_sync_write fs/read_write.c:518 [inline]
 vfs_write+0x770/0xd70 fs/read_write.c:605
 ksys_write+0x14a/0x260 fs/read_write.c:658
 __do_sys_write fs/read_write.c:670 [inline]
 __se_sys_write fs/read_write.c:667 [inline]
 __x64_sys_write+0x7b/0x90 fs/read_write.c:667
 do_syscall_64+0x31/0x40 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x61/0xcb
RIP: 0033:0x7f3e07bd498e
Code: 08 0f 85 a5 a8 ff ff 49 89 fb 48 89 f0 48 89 d7 48 89 ce 4c 89 c2 4d 89 ca 4c 8b 44 24 08 4c 8b 4c 24 10 4c 89 5c 24 08 0f 05 <c3> 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 80 00 00 00 00 48 83 ec 08
RSP: 002b:00007f3e0666cfb8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007f3e0666d6c0 RCX: 00007f3e07bd498e
RDX: 000000000000004c RSI: 0000200000000140 RDI: 00000000000000c8
RBP: 00007f3e07cab06b R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f3e07e9c038 R14: 00007f3e07e9bfa0 R15: 00007ffc09968e58

Allocated by task 615:
 kasan_save_stack mm/kasan/common.c:38 [inline]
 kasan_set_track mm/kasan/common.c:45 [inline]
 set_alloc_info mm/kasan/common.c:430 [inline]
 ____kasan_kmalloc mm/kasan/common.c:509 [inline]
 __kasan_kmalloc+0xd4/0x100 mm/kasan/common.c:518
 kasan_kmalloc include/linux/kasan.h:254 [inline]
 __kmalloc+0x19f/0x330 mm/slub.c:4038
 kmalloc include/linux/slab.h:560 [inline]
 kzalloc include/linux/slab.h:667 [inline]
 l2tp_session_create+0x39/0xb60 net/l2tp/l2tp_core.c:1616
 pppol2tp_connect+0xbf5/0x1640 net/l2tp/l2tp_ppp.c:772
 __sys_connect_file net/socket.c:1867 [inline]
 __sys_connect+0x3ce/0x450 net/socket.c:1884
 __do_sys_connect net/socket.c:1894 [inline]
 __se_sys_connect net/socket.c:1891 [inline]
 __x64_sys_connect+0x7a/0x90 net/socket.c:1891
 do_syscall_64+0x31/0x40 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x61/0xcb

Freed by task 615:
 kasan_save_stack mm/kasan/common.c:38 [inline]
 kasan_set_track+0x4a/0x70 mm/kasan/common.c:45
 kasan_set_free_info+0x23/0x40 mm/kasan/generic.c:370
 ____kasan_slab_free+0x125/0x160 mm/kasan/common.c:362
 __kasan_slab_free+0x11/0x20 mm/kasan/common.c:370
 kasan_slab_free include/linux/kasan.h:220 [inline]
 slab_free_hook mm/slub.c:1600 [inline]
 slab_free_freelist_hook+0xc5/0x190 mm/slub.c:1626
 slab_free mm/slub.c:3208 [inline]
 kfree+0xc0/0x270 mm/slub.c:4196
 l2tp_session_free net/l2tp/l2tp_core.c:168 [inline]
 l2tp_session_put+0xb2/0x1a0 net/l2tp/l2tp_core.c:193
 pppol2tp_ioctl+0x748/0x960 net/l2tp/l2tp_ppp.c:-1
 pppox_ioctl+0x1e4/0x320 drivers/net/ppp/pppox.c:93
 sock_do_ioctl+0xda/0x380 net/socket.c:1068
 sock_ioctl+0x472/0x740 net/socket.c:1208
 vfs_ioctl fs/ioctl.c:48 [inline]
 __do_sys_ioctl fs/ioctl.c:753 [inline]
 __se_sys_ioctl+0x121/0x1a0 fs/ioctl.c:739
 __x64_sys_ioctl+0x7b/0x90 fs/ioctl.c:739
 do_syscall_64+0x31/0x40 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x61/0xcb

The buggy address belongs to the object at ffff8881184fd800
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 24 bytes inside of
 512-byte region [ffff8881184fd800, ffff8881184fda00)
The buggy address belongs to the page:
page:ffffea0004613f00 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x1184fc
head:ffffea0004613f00 order:2 compound_mapcount:0 compound_pincount:0
flags: 0x4000000000010200(slab|head)
raw: 4000000000010200 ffffea00045ed900 0000000400000004 ffff888100043080
