CVE-2025-40257: 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---