[v5.15] possible deadlock in ip_mc_drop_socket

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 3330a8d33e08 Linux 5.15.201
git tree: linux-5.15.y
console output: https://syzkaller.appspot.com/x/log.txt?x=134f3eaa580000
kernel config: https://syzkaller.appspot.com/x/.config?x=353ae28c40b35af5
dashboard link: https://syzkaller.appspot.com/bug?extid=418dfdde23b79dee8526
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/b65fe19af112/disk-3330a8d3.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/351948519a0d/vmlinux-3330a8d3.xz
kernel image: https://storage.googleapis.com/syzbot-assets/a3b3f8477595/bzImage-3330a8d3.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+418dfd...@syzkaller.appspotmail.com

netdevsim netdevsim1 netdevsim1: unset [0, 0] type 1 family 0 port 8472 - 0
netdevsim netdevsim1 netdevsim2: unset [0, 0] type 1 family 0 port 8472 - 0
netdevsim netdevsim1 netdevsim3: unset [0, 0] type 1 family 0 port 8472 - 0
============================================
WARNING: possible recursive locking detected
syzkaller #0 Not tainted
--------------------------------------------
syz.1.413/5836 is trying to acquire lock:
ffffffff8d43c848 (rtnl_mutex){+.+.}-{3:3}, at: ip_mc_drop_socket+0x7b/0x270 net/ipv4/igmp.c:2702

but task is already holding lock:
ffffffff8d43c848 (rtnl_mutex){+.+.}-{3:3}, at: rtnl_lock net/core/rtnetlink.c:72 [inline]
ffffffff8d43c848 (rtnl_mutex){+.+.}-{3:3}, at: rtnetlink_rcv_msg+0x7ee/0xf30 net/core/rtnetlink.c:5681

other info that might help us debug this:
Possible unsafe locking scenario:

CPU0
----
lock(rtnl_mutex);
lock(rtnl_mutex);

*** DEADLOCK ***

May be due to missing lock nesting notation

1 lock held by syz.1.413/5836:
#0: ffffffff8d43c848 (rtnl_mutex){+.+.}-{3:3}, at: rtnl_lock net/core/rtnetlink.c:72 [inline]
#0: ffffffff8d43c848 (rtnl_mutex){+.+.}-{3:3}, at: rtnetlink_rcv_msg+0x7ee/0xf30 net/core/rtnetlink.c:5681

stack backtrace:
CPU: 1 PID: 5836 Comm: syz.1.413 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
Call Trace:
<TASK>
dump_stack_lvl+0x188/0x250 lib/dump_stack.c:106
__lock_acquire+0x1236/0x7d10 kernel/locking/lockdep.c:-1
lock_acquire+0x19e/0x400 kernel/locking/lockdep.c:5623
__mutex_lock_common+0x1e3/0x2400 kernel/locking/mutex.c:596
__mutex_lock kernel/locking/mutex.c:729 [inline]
mutex_lock_nested+0x17/0x20 kernel/locking/mutex.c:743
ip_mc_drop_socket+0x7b/0x270 net/ipv4/igmp.c:2702
inet_release+0x47/0x180 net/ipv4/af_inet.c:419
__sock_release net/socket.c:651 [inline]
sock_release+0x7b/0x140 net/socket.c:679
vxlan_sock_release+0x385/0x400 drivers/net/vxlan/vxlan_core.c:1637
vxlan_stop+0xf62/0x1090 drivers/net/vxlan/vxlan_core.c:3149
__dev_close_many+0x1d3/0x2b0 net/core/dev.c:1634
__dev_close net/core/dev.c:1646 [inline]
__dev_change_flags+0x2d8/0x6a0 net/core/dev.c:8864
dev_change_flags+0x82/0x1a0 net/core/dev.c:8937
do_setlink+0xcdb/0x3d60 net/core/rtnetlink.c:2781
rtnl_group_changelink net/core/rtnetlink.c:3298 [inline]
__rtnl_newlink net/core/rtnetlink.c:3462 [inline]
rtnl_newlink+0xf8d/0x1a50 net/core/rtnetlink.c:3574
rtnetlink_rcv_msg+0x844/0xf30 net/core/rtnetlink.c:5684
netlink_rcv_skb+0x1f5/0x440 net/netlink/af_netlink.c:2507
netlink_unicast_kernel net/netlink/af_netlink.c:1318 [inline]
netlink_unicast+0x774/0x920 net/netlink/af_netlink.c:1344
netlink_sendmsg+0x8ba/0xbe0 net/netlink/af_netlink.c:1918
sock_sendmsg_nosec net/socket.c:706 [inline]
__sock_sendmsg net/socket.c:718 [inline]
____sys_sendmsg+0x5b7/0x8f0 net/socket.c:2445
___sys_sendmsg+0x236/0x2e0 net/socket.c:2499
__sys_sendmsg net/socket.c:2528 [inline]
__do_sys_sendmsg net/socket.c:2537 [inline]
__se_sys_sendmsg+0x1af/0x290 net/socket.c:2535
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x4c/0xa0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x66/0xd0
RIP: 0033:0x7fc4f0dfc629
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:00007fc4ef056028 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00007fc4f1075fa0 RCX: 00007fc4f0dfc629
RDX: 0000000000000000 RSI: 0000200000000280 RDI: 000000000000000c
RBP: 00007fc4f0e92b39 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fc4f1076038 R14: 00007fc4f1075fa0 R15: 00007fffeaf15568
</TASK>


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