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 101, ts 6074031482, free_ts 0
 set_page_owner include/linux/page_owner.h:35 [inline]
 post_alloc_hook mm/page_alloc.c:2456 [inline]
 prep_new_page+0x176/0x190 mm/page_alloc.c:2462
 get_page_from_freelist+0x225f/0x23f0 mm/page_alloc.c:4254
 __alloc_pages_nodemask+0x29a/0x640 mm/page_alloc.c:5384
 alloc_slab_page mm/slub.c:-1 [inline]
 allocate_slab mm/slub.c:1813 [inline]
 new_slab+0x84/0x3f0 mm/slub.c:1874
 new_slab_objects mm/slub.c:2632 [inline]
 ___slab_alloc+0x2f8/0x4c0 mm/slub.c:2796
 __slab_alloc+0x63/0xa0 mm/slub.c:2836
 slab_alloc_node mm/slub.c:2918 [inline]
 slab_alloc mm/slub.c:2960 [inline]
 __kmalloc_track_caller+0x1e4/0x310 mm/slub.c:4541
 __kmalloc_reserve net/core/skbuff.c:144 [inline]
 __alloc_skb+0xdc/0x520 net/core/skbuff.c:212
 alloc_skb include/linux/skbuff.h:1126 [inline]
 alloc_skb_with_frags+0xa3/0x560 net/core/skbuff.c:6045
 sock_alloc_send_pskb+0x87f/0x9a0 net/core/sock.c:2392
 unix_dgram_sendmsg+0x6f3/0x18f0 net/unix/af_unix.c:1725
 sock_sendmsg_nosec net/socket.c:652 [inline]
 __sock_sendmsg net/socket.c:664 [inline]
 sock_write_iter+0x2a6/0x3a0 net/socket.c:1019
 call_write_iter include/linux/fs.h:2072 [inline]
 new_sync_write fs/read_write.c:518 [inline]
 vfs_write+0x770/0xd70 fs/read_write.c:605
 ksys_write+0x14a/0x260 fs/read_write.c:658
 __do_sys_write fs/read_write.c:670 [inline]
 __se_sys_write fs/read_write.c:667 [inline]
 __x64_sys_write+0x7b/0x90 fs/read_write.c:667
 do_syscall_64+0x31/0x40 arch/x86/entry/common.c:46
page_owner free stack trace missing

Memory state around the buggy address:
 ffff8881184fd700: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff8881184fd780: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff8881184fd800: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                            ^
 ffff8881184fd880: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8881184fd900: 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: 615 at lib/refcount.c:25 refcount_warn_saturate+0x104/0x1a0 lib/refcount.c:25
Modules linked in:
CPU: 1 PID: 615 Comm: syz.4.61 Tainted: G    B             syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/16/2026
RIP: 0010:refcount_warn_saturate+0x104/0x1a0 lib/refcount.c:25
Code: 04 01 48 c7 c7 80 52 40 85 e8 d8 60 3b 02 0f 0b eb df e8 af 93 26 ff c6 05 9d 78 82 04 01 48 c7 c7 c0 51 40 85 e8 bc 60 3b 02 <0f> 0b eb c3 e8 93 93 26 ff c6 05 82 78 82 04 01 48 c7 c7 20 52 40
RSP: 0018:ffffc90000d771f8 EFLAGS: 00010246
RAX: 78c04cee2eba4500 RBX: 0000000000000002 RCX: 0000000000080000
RDX: ffffc900034f3000 RSI: 000000000007ffff RDI: 0000000000080000
RBP: ffffc90000d77208 R08: ffffc90000d76b87 R09: fffff520001aeda4
R10: dffffc0000000000 R11: fffff520001aed71 R12: 0000000000000000
R13: dffffc0000000000 R14: 0000000000000002 R15: ffff8881184fd888
FS:  00007f3e0666d6c0(0000) GS:ffff8881f7100000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000110c300014 CR3: 0000000118c22000 CR4: 00000000003526a0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 __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+0x161/0x170 net/l2tp/l2tp_core.c:271
 l2tp_udp_recv_core net/l2tp/l2tp_core.c:875 [inline]
 l2tp_udp_encap_recv+0x318/0x7e0 net/l2tp/l2tp_core.c:929
 udpv6_queue_rcv_one_skb+0x11ea/0x1350 net/ipv6/udp.c:712
 udpv6_queue_rcv_skb+0x470/0x680 net/ipv6/udp.c:770
 udp6_unicast_rcv_skb+0x1e0/0x310 net/ipv6/udp.c:912
 __udp6_lib_rcv+0xa45/0xe10 net/ipv6/udp.c:996
 udpv6_rcv+0x21/0x30 net/ipv6/udp.c:1098
 ip6_protocol_deliver_rcu+0x8ad/0x1380 net/ipv6/ip6_input.c:427
 ip6_input_finish net/ipv6/ip6_input.c:468 [inline]
 NF_HOOK include/linux/netfilter.h:299 [inline]
 ip6_input+0xda/0x210 net/ipv6/ip6_input.c:477
 dst_input include/net/dst.h:461 [inline]
 ip6_rcv_finish+0x18c/0x360 net/ipv6/ip6_input.c:80
 NF_HOOK include/linux/netfilter.h:299 [inline]
 ipv6_rcv+0xf0/0x230 net/ipv6/ip6_input.c:301
 __netif_receive_skb_one_core net/core/dev.c:5413 [inline]
 __netif_receive_skb+0xc8/0x280 net/core/dev.c:5527
 netif_receive_skb_internal net/core/dev.c:5632 [inline]
 netif_receive_skb+0x9b/0x3e0 net/core/dev.c:5691
 tun_rx_batched+0x600/0x750 drivers/net/tun.c:-1
 tun_get_user+0x270e/0x31c0 drivers/net/tun.c:2026
 tun_chr_write_iter+0x1c1/0x270 drivers/net/tun.c:2059
 call_write_iter include/linux/fs.h:2072 [inline]
 new_sync_write fs/read_write.c:518 [inline]
 vfs_write+0x770/0xd70 fs/read_write.c:605
 ksys_write+0x14a/0x260 fs/read_write.c:658
 __do_sys_write fs/read_write.c:670 [inline]
 __se_sys_write fs/read_write.c:667 [inline]
 __x64_sys_write+0x7b/0x90 fs/read_write.c:667
 do_syscall_64+0x31/0x40 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x61/0xcb
RIP: 0033:0x7f3e07bd498e
Code: 08 0f 85 a5 a8 ff ff 49 89 fb 48 89 f0 48 89 d7 48 89 ce 4c 89 c2 4d 89 ca 4c 8b 44 24 08 4c 8b 4c 24 10 4c 89 5c 24 08 0f 05 <c3> 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 80 00 00 00 00 48 83 ec 08
RSP: 002b:00007f3e0666cfb8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007f3e0666d6c0 RCX: 00007f3e07bd498e
RDX: 000000000000004c RSI: 0000200000000140 RDI: 00000000000000c8
RBP: 00007f3e07cab06b R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f3e07e9c038 R14: 00007f3e07e9bfa0 R15: 00007ffc09968e58
---[ end trace 8fd9338f2c29ce8e ]---
------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: CPU: 1 PID: 615 at lib/refcount.c:28 refcount_warn_saturate+0x120/0x1a0 lib/refcount.c:28
Modules linked in:
CPU: 1 PID: 615 Comm: syz.4.61 Tainted: G    B   W         syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/16/2026
RIP: 0010:refcount_warn_saturate+0x120/0x1a0 lib/refcount.c:28
Code: 04 01 48 c7 c7 c0 51 40 85 e8 bc 60 3b 02 0f 0b eb c3 e8 93 93 26 ff c6 05 82 78 82 04 01 48 c7 c7 20 52 40 85 e8 a0 60 3b 02 <0f> 0b eb a7 e8 77 93 26 ff c6 05 63 78 82 04 01 48 c7 c7 40 51 40
RSP: 0018:ffffc90000d77208 EFLAGS: 00010246

RAX: 78c04cee2eba4500 RBX: 0000000000000003 RCX: 0000000000080000
RDX: ffffc900034f3000 RSI: 000000000007ffff RDI: 0000000000080000
RBP: ffffc90000d77218 R08: ffff8881f7152fbf R09: 1ffff1103ee2a5f7
R10: dffffc0000000000 R11: ffffed103ee2a5f8 R12: ffff8881199c807e
R13: 1ffff11022ffa9c6 R14: 0000000000000003 R15: 00000000c0000000
FS:  00007f3e0666d6c0(0000) GS:ffff8881f7100000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000110c300014 CR3: 0000000118c22000 CR4: 00000000003506a0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 __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+0xcb/0x1a0 net/l2tp/l2tp_core.c:192
