mptcp: init: protect sched with rcu_read_lock

JIRA: https://issues.redhat.com/browse/RHEL-68179
CVE: CVE-2024-53047

commit 3deb12c
Author: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Date:   Mon Oct 21 12:25:26 2024 +0200

    mptcp: init: protect sched with rcu_read_lock

    Enabling CONFIG_PROVE_RCU_LIST with its dependence CONFIG_RCU_EXPERT
    creates this splat when an MPTCP socket is created:

      =============================
      WARNING: suspicious RCU usage
      6.12.0-rc2+ #11 Not tainted
      -----------------------------
      net/mptcp/sched.c:44 RCU-list traversed in non-reader section!!

      other info that might help us debug this:

      rcu_scheduler_active = 2, debug_locks = 1
      no locks held by mptcp_connect/176.

      stack backtrace:
      CPU: 0 UID: 0 PID: 176 Comm: mptcp_connect Not tainted 6.12.0-rc2+ #11
      Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
      Call Trace:
       <TASK>
       dump_stack_lvl (lib/dump_stack.c:123)
       lockdep_rcu_suspicious (kernel/locking/lockdep.c:6822)
       mptcp_sched_find (net/mptcp/sched.c:44 (discriminator 7))
       mptcp_init_sock (net/mptcp/protocol.c:2867 (discriminator 1))
       ? sock_init_data_uid (arch/x86/include/asm/atomic.h:28)
       inet_create.part.0.constprop.0 (net/ipv4/af_inet.c:386)
       ? __sock_create (include/linux/rcupdate.h:347 (discriminator 1))
       __sock_create (net/socket.c:1576)
       __sys_socket (net/socket.c:1671)
       ? __pfx___sys_socket (net/socket.c:1712)
       ? do_user_addr_fault (arch/x86/mm/fault.c:1419 (discriminator 1))
       __x64_sys_socket (net/socket.c:1728)
       do_syscall_64 (arch/x86/entry/common.c:52 (discriminator 1))
       entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)

    That's because when the socket is initialised, rcu_read_lock() is not
    used despite the explicit comment written above the declaration of
    mptcp_sched_find() in sched.c. Adding the missing lock/unlock avoids the
    warning.

    Fixes: 1730b2b ("mptcp: add sched in mptcp_sock")
    Cc: stable@vger.kernel.org
    Closes: multipath-tcp/mptcp_net-next#523
    Reviewed-by: Geliang Tang <geliang@kernel.org>
    Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
    Reviewed-by: Simon Horman <horms@kernel.org>
    Link: https://patch.msgid.link/20241021-net-mptcp-sched-lock-v1-1-637759cf061c@kernel.org
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Signed-off-by: CKI Backport Bot <cki-ci-bot+cki-gitlab-backport-bot@redhat.com>

Author: CKI Backport Bot

net/mptcp/protocol.c

@@ -2845,8 +2845,10 @@ static int mptcp_init_sock(struct sock *sk)
 	if (unlikely(!net->mib.mptcp_statistics) && !mptcp_mib_alloc(net))
 		return -ENOMEM;
 
+	rcu_read_lock();
 	ret = mptcp_init_sched(mptcp_sk(sk),
 			       mptcp_sched_find(mptcp_get_scheduler(net)));
+	rcu_read_unlock();
 	if (ret)
 		return ret;