[PATCH 5.10.y 1/3] mptcp: fix NULL pointer in can_accept_new_subflow

["Matthieu Baerts (NGI0)" <matttbe@kernel.org>]

From: Gang Yan <yangang@kylinos.cn>

commit 443041deb5ef6a1289a99ed95015ec7442f141dc upstream.

When testing valkey benchmark tool with MPTCP, the kernel panics in
'mptcp_can_accept_new_subflow' because subflow_req->msk is NULL.

Call trace:

  mptcp_can_accept_new_subflow (./net/mptcp/subflow.c:63 (discriminator 4)) (P)
  subflow_syn_recv_sock (./net/mptcp/subflow.c:854)
  tcp_check_req (./net/ipv4/tcp_minisocks.c:863)
  tcp_v4_rcv (./net/ipv4/tcp_ipv4.c:2268)
  ip_protocol_deliver_rcu (./net/ipv4/ip_input.c:207)
  ip_local_deliver_finish (./net/ipv4/ip_input.c:234)
  ip_local_deliver (./net/ipv4/ip_input.c:254)
  ip_rcv_finish (./net/ipv4/ip_input.c:449)
  ...

According to the debug log, the same req received two SYN-ACK in a very
short time, very likely because the client retransmits the syn ack due
to multiple reasons.

Even if the packets are transmitted with a relevant time interval, they
can be processed by the server on different CPUs concurrently). The
'subflow_req->msk' ownership is transferred to the subflow the first,
and there will be a risk of a null pointer dereference here.

This patch fixes this issue by moving the 'subflow_req->msk' under the
`own_req == true` conditional.

Note that the !msk check in subflow_hmac_valid() can be dropped, because
the same check already exists under the own_req mpj branch where the
code has been moved to.

Fixes: 9466a1ccebbe ("mptcp: enable JOIN requests even if cookies are in use")
Cc: stable@vger.kernel.org
Suggested-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Gang Yan <yangang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250328-net-mptcp-misc-fixes-6-15-v1-1-34161a482a7f@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[ Conflict in subflow.c because commit 74c7dfbee3e1 ("mptcp: consolidate
  in_opt sub-options fields in a bitmask") is not in this version. The
  conflict is in the context, and the modification can still be applied.
  Note that subflow_add_reset_reason() is not needed here, because the
  related feature is not supported in this version. ]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 net/mptcp/subflow.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index c3434069fb0a..5403292d4473 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -454,8 +454,6 @@ static bool subflow_hmac_valid(const struct request_sock *req,
 
 	subflow_req = mptcp_subflow_rsk(req);
 	msk = subflow_req->msk;
-	if (!msk)
-		return false;
 
 	subflow_generate_hmac(msk->remote_key, msk->local_key,
 			      subflow_req->remote_nonce,
@@ -578,11 +576,8 @@ static struct sock *subflow_syn_recv_sock(const struct sock *sk,
 			fallback = true;
 	} else if (subflow_req->mp_join) {
 		mptcp_get_options(skb, &mp_opt);
-		if (!mp_opt.mp_join || !subflow_hmac_valid(req, &mp_opt) ||
-		    !mptcp_can_accept_new_subflow(subflow_req->msk)) {
-			SUBFLOW_REQ_INC_STATS(req, MPTCP_MIB_JOINACKMAC);
+		if (!mp_opt.mp_join)
 			fallback = true;
-		}
 	}
 
 create_child:
@@ -636,6 +631,12 @@ static struct sock *subflow_syn_recv_sock(const struct sock *sk,
 			if (!owner)
 				goto dispose_child;
 
+			if (!subflow_hmac_valid(req, &mp_opt) ||
+			    !mptcp_can_accept_new_subflow(subflow_req->msk)) {
+				SUBFLOW_REQ_INC_STATS(req, MPTCP_MIB_JOINACKMAC);
+				goto dispose_child;
+			}
+
 			/* move the msk reference ownership to the subflow */
 			subflow_req->msk = NULL;
 			ctx->conn = (struct sock *)owner;
-- 
2.48.1