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: 245500d853e9 ("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>

1 files changed, 1 insertions, 0 deletions

diff --git a/net/rxrpc/ar-internal.h b/net/rxrpc/ar-internal.h
index 1ad0ec5afb50..8499ceb7719c 100644
--- a/net/rxrpc/ar-internal.h
+++ b/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 */