[PATCH net-next 2/4] net: psp: require an established connection for association setup

[Daniel Zahka <daniel.zahka@gmail.com>]

Return -ENOTCONN if sk_state is not TCP_ESTABLISHED when read under the
socket lock in the rx and tx assoc paths.

Nothing useful can be done after association setup on closed or listen
sockets today. Listen sockets could accept a PSP encrypted TCP SYN, but
the child socket will not inherit any PSP state. On the other side,
establishing PSP state prior to connect() will result in a PSP encrypted
TCP SYN sent to a listening peer that will in turn have the
aforementioned limitations. That implies that there cannot be any users
of this feature, and thus it should be safe to remove doing so as a
feature of the PSP uapi.

These can be reintroduced when there is a use case and a design that
takes into account all of the socket states that are reachable by
relaxing this constraint.

In theory, the check in the tx-assoc path is more restrictive than
necessary. FIN_WAIT1/2, CLOSING, LAST_ACK, CLOSE_WAIT could be allowed
and the peer would accept PSP encrypted ACKs in the post FIN sent
states, or data in the half close case, but for now we can just document
that connection upgrade protocol should avoid these.

The check in the tx-assoc path fixes a bug in commit 6b46ca260e22 ("net:
psp: add socket security association code") where an unsynchronized
write can be performed an assoc shared with a timewait socket when the
socket is in TCP_CLOSE after shutdown. This commit is not included in
net, because its premise of preventing listen sockets from holding assoc
state depends on the net-next commit 8cc3aef0cb19 ("tcp: Do not allow
buggy transitions between ehash and lhash2.")

Signed-off-by: Daniel Zahka <daniel.zahka@gmail.com>
---
 Documentation/networking/psp.rst | 13 +++++++++++++
 net/psp/psp_sock.c               | 12 ++++++++++++
 2 files changed, 25 insertions(+)

diff --git a/Documentation/networking/psp.rst b/Documentation/networking/psp.rst
index 4ac09e64e95a..2aa971e16d95 100644
--- a/Documentation/networking/psp.rst
+++ b/Documentation/networking/psp.rst
@@ -132,6 +132,19 @@ numbers in a way that deletes a prefix of the PSP protected part of
 the TCP stream. If userspace cares to mitigate this type of attack, a
 special "start of PSP" message should be exchanged after ``tx-assoc``.
 
+Upgrade to PSP must be done on established TCP connections.
+``rx-assoc`` and ``tx-assoc`` will return ``-ENOTCONN`` if
+``sk_state`` is not ``TCP_ESTABLISHED``.
+
+Disconnecting a socket with PSP assoc state (``connect()`` with a
+family of ``AF_UNSPEC``) will succeed, but should be considered
+unsupported. Disconnect does not reset the PSP assoc state of a
+socket to avoid potential for clear text leak. Disconnect on a socket
+after ``rx-assoc`` will leave a socket that can be reconnected, but
+with potentially stale PSP assoc state present and a reduced MSS.
+Disconnect after ``tx-assoc`` will likely result in a dead socket, as
+the subsequent ``connect()`` will send a PSP encapsulated SYN.
+
 Rotation notifications
 ----------------------
 
diff --git a/net/psp/psp_sock.c b/net/psp/psp_sock.c
index a9cfeebe4ba1..a6b1c42dd626 100644
--- a/net/psp/psp_sock.c
+++ b/net/psp/psp_sock.c
@@ -159,6 +159,12 @@ int psp_sock_assoc_set_rx(struct sock *sk, struct psp_assoc *pas,
 
 	lock_sock(sk);
 
+	if (sk->sk_state != TCP_ESTABLISHED) {
+		NL_SET_ERR_MSG(extack, "Socket must be in established state");
+		err = -ENOTCONN;
+		goto exit_unlock;
+	}
+
 	if (psp_sk_assoc(sk)) {
 		NL_SET_ERR_MSG(extack, "Socket already has PSP state");
 		err = -EBUSY;
@@ -252,6 +258,12 @@ int psp_sock_assoc_set_tx(struct sock *sk, struct psp_dev *psd,
 
 	lock_sock(sk);
 
+	if (sk->sk_state != TCP_ESTABLISHED) {
+		NL_SET_ERR_MSG(extack, "Socket must be in established state");
+		err = -ENOTCONN;
+		goto exit_unlock;
+	}
+
 	pas = psp_sk_assoc(sk);
 	if (!pas) {
 		NL_SET_ERR_MSG(extack, "Socket has no Rx key");

-- 
2.52.0