rxrpc: rxgk: Implement connection rekeying

Implement rekeying of connections with the RxGK security class.  This
involves regenerating the keys with a different key number as part of the
input data after a certain amount of time or a certain amount of bytes
encrypted.  Rekeying may be triggered by either end.

The LSW of the key number is inserted into the security-specific field in
the RX header, and we try and expand it to 32-bits to make it last longer.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Herbert Xu <herbert@gondor.apana.org.au>
cc: Chuck Lever <chuck.lever@oracle.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
Link: https://patch.msgid.link/20250411095303.2316168-10-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/trace/events/rxrpc.h

@@ -2725,6 +2725,30 @@ TRACE_EVENT(rxrpc_rack_timer,
 		      ktime_to_us(__entry->delay))
 	    );
 
+TRACE_EVENT(rxrpc_rxgk_rekey,
+	    TP_PROTO(struct rxrpc_connection *conn,
+		     unsigned int current_key, unsigned int requested_key),
+
+	    TP_ARGS(conn, current_key, requested_key),
+
+	    TP_STRUCT__entry(
+		    __field(unsigned int,	conn)
+		    __field(unsigned int,	current_key)
+		    __field(unsigned int,	requested_key)
+			     ),
+
+	    TP_fast_assign(
+		    __entry->conn		= conn->debug_id;
+		    __entry->current_key	= current_key;
+		    __entry->requested_key	= requested_key;
+			   ),
+
+	    TP_printk("C=%08x cur=%x req=%x",
+		      __entry->conn,
+		      __entry->current_key,
+		      __entry->requested_key)
+	    );
+
 #undef EM
 #undef E_
 

net/rxrpc/ar-internal.h

@@ -565,13 +565,16 @@ struct rxrpc_connection {
 			u32	nonce;		/* response re-use preventer */
 		} rxkad;
 		struct {
-			struct rxgk_context *keys[1];
+			struct rxgk_context *keys[4]; /* (Re-)keying buffer */
 			u64	start_time;	/* The start time for TK derivation */
 			u8	nonce[20];	/* Response re-use preventer */
 			u32	enctype;	/* Kerberos 5 encoding type */
+			u32	key_number;	/* Current key number */
 		} rxgk;
 	};
+	rwlock_t		security_use_lock; /* Security use/modification lock */
 	struct sk_buff		*tx_response;	/* Response packet to be transmitted */
+
 	unsigned long		flags;
 	unsigned long		events;
 	unsigned long		idle_timestamp;	/* Time at which last became idle */

net/rxrpc/conn_object.c

@@ -73,6 +73,7 @@ struct rxrpc_connection *rxrpc_alloc_connection(struct rxrpc_net *rxnet,
 		skb_queue_head_init(&conn->rx_queue);
 		conn->rxnet = rxnet;
 		conn->security = &rxrpc_no_security;
+		rwlock_init(&conn->security_use_lock);
 		spin_lock_init(&conn->state_lock);
 		conn->debug_id = atomic_inc_return(&rxrpc_debug_id);
 		conn->idle_timestamp = jiffies;

net/rxrpc/rxgk.c

@@ -76,11 +76,155 @@ static void rxgk_describe_server_key(const struct key *key, struct seq_file *m)
 		seq_printf(m, ": %s", krb5->name);
 }
 
