tcp: add datapath logic for PSP with inline key exchange

Add validation points and state propagation to support PSP key
exchange inline, on TCP connections. The expectation is that
application will use some well established mechanism like TLS
handshake to establish a secure channel over the connection and
if both endpoints are PSP-capable - exchange and install PSP keys.
Because the connection can existing in PSP-unsecured and PSP-secured
state we need to make sure that there are no race conditions or
retransmission leaks.

On Tx - mark packets with the skb->decrypted bit when PSP key
is at the enqueue time. Drivers should only encrypt packets with
this bit set. This prevents retransmissions getting encrypted when
original transmission was not. Similarly to TLS, we'll use
sk->sk_validate_xmit_skb to make sure PSP skbs can't "escape"
via a PSP-unaware device without being encrypted.

On Rx - validation is done under socket lock. This moves the validation
point later than xfrm, for example. Please see the documentation patch
for more details on the flow of securing a connection, but for
the purpose of this patch what's important is that we want to
enforce the invariant that once connection is secured any skb
in the receive queue has been encrypted with PSP.

Add GRO and coalescing checks to prevent PSP authenticated data from
being combined with cleartext data, or data with non-matching PSP
state. On Rx, check skb's with psp_skb_coalesce_diff() at points
before psp_sk_rx_policy_check(). After skb's are policy checked and on
the socket receive queue, skb_cmp_decrypted() is sufficient for
checking for coalescable PSP state. On Tx, tcp_write_collapse_fence()
should be called when transitioning a socket into PSP Tx state to
prevent data sent as cleartext from being coalesced with PSP
encapsulated data.

This change only adds the validation points, for ease of review.
Subsequent change will add the ability to install keys, and flesh
the enforcement logic out

Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Co-developed-by: Daniel Zahka <daniel.zahka@gmail.com>
Signed-off-by: Daniel Zahka <daniel.zahka@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250917000954.859376-5-daniel.zahka@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

1 files changed, 13 insertions, 1 deletions

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 6a63be1f6461..f27f6f865a48 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -75,6 +75,7 @@
 #include <net/secure_seq.h>
 #include <net/busy_poll.h>
 #include <net/rstreason.h>
+#include <net/psp.h>
 
 #include <linux/inet.h>
 #include <linux/ipv6.h>
@@ -1907,6 +1908,10 @@ int tcp_v4_do_rcv(struct sock *sk, struct sk_buff *skb)
 	enum skb_drop_reason reason;
 	struct sock *rsk;
 
+	reason = psp_sk_rx_policy_check(sk, skb);
+	if (reason)
+		goto err_discard;
+
 	if (sk->sk_state == TCP_ESTABLISHED) { /* Fast path */
 		struct dst_entry *dst;
 
@@ -1968,6 +1973,7 @@ csum_err:
 	reason = SKB_DROP_REASON_TCP_CSUM;
 	trace_tcp_bad_csum(skb);
 	TCP_INC_STATS(sock_net(sk), TCP_MIB_CSUMERRORS);
+err_discard:
 	TCP_INC_STATS(sock_net(sk), TCP_MIB_INERRS);
 	goto discard;
 }
@@ -2069,7 +2075,9 @@ bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb,
 	     (TCPHDR_ECE | TCPHDR_CWR | TCPHDR_AE)) ||
 	    !tcp_skb_can_collapse_rx(tail, skb) ||
 	    thtail->doff != th->doff ||
-	    memcmp(thtail + 1, th + 1, hdrlen - sizeof(*th)))
+	    memcmp(thtail + 1, th + 1, hdrlen - sizeof(*th)) ||
+	    /* prior to PSP Rx policy check, retain exact PSP metadata */
+	    psp_skb_coalesce_diff(tail, skb))
 		goto no_coalesce;
 
 	__skb_pull(skb, hdrlen);
@@ -2437,6 +2445,10 @@ do_time_wait:
 			__this_cpu_write(tcp_tw_isn, isn);
 			goto process;
 		}
+
+		drop_reason = psp_twsk_rx_policy_check(inet_twsk(sk), skb);
+		if (drop_reason)
+			break;
 	}
 		/* to ACK */
 		fallthrough;