mptcp: pm: in-kernel: always set ID as avail when rm endp

In the Linux kernel, the following vulnerability has been resolved:

mptcp: pm: in-kernel: always set ID as avail when rm endp

Syzkaller managed to find a combination of actions that was generating
this warning:

WARNING: net/mptcp/pm_kernel.c:1074 at __mark_subflow_endp_available net/mptcp/pm_kernel.c:1074 [inline], CPU#1: syz.7.48/2535
WARNING: net/mptcp/pm_kernel.c:1074 at mptcp_pm_nl_fullmesh net/mptcp/pm_kernel.c:1446 [inline], CPU#1: syz.7.48/2535
WARNING: net/mptcp/pm_kernel.c:1074 at mptcp_pm_nl_set_flags_all net/mptcp/pm_kernel.c:1474 [inline], CPU#1: syz.7.48/2535
WARNING: net/mptcp/pm_kernel.c:1074 at mptcp_pm_nl_set_flags+0x5de/0x640 net/mptcp/pm_kernel.c:1538, CPU#1: syz.7.48/2535
Modules linked in:
CPU: 1 UID: 0 PID: 2535 Comm: syz.7.48 Not tainted 6.18.0-03987-gea5f5e676cf5 #17 PREEMPT(voluntary)
Hardware name: QEMU Ubuntu 25.10 PC (i440FX + PIIX, 1996), BIOS 1.17.0-debian-1.17.0-1 04/01/2014
RIP: 0010:__mark_subflow_endp_available net/mptcp/pm_kernel.c:1074 [inline]
RIP: 0010:mptcp_pm_nl_fullmesh net/mptcp/pm_kernel.c:1446 [inline]
RIP: 0010:mptcp_pm_nl_set_flags_all net/mptcp/pm_kernel.c:1474 [inline]
RIP: 0010:mptcp_pm_nl_set_flags+0x5de/0x640 net/mptcp/pm_kernel.c:1538
Code: 89 c7 e8 c5 8c 73 fe e9 f7 fd ff ff 49 83 ef 80 e8 b7 8c 73 fe 4c 89 ff be 03 00 00 00 e8 4a 29 e3 fe eb ac e8 a3 8c 73 fe 90 <0f> 0b 90 e9 3d ff ff ff e8 95 8c 73 fe b8 a1 ff ff ff eb 1a e8 89
RSP: 0018:ffffc9001535b820 EFLAGS: 00010287
netdevsim0: tun_chr_ioctl cmd 1074025677
RAX: ffffffff82da294d RBX: 0000000000000001 RCX: 0000000000080000
RDX: ffffc900096d0000 RSI: 00000000000006d6 RDI: 00000000000006d7
netdevsim0: linktype set to 823
RBP: ffff88802cdb2240 R08: 00000000000104ae R09: ffffffffffffffff
R10: ffffffff82da27d4 R11: 0000000000000000 R12: 0000000000000000
R13: ffff88801246d8c0 R14: ffffc9001535b8b8 R15: ffff88802cdb1800
FS: 00007fc6ac5a76c0(0000) GS:ffff8880f90c8000(0000) knlGS:0000000000000000
netlink: 'syz.3.50': attribute type 5 has an invalid length.
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
netlink: 1232 bytes leftover after parsing attributes in process `syz.3.50'.
CR2: 0000200000010000 CR3: 0000000025b1a000 CR4: 0000000000350ef0
Call Trace:
<TASK>
mptcp_pm_set_flags net/mptcp/pm_netlink.c:277 [inline]
mptcp_pm_nl_set_flags_doit+0x1d7/0x210 net/mptcp/pm_netlink.c:282
genl_family_rcv_msg_doit+0x117/0x180 net/netlink/genetlink.c:1115
genl_family_rcv_msg net/netlink/genetlink.c:1195 [inline]
genl_rcv_msg+0x3a8/0x3f0 net/netlink/genetlink.c:1210
netlink_rcv_skb+0x16d/0x240 net/netlink/af_netlink.c:2550
genl_rcv+0x28/0x40 net/netlink/genetlink.c:1219
netlink_unicast_kernel net/netlink/af_netlink.c:1318 [inline]
netlink_unicast+0x3e9/0x4c0 net/netlink/af_netlink.c:1344
netlink_sendmsg+0x4ab/0x5b0 net/netlink/af_netlink.c:1894
sock_sendmsg_nosec net/socket.c:718 [inline]
__sock_sendmsg+0xc9/0xf0 net/socket.c:733
____sys_sendmsg+0x272/0x3b0 net/socket.c:2608
___sys_sendmsg+0x2de/0x320 net/socket.c:2662
__sys_sendmsg net/socket.c:2694 [inline]
__do_sys_sendmsg net/socket.c:2699 [inline]
__se_sys_sendmsg net/socket.c:2697 [inline]
__x64_sys_sendmsg+0x110/0x1a0 net/socket.c:2697
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xed/0x360 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fc6adb66f6d
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 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:00007fc6ac5a6ff8 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00007fc6addf5fa0 RCX: 00007fc6adb66f6d
RDX: 0000000000048084 RSI: 00002000000002c0 RDI: 000000000000000e
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 000000000000
---truncated---