rxrpc: Fix race between conn bundle lookup and bundle removal [ZDI-CAN-15975]

After rxrpc_unbundle_conn() has removed a connection from a bundle, it
checks to see if there are any conns with available channels and, if not,
removes and attempts to destroy the bundle.

Whilst it does check after grabbing client_bundles_lock that there are no
connections attached, this races with rxrpc_look_up_bundle() retrieving the
bundle, but not attaching a connection for the connection to be attached
later.

There is therefore a window in which the bundle can get destroyed before we
manage to attach a new connection to it.

Fix this by adding an "active" counter to struct rxrpc_bundle:

 (1) rxrpc_connect_call() obtains an active count by prepping/looking up a
     bundle and ditches it before returning.

 (2) If, during rxrpc_connect_call(), a connection is added to the bundle,
     this obtains an active count, which is held until the connection is
     discarded.

 (3) rxrpc_deactivate_bundle() is created to drop an active count on a
     bundle and destroy it when the active count reaches 0.  The active
     count is checked inside client_bundles_lock() to prevent a race with
     rxrpc_look_up_bundle().

 (4) rxrpc_unbundle_conn() then calls rxrpc_deactivate_bundle().

Fixes: 245500d8 ("rxrpc: Rewrite the client connection manager")
Reported-by: zdi-disclosures@trendmicro.com # ZDI-CAN-15975
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: zdi-disclosures@trendmicro.com
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Signed-off-by: David S. Miller <davem@davemloft.net>

net/rxrpc/ar-internal.h

@@ -399,6 +399,7 @@ enum rxrpc_conn_proto_state {
 struct rxrpc_bundle {
 	struct rxrpc_conn_parameters params;
 	refcount_t		ref;
+	atomic_t		active;		/* Number of active users */
 	unsigned int		debug_id;
 	bool			try_upgrade;	/* True if the bundle is attempting upgrade */
 	bool			alloc_conn;	/* True if someone's getting a conn */

net/rxrpc/conn_client.c

@@ -40,6 +40,8 @@ __read_mostly unsigned long rxrpc_conn_idle_client_fast_expiry = 2 * HZ;
 DEFINE_IDR(rxrpc_client_conn_ids);
 static DEFINE_SPINLOCK(rxrpc_conn_id_lock);
 
+static void rxrpc_deactivate_bundle(struct rxrpc_bundle *bundle);
+
 /*
  * Get a connection ID and epoch for a client connection from the global pool.
  * The connection struct pointer is then recorded in the idr radix tree.  The
@@ -123,6 +125,7 @@ static struct rxrpc_bundle *rxrpc_alloc_bundle(struct rxrpc_conn_parameters *cp,
 		bundle->params = *cp;
 		rxrpc_get_peer(bundle->params.peer);
 		refcount_set(&bundle->ref, 1);
+		atomic_set(&bundle->active, 1);
 		spin_lock_init(&bundle->channel_lock);
 		INIT_LIST_HEAD(&bundle->waiting_calls);
 	}
@@ -149,7 +152,7 @@ void rxrpc_put_bundle(struct rxrpc_bundle *bundle)
 
 	dead = __refcount_dec_and_test(&bundle->ref, &r);
 
-	_debug("PUT B=%x %d", d, r);
+	_debug("PUT B=%x %d", d, r - 1);
 	if (dead)
 		rxrpc_free_bundle(bundle);
 }
@@ -338,6 +341,7 @@ static struct rxrpc_bundle *rxrpc_look_up_bundle(struct rxrpc_conn_parameters *c
 	rxrpc_free_bundle(candidate);
 found_bundle:
 	rxrpc_get_bundle(bundle);
+	atomic_inc(&bundle->active);
 	spin_unlock(&local->client_bundles_lock);
 	_leave(" = %u [found]", bundle->debug_id);
 	return bundle;
@@ -435,6 +439,7 @@ static void rxrpc_add_conn_to_bundle(struct rxrpc_bundle *bundle, gfp_t gfp)
 			if (old)
 				trace_rxrpc_client(old, -1, rxrpc_client_replace);
 			candidate->bundle_shift = shift;
+			atomic_inc(&bundle->active);
 			bundle->conns[i] = candidate;
 			for (j = 0; j < RXRPC_MAXCALLS; j++)
 				set_bit(shift + j, &bundle->avail_chans);
@@ -725,6 +730,7 @@ int rxrpc_connect_call(struct rxrpc_sock *rx,
 	smp_rmb();
 
 out_put_bundle:
+	rxrpc_deactivate_bundle(bundle);
 	rxrpc_put_bundle(bundle);
 out:
 	_leave(" = %d", ret);
@@ -900,9 +906,8 @@ void rxrpc_disconnect_client_call(struct rxrpc_bundle *bundle, struct rxrpc_call
 static void rxrpc_unbundle_conn(struct rxrpc_connection *conn)
 {
 	struct rxrpc_bundle *bundle = conn->bundle;
-	struct rxrpc_local *local = bundle->params.local;
 	unsigned int bindex;
-	bool need_drop = false, need_put = false;
+	bool need_drop = false;
 	int i;
 
 	_enter("C=%x", conn->debug_id);
@@ -921,15 +926,22 @@ static void rxrpc_unbundle_conn(struct rxrpc_connection *conn)
 	}
 	spin_unlock(&bundle->channel_lock);
 
-	/* If there are no more connections, remove the bundle */
-	if (!bundle->avail_chans) {
-		_debug("maybe unbundle");
-		spin_lock(&local->client_bundles_lock);
+	if (need_drop) {
+		rxrpc_deactivate_bundle(bundle);
+		rxrpc_put_connection(conn);
+	}
+}
 
-		for (i = 0; i < ARRAY_SIZE(bundle->conns); i++)
-			if (bundle->conns[i])
-				break;
-		if (i == ARRAY_SIZE(bundle->conns) && !bundle->params.exclusive) {
+/*
+ * Drop the active count on a bundle.
+ */
+static void rxrpc_deactivate_bundle(struct rxrpc_bundle *bundle)
+{
+	struct rxrpc_local *local = bundle->params.local;
+	bool need_put = false;
+
+	if (atomic_dec_and_lock(&bundle->active, &local->client_bundles_lock)) {
+		if (!bundle->params.exclusive) {
 			_debug("erase bundle");
 			rb_erase(&bundle->local_node, &local->client_bundles);
 			need_put = true;
@@ -939,10 +951,6 @@ static void rxrpc_unbundle_conn(struct rxrpc_connection *conn)
 		if (need_put)
 			rxrpc_put_bundle(bundle);
 	}
-
-	if (need_drop)
-		rxrpc_put_connection(conn);
-	_leave("");
 }
 
 /*