-static struct rxgk_context *rxgk_get_key(struct rxrpc_connection *conn,
+/*
-					 u16 *specific_key_number)
+ * Handle rekeying the connection when we see our limits overrun or when the
+ * far side decided to rekey.
+ *
+ * Returns a ref on the context if successful or -ESTALE if the key is out of
+ * date.
+ */
+static struct rxgk_context *rxgk_rekey(struct rxrpc_connection *conn,
+				       const u16 *specific_key_number)
 {
-	refcount_inc(&conn->rxgk.keys[0]->usage);
+	struct rxgk_context *gk, *dead = NULL;
-	return conn->rxgk.keys[0];
+	unsigned int key_number, current_key, mask = ARRAY_SIZE(conn->rxgk.keys) - 1;
+	bool crank = false;
+
+	_enter("%d", specific_key_number ? *specific_key_number : -1);
+
+	mutex_lock(&conn->security_lock);
+
+	current_key = conn->rxgk.key_number;
+	if (!specific_key_number) {
+		key_number = current_key;
+	} else {
+		if (*specific_key_number == (u16)current_key)
+			key_number = current_key;
+		else if (*specific_key_number == (u16)(current_key - 1))
+			key_number = current_key - 1;
+		else if (*specific_key_number == (u16)(current_key + 1))
+			goto crank_window;
+		else
+			goto bad_key;
+	}
+
+	gk = conn->rxgk.keys[key_number & mask];
+	if (!gk)
+		goto generate_key;
+	if (!specific_key_number &&
+	    test_bit(RXGK_TK_NEEDS_REKEY, &gk->flags))
+		goto crank_window;
+
+grab:
+	refcount_inc(&gk->usage);
+	mutex_unlock(&conn->security_lock);
+	rxgk_put(dead);
+	return gk;
+
+crank_window:
+	trace_rxrpc_rxgk_rekey(conn, current_key,
+			       specific_key_number ? *specific_key_number : -1);
+	if (current_key == UINT_MAX)
+		goto bad_key;
+	if (current_key + 1 == UINT_MAX)
+		set_bit(RXRPC_CONN_DONT_REUSE, &conn->flags);
+
+	key_number = current_key + 1;
+	if (WARN_ON(conn->rxgk.keys[key_number & mask]))
+		goto bad_key;
+	crank = true;
+
+generate_key:
+	gk = conn->rxgk.keys[current_key & mask];
+	gk = rxgk_generate_transport_key(conn, gk->key, key_number, GFP_NOFS);
+	if (IS_ERR(gk)) {
+		mutex_unlock(&conn->security_lock);
+		return gk;
+	}
+
+	write_lock(&conn->security_use_lock);
+	if (crank) {
+		current_key++;
+		conn->rxgk.key_number = current_key;
+		dead = conn->rxgk.keys[(current_key - 2) & mask];
+		conn->rxgk.keys[(current_key - 2) & mask] = NULL;
+	}
+	conn->rxgk.keys[current_key & mask] = gk;
+	write_unlock(&conn->security_use_lock);
+	goto grab;
+
+bad_key:
+	mutex_unlock(&conn->security_lock);
+	return ERR_PTR(-ESTALE);
+}
+
+/*
+ * Get the specified keying context.
+ *
+ * Returns a ref on the context if successful or -ESTALE if the key is out of
+ * date.
+ */
+static struct rxgk_context *rxgk_get_key(struct rxrpc_connection *conn,
+					 const u16 *specific_key_number)
+{
+	struct rxgk_context *gk;
+	unsigned int key_number, current_key, mask = ARRAY_SIZE(conn->rxgk.keys) - 1;
+
+	_enter("{%u},%d",
+	       conn->rxgk.key_number, specific_key_number ? *specific_key_number : -1);
+
+	read_lock(&conn->security_use_lock);
+
+	current_key = conn->rxgk.key_number;
+	if (!specific_key_number) {
+		key_number = current_key;
+	} else {
+		/* Only the bottom 16 bits of the key number are exposed in the
+		 * header, so we try and keep the upper 16 bits in step.  The
+		 * whole 32 bits are used to generate the TK.
+		 */
+		if (*specific_key_number == (u16)current_key)
+			key_number = current_key;
+		else if (*specific_key_number == (u16)(current_key - 1))
+			key_number = current_key - 1;
+		else if (*specific_key_number == (u16)(current_key + 1))
+			goto rekey;
+		else
+			goto bad_key;
+	}
+
+	gk = conn->rxgk.keys[key_number & mask];
+	if (!gk)
+		goto slow_path;
+	if (!specific_key_number &&
+	    key_number < UINT_MAX) {
+		if (time_after(jiffies, gk->expiry) ||
+		    gk->bytes_remaining < 0) {
+			set_bit(RXGK_TK_NEEDS_REKEY, &gk->flags);
+			goto slow_path;
+		}
+
+		if (test_bit(RXGK_TK_NEEDS_REKEY, &gk->flags))
+			goto slow_path;
+	}
+
+	refcount_inc(&gk->usage);
+	read_unlock(&conn->security_use_lock);
+	return gk;
+
+rekey:
+	_debug("rekey");
+	if (current_key == UINT_MAX)
+		goto bad_key;
+	gk = conn->rxgk.keys[current_key & mask];
+	if (gk)
+		set_bit(RXGK_TK_NEEDS_REKEY, &gk->flags);
+slow_path:
+	read_unlock(&conn->security_use_lock);
+	return rxgk_rekey(conn, specific_key_number);
+bad_key:
+	read_unlock(&conn->security_use_lock);
+	return ERR_PTR(-ESTALE);
 }
 
 /*
@@ -92,7 +236,8 @@ static int rxgk_init_connection_security(struct rxrpc_connection *conn,
 	struct rxgk_context *gk;
 	int ret;
 
-	_enter("{%d},{%x}", conn->debug_id, key_serial(conn->key));
+	_enter("{%d,%u},{%x}",
+	       conn->debug_id, conn->rxgk.key_number, key_serial(conn->key));
 
 	conn->security_ix = token->security_index;
 	conn->security_level = token->rxgk->level;
@@ -102,11 +247,12 @@ static int rxgk_init_connection_security(struct rxrpc_connection *conn,
 		do_div(conn->rxgk.start_time, 100);
 	}
 
-	gk = rxgk_generate_transport_key(conn, token->rxgk, 0, GFP_NOFS);
+	gk = rxgk_generate_transport_key(conn, token->rxgk, conn->rxgk.key_number,
+					 GFP_NOFS);
 	if (IS_ERR(gk))
 		return PTR_ERR(gk);
 	conn->rxgk.enctype = gk->krb5->etype;
-	conn->rxgk.keys[0] = gk;
+	conn->rxgk.keys[gk->key_number & 3] = gk;
 
 	switch (conn->security_level) {
 	case RXRPC_SECURITY_PLAIN: