rxrpc: Fix incoming call setup race

An incoming call can race with rxrpc socket destruction, leading to a
leaked call.  This may result in an oops when the call timer eventually
expires:

   BUG: kernel NULL pointer dereference, address: 0000000000000874
   RIP: 0010:_raw_spin_lock_irqsave+0x2a/0x50
   Call Trace:
    <IRQ>
    try_to_wake_up+0x59/0x550
    ? __local_bh_enable_ip+0x37/0x80
    ? rxrpc_poke_call+0x52/0x110 [rxrpc]
    ? rxrpc_poke_call+0x110/0x110 [rxrpc]
    ? rxrpc_poke_call+0x110/0x110 [rxrpc]
    call_timer_fn+0x24/0x120

with a warning in the kernel log looking something like:

   rxrpc: Call 00000000ba5e571a still in use (1,SvAwtACK,1061d,0)!

incurred during rmmod of rxrpc.  The 1061d is the call flags:

   RECVMSG_READ_ALL, RX_HEARD, BEGAN_RX_TIMER, RX_LAST, EXPOSED,
   IS_SERVICE, RELEASED

but no DISCONNECTED flag (0x800), so it's an incoming (service) call and
it's still connected.

The race appears to be that:

 (1) rxrpc_new_incoming_call() consults the service struct, checks sk_state
     and allocates a call - then pauses, possibly for an interrupt.

 (2) rxrpc_release_sock() sets RXRPC_CLOSE, nulls the service pointer,
     discards the prealloc and releases all calls attached to the socket.

 (3) rxrpc_new_incoming_call() resumes, launching the new call, including
     its timer and attaching it to the socket.

Fix this by read-locking local->services_lock to access the AF_RXRPC socket
providing the service rather than RCU in rxrpc_new_incoming_call().
There's no real need to use RCU here as local->services_lock is only
write-locked by the socket side in two places: when binding and when
shutting down.

Fixes: 5e6ef4f1017c ("rxrpc: Make the I/O thread take over the call and local processor work")
Reported-by: Marc Dionne <marc.dionne@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-afs@lists.infradead.org

1 files changed, 4 insertions, 4 deletions

diff --git a/net/rxrpc/af_rxrpc.c b/net/rxrpc/af_rxrpc.c
index cf200e4e0eae..ebbd4a1c3f86 100644
--- a/net/rxrpc/af_rxrpc.c
+++ b/net/rxrpc/af_rxrpc.c