netdev - [PATCH net 1/3] mptcp: init: protect sched with rcu_read

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Message-Id: <20241021-net-mptcp-sched-lock-v1-1-637759cf061c@kernel.org>
Date: Mon, 21 Oct 2024 12:25:26 +0200
From: "Matthieu Baerts (NGI0)" <matttbe@...nel.org>
To: mptcp@...ts.linux.dev, Mat Martineau <martineau@...nel.org>, 
 Geliang Tang <geliang@...nel.org>, "David S. Miller" <davem@...emloft.net>, 
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
 Gregory Detal <gregory.detal@...il.com>, Shuah Khan <shuah@...nel.org>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
 linux-kselftest@...r.kernel.org, 
 "Matthieu Baerts (NGI0)" <matttbe@...nel.org>, stable@...r.kernel.org
Subject: [PATCH net 1/3] 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: 1730b2b2c5a5 ("mptcp: add sched in mptcp_sock")
Cc: stable@...r.kernel.org
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/523
Reviewed-by: Geliang Tang <geliang@...nel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@...nel.org>
---
 net/mptcp/protocol.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 6d0e201c3eb26aa6ca0ff27e5a65cb6f911012f2..d263091659e076587bc3406dfdcb4409adb3247e 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -2864,8 +2864,10 @@ 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;

-- 
2.45.2