[PATCH LSM 1/2] security, lsm: Introduce security_mptcp_add_subflow()

[Matthieu Baerts]

From: Paolo Abeni <pabeni@xxxxxxxxxx>

MPTCP can create subflows in kernel context, and later indirectly
expose them to user-space, via the owning mptcp socket.

As discussed in the reported link, the above causes unexpected failures
for server, MPTCP-enabled applications.

Let's introduce a new LSM hook to allow the security module to relabel
the subflow according to the owing process.

Note that the new hook requires both the mptcp socket and the new
subflow. This could allow future extensions, e.g. explicitly validating
the mptcp <-> subflow linkage.

Link: https://lore.kernel.org/mptcp/CAHC9VhTNh-YwiyTds=P1e3rixEDqbRTFj22bpya=+qJqfcaMfg@xxxxxxxxxxxxxx/
Signed-off-by: Paolo Abeni <pabeni@xxxxxxxxxx>
Acked-by: Matthieu Baerts <matthieu.baerts@xxxxxxxxxxxx>
Signed-off-by: Matthieu Baerts <matthieu.baerts@xxxxxxxxxxxx>
---
 include/linux/lsm_hook_defs.h |  1 +
 include/linux/security.h      |  6 ++++++
 net/mptcp/subflow.c           |  6 ++++++
 security/security.c           | 15 +++++++++++++++
 4 files changed, 28 insertions(+)

diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index 6bb55e61e8e8..7308a1a7599b 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -343,6 +343,7 @@ LSM_HOOK(void, LSM_RET_VOID, sctp_sk_clone, struct sctp_association *asoc,
 	 struct sock *sk, struct sock *newsk)
 LSM_HOOK(int, 0, sctp_assoc_established, struct sctp_association *asoc,
 	 struct sk_buff *skb)
+LSM_HOOK(int, 0, mptcp_add_subflow, struct sock *sk, struct sock *ssk)
 #endif /* CONFIG_SECURITY_NETWORK */
 
 #ifdef CONFIG_SECURITY_INFINIBAND
diff --git a/include/linux/security.h b/include/linux/security.h
index cd23221ce9e6..80a0b37a9f26 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -1465,6 +1465,7 @@ void security_sctp_sk_clone(struct sctp_association *asoc, struct sock *sk,
 			    struct sock *newsk);
 int security_sctp_assoc_established(struct sctp_association *asoc,
 				    struct sk_buff *skb);
+int security_mptcp_add_subflow(struct sock *sk, struct sock *ssk);
 
 #else	/* CONFIG_SECURITY_NETWORK */
 static inline int security_unix_stream_connect(struct sock *sock,
@@ -1692,6 +1693,11 @@ static inline int security_sctp_assoc_established(struct sctp_association *asoc,
 {
 	return 0;
 }
+
+static inline int security_mptcp_add_subflow(struct sock *sk, struct sock *ssk)
+{
+	return 0;
+}
 #endif	/* CONFIG_SECURITY_NETWORK */
 
 #ifdef CONFIG_SECURITY_INFINIBAND
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 4ae1a7304cf0..d361749cabff 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -1692,6 +1692,10 @@ int mptcp_subflow_create_socket(struct sock *sk, unsigned short family,
 
 	lock_sock_nested(sf->sk, SINGLE_DEPTH_NESTING);
 
+	err = security_mptcp_add_subflow(sk, sf->sk);
+	if (err)
+		goto release_ssk;
+
 	/* the newly created socket has to be in the same cgroup as its parent */
 	mptcp_attach_cgroup(sk, sf->sk);
 
@@ -1704,6 +1708,8 @@ int mptcp_subflow_create_socket(struct sock *sk, unsigned short family,
 	get_net_track(net, &sf->sk->ns_tracker, GFP_KERNEL);
 	sock_inuse_add(net, 1);
 	err = tcp_set_ulp(sf->sk, "mptcp");
+
+release_ssk:
 	release_sock(sf->sk);
 
 	if (err) {
diff --git a/security/security.c b/security/security.c
index f4170efcddda..24cf2644a4b9 100644
--- a/security/security.c
+++ b/security/security.c
@@ -4667,6 +4667,21 @@ int security_sctp_assoc_established(struct sctp_association *asoc,
 }
 EXPORT_SYMBOL(security_sctp_assoc_established);
 
+/**
+ * security_mptcp_add_subflow() - Inherit the LSM label from the MPTCP socket
+ * @sk: the owning MPTCP socket
+ * @ssk: the new subflow
+ *
+ * Update the labeling for the given MPTCP subflow, to match the one of the
+ * owning MPTCP socket.
+ *
+ * Return: Returns 0 on success or a negative error code on failure.
+ */
+int security_mptcp_add_subflow(struct sock *sk, struct sock *ssk)
+{
+	return call_int_hook(mptcp_add_subflow, 0, sk, ssk);
+}
+
 #endif	/* CONFIG_SECURITY_NETWORK */
 
 #ifdef CONFIG_SECURITY_INFINIBAND

-- 
2.39.2