 l2tp_udp_recv_core net/l2tp/l2tp_core.c:893 [inline]
 l2tp_udp_encap_recv+0x555/0x7e0 net/l2tp/l2tp_core.c:929
 udpv6_queue_rcv_one_skb+0x11ea/0x1350 net/ipv6/udp.c:712
 udpv6_queue_rcv_skb+0x470/0x680 net/ipv6/udp.c:770
 udp6_unicast_rcv_skb+0x1e0/0x310 net/ipv6/udp.c:912
 __udp6_lib_rcv+0xa45/0xe10 net/ipv6/udp.c:996
 udpv6_rcv+0x21/0x30 net/ipv6/udp.c:1098
 ip6_protocol_deliver_rcu+0x8ad/0x1380 net/ipv6/ip6_input.c:427
 ip6_input_finish net/ipv6/ip6_input.c:468 [inline]
 NF_HOOK include/linux/netfilter.h:299 [inline]
 ip6_input+0xda/0x210 net/ipv6/ip6_input.c:477
 dst_input include/net/dst.h:461 [inline]
 ip6_rcv_finish+0x18c/0x360 net/ipv6/ip6_input.c:80
 NF_HOOK include/linux/netfilter.h:299 [inline]
 ipv6_rcv+0xf0/0x230 net/ipv6/ip6_input.c:301
 __netif_receive_skb_one_core net/core/dev.c:5413 [inline]
 __netif_receive_skb+0xc8/0x280 net/core/dev.c:5527
 netif_receive_skb_internal net/core/dev.c:5632 [inline]
 netif_receive_skb+0x9b/0x3e0 net/core/dev.c:5691
 tun_rx_batched+0x600/0x750 drivers/net/tun.c:-1
 tun_get_user+0x270e/0x31c0 drivers/net/tun.c:2026
 tun_chr_write_iter+0x1c1/0x270 drivers/net/tun.c:2059
 call_write_iter include/linux/fs.h:2072 [inline]
 new_sync_write fs/read_write.c:518 [inline]
 vfs_write+0x770/0xd70 fs/read_write.c:605
 ksys_write+0x14a/0x260 fs/read_write.c:658
 __do_sys_write fs/read_write.c:670 [inline]
 __se_sys_write fs/read_write.c:667 [inline]
 __x64_sys_write+0x7b/0x90 fs/read_write.c:667
 do_syscall_64+0x31/0x40 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x61/0xcb
RIP: 0033:0x7f3e07bd498e
Code: 08 0f 85 a5 a8 ff ff 49 89 fb 48 89 f0 48 89 d7 48 89 ce 4c 89 c2 4d 89 ca 4c 8b 44 24 08 4c 8b 4c 24 10 4c 89 5c 24 08 0f 05 <c3> 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 80 00 00 00 00 48 83 ec 08
RSP: 002b:00007f3e0666cfb8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007f3e0666d6c0 RCX: 00007f3e07bd498e
RDX: 000000000000004c RSI: 0000200000000140 RDI: 00000000000000c8
RBP: 00007f3e07cab06b R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f3e07e9c038 R14: 00007f3e07e9bfa0 R15: 00007ffc09968e58
---[ end trace 8fd9338f2c29ce8f ]---

Crashes (5):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2026/09/26 19:48 android13-5.10-lts c5d8ec7afea2 a3c7249d .config console log report info [disk image] [vmlinux] [kernel image] ci2-android-5-10 KASAN: use-after-free Read in l2tp_tunnel_get_session
2026/09/13 17:17 android13-5.10-lts 452567d1985b 70a60e12 .config console log report info [disk image] [vmlinux] [kernel image] ci2-android-5-10 KASAN: use-after-free Read in l2tp_tunnel_get_session
2026/09/13 10:14 android13-5.10-lts 452567d1985b 70a60e12 .config console log report info [disk image] [vmlinux] [kernel image] ci2-android-5-10 KASAN: use-after-free Read in l2tp_tunnel_get_session
2026/08/08 19:49 android13-5.10-lts 447db0046f21 5d63f80b .config console log report info [disk image] [vmlinux] [kernel image] ci2-android-5-10 KASAN: use-after-free Read in l2tp_tunnel_get_session
2026/07/05 05:20 android13-5.10-lts 1bb06dbd8c78 fcece630 .config console log report info [disk image] [vmlinux] [kernel image] ci2-android-5-10 KASAN: use-after-free Read in l2tp_tunnel_get_session
* Struck through repros no longer work on HEAD.