mptcp: fix a race in mptcp_pm_del_add_timer()

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

mptcp: fix a race in mptcp_pm_del_add_timer()

mptcp_pm_del_add_timer() can call sk_stop_timer_sync(sk, &entry->add_timer)
while another might have free entry already, as reported by syzbot.

Add RCU protection to fix this issue.

Also change confusing add_timer variable with stop_timer boolean.

syzbot report:

BUG: KASAN: slab-use-after-free in __timer_delete_sync+0x372/0x3f0 kernel/time/timer.c:1616
Read of size 4 at addr ffff8880311e4150 by task kworker/1:1/44

CPU: 1 UID: 0 PID: 44 Comm: kworker/1:1 Not tainted syzkaller #0 PREEMPT_{RT,(full)}
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025
Workqueue: events mptcp_worker
Call Trace:
<TASK>
dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
print_address_description mm/kasan/report.c:378 [inline]
print_report+0xca/0x240 mm/kasan/report.c:482
kasan_report+0x118/0x150 mm/kasan/report.c:595
__timer_delete_sync+0x372/0x3f0 kernel/time/timer.c:1616
sk_stop_timer_sync+0x1b/0x90 net/core/sock.c:3631
mptcp_pm_del_add_timer+0x283/0x310 net/mptcp/pm.c:362
mptcp_incoming_options+0x1357/0x1f60 net/mptcp/options.c:1174
tcp_data_queue+0xca/0x6450 net/ipv4/tcp_input.c:5361
tcp_rcv_established+0x1335/0x2670 net/ipv4/tcp_input.c:6441
tcp_v4_do_rcv+0x98b/0xbf0 net/ipv4/tcp_ipv4.c:1931
tcp_v4_rcv+0x252a/0x2dc0 net/ipv4/tcp_ipv4.c:2374
ip_protocol_deliver_rcu+0x221/0x440 net/ipv4/ip_input.c:205
ip_local_deliver_finish+0x3bb/0x6f0 net/ipv4/ip_input.c:239
NF_HOOK+0x30c/0x3a0 include/linux/netfilter.h:318
NF_HOOK+0x30c/0x3a0 include/linux/netfilter.h:318
__netif_receive_skb_one_core net/core/dev.c:6079 [inline]
__netif_receive_skb+0x143/0x380 net/core/dev.c:6192
process_backlog+0x31e/0x900 net/core/dev.c:6544
__napi_poll+0xb6/0x540 net/core/dev.c:7594
napi_poll net/core/dev.c:7657 [inline]
net_rx_action+0x5f7/0xda0 net/core/dev.c:7784
handle_softirqs+0x22f/0x710 kernel/softirq.c:622
__do_softirq kernel/softirq.c:656 [inline]
__local_bh_enable_ip+0x1a0/0x2e0 kernel/softirq.c:302
mptcp_pm_send_ack net/mptcp/pm.c:210 [inline]
mptcp_pm_addr_send_ack+0x41f/0x500 net/mptcp/pm.c:-1
mptcp_pm_worker+0x174/0x320 net/mptcp/pm.c:1002
mptcp_worker+0xd5/0x1170 net/mptcp/protocol.c:2762
process_one_work kernel/workqueue.c:3263 [inline]
process_scheduled_works+0xae1/0x17b0 kernel/workqueue.c:3346
worker_thread+0x8a0/0xda0 kernel/workqueue.c:3427
kthread+0x711/0x8a0 kernel/kthread.c:463
ret_from_fork+0x4bc/0x870 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
</TASK>

Allocated by task 44:
kasan_save_stack mm/kasan/common.c:56 [inline]
kasan_save_track+0x3e/0x80 mm/kasan/common.c:77
poison_kmalloc_redzone mm/kasan/common.c:400 [inline]
__kasan_kmalloc+0x93/0xb0 mm/kasan/common.c:417
kasan_kmalloc include/linux/kasan.h:262 [inline]
__kmalloc_cache_noprof+0x1ef/0x6c0 mm/slub.c:5748
kmalloc_noprof include/linux/slab.h:957 [inline]
mptcp_pm_alloc_anno_list+0x104/0x460 net/mptcp/pm.c:385
mptcp_pm_create_subflow_or_signal_addr+0xf9d/0x1360 net/mptcp/pm_kernel.c:355
mptcp_pm_nl_fully_established net/mptcp/pm_kernel.c:409 [inline]
__mptcp_pm_kernel_worker+0x417/0x1ef0 net/mptcp/pm_kernel.c:1529
mptcp_pm_worker+0x1ee/0x320 net/mptcp/pm.c:1008
mptcp_worker+0xd5/0x1170 net/mptcp/protocol.c:2762
process_one_work kernel/workqueue.c:3263 [inline]
process_scheduled_works+0xae1/0x17b0 kernel/workqueue.c:3346
worker_thread+0x8a0/0xda0 kernel/workqueue.c:3427
kthread+0x711/0x8a0 kernel/kthread.c:463
ret_from_fork+0x4bc/0x870 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

Freed by task 6630:
kasan_save_stack mm/kasan/common.c:56 [inline]
kasan_save_track+0x3e/0x80 mm/kasan/common.c:77
__kasan_save_free_info+0x46/0x50 mm/kasan/generic.c:587
kasan_save_free_info mm/kasan/kasan.h:406 [inline]
poison_slab_object m
---truncated---