mptcp: fix deadlock in __mptcp_push_pending()

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

mptcp: fix deadlock in __mptcp_push_pending()

__mptcp_push_pending() may call mptcp_flush_join_list() with subflow

socket lock held. If such call hits mptcp_sockopt_sync_all() then

subsequently __mptcp_sockopt_sync() could try to lock the subflow

socket for itself, causing a deadlock.

sysrq: Show Blocked State

task:ss-server state:D stack: 0 pid: 938 ppid: 1 flags:0x00000000

Call Trace:

<TASK>

__schedule+0x2d6/0x10c0

? __mod_memcg_state+0x4d/0x70

? csum_partial+0xd/0x20

? _raw_spin_lock_irqsave+0x26/0x50

schedule+0x4e/0xc0

__lock_sock+0x69/0x90

? do_wait_intr_irq+0xa0/0xa0

__lock_sock_fast+0x35/0x50

mptcp_sockopt_sync_all+0x38/0xc0

__mptcp_push_pending+0x105/0x200

mptcp_sendmsg+0x466/0x490

sock_sendmsg+0x57/0x60

__sys_sendto+0xf0/0x160

? do_wait_intr_irq+0xa0/0xa0

? fpregs_restore_userregs+0x12/0xd0

__x64_sys_sendto+0x20/0x30

do_syscall_64+0x38/0x90

entry_SYSCALL_64_after_hwframe+0x44/0xae

RIP: 0033:0x7f9ba546c2d0

RSP: 002b:00007ffdc3b762d8 EFLAGS: 00000246 ORIG_RAX: 000000000000002c

RAX: ffffffffffffffda RBX: 00007f9ba56c8060 RCX: 00007f9ba546c2d0

RDX: 000000000000077a RSI: 0000000000e5e180 RDI: 0000000000000234

RBP: 0000000000cc57f0 R08: 0000000000000000 R09: 0000000000000000

R10: 0000000000000000 R11: 0000000000000246 R12: 00007f9ba56c8060

R13: 0000000000b6ba60 R14: 0000000000cc7840 R15: 41d8685b1d7901b8

</TASK>

Fix the issue by using __mptcp_flush_join_list() instead of plain

mptcp_flush_join_list() inside __mptcp_push_pending(), as suggested by

Florian. The sockopt sync will be deferred to the workqueue.