[MPTCP] [PATCH net-next 2/2] mptcp: mptcp_close: avoid a lockdep splat when mcast group was joined

[MPTCP] [PATCH net-next 2/2] mptcp: mptcp_close: avoid a lockdep splat when mcast group was joined

[Florian Westphal]

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syzbot triggered following lockdep splat:

ffffffff82d2cd40 (rtnl_mutex){+.+.}, at: ip_mc_drop_socket+0x52/0x180
but task is already holding lock:
ffff8881187a2310 (sk_lock-AF_INET){+.+.}, at: mptcp_close+0x18/0x30

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:
-> #1 (sk_lock-AF_INET){+.+.}:
       lock_acquire+0xee/0x230
       lock_sock_nested+0x89/0xc0
       do_ip_setsockopt.isra.0+0x335/0x22f0
       ip_setsockopt+0x35/0x60
       tcp_setsockopt+0x5d/0x90
       __sys_setsockopt+0xf3/0x190
       __x64_sys_setsockopt+0x61/0x70
       do_syscall_64+0x72/0x300
       entry_SYSCALL_64_after_hwframe+0x49/0xbe
-> #0 (rtnl_mutex){+.+.}:
       check_prevs_add+0x2b7/0x1210
       __lock_acquire+0x10b6/0x1400
       lock_acquire+0xee/0x230
       __mutex_lock+0x120/0xc70
       ip_mc_drop_socket+0x52/0x180
       inet_release+0x36/0xe0
       __sock_release+0xfd/0x130
       __mptcp_close+0xa8/0x1f0
       inet_release+0x7f/0xe0
       __sock_release+0x69/0x130
       sock_close+0x18/0x20
       __fput+0x179/0x400
       task_work_run+0xd5/0x110
       do_exit+0x685/0x1510
       do_group_exit+0x7e/0x170
       __x64_sys_exit_group+0x28/0x30
       do_syscall_64+0x72/0x300
       entry_SYSCALL_64_after_hwframe+0x49/0xbe

The trigger is:
  socket(AF_INET, SOCK_STREAM, 0x106 /* IPPROTO_MPTCP */) = 4
  setsockopt(4, SOL_IP, MCAST_JOIN_GROUP, {gr_interface=7, gr_group={sa_family=AF_INET, sin_port=htons(20003), sin_addr=inet_addr("224.0.0.2")}}, 136) = 0
  exit(0)

Which results in a call to rtnl_lock while we are holding
the parent mptcp socket lock via
mptcp_close -> lock_sock(msk) -> inet_release -> ip_mc_drop_socket -> rtnl_lock().

From lockdep point of view we thus have both
'rtnl_lock; lock_sock' and 'lock_sock; rtnl_lock'.

Fix this by stealing the msk conn_list and doing the subflow close
without holding the msk lock.

Fixes: cec37a6e41aae7bf ("mptcp: Handle MP_CAPABLE options for outgoing connections")
Reported-by: Christoph Paasch <cpaasch(a)apple.com>
Signed-off-by: Florian Westphal <fw(a)strlen.de>
---
 net/mptcp/protocol.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index f8232e9f08e7..120b3305a594 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -644,11 +644,16 @@ static void __mptcp_close(struct sock *sk, long timeout)
 {
 	struct mptcp_subflow_context *subflow, *tmp;
 	struct mptcp_sock *msk = mptcp_sk(sk);
+	LIST_HEAD(conn_list);
 
 	mptcp_token_destroy(msk->token);
 	inet_sk_state_store(sk, TCP_CLOSE);
 
-	list_for_each_entry_safe(subflow, tmp, &msk->conn_list, node) {
+	list_splice_init(&msk->conn_list, &conn_list);
+
+	release_sock(sk);
+
+	list_for_each_entry_safe(subflow, tmp, &conn_list, node) {
 		struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
 
 		__mptcp_close_ssk(sk, ssk, subflow, timeout);
@@ -656,7 +661,7 @@ static void __mptcp_close(struct sock *sk, long timeout)
 
 	if (msk->cached_ext)
 		__skb_ext_put(msk->cached_ext);
-	release_sock(sk);
+
 	sk_common_release(sk);
 }
 
-- 
2.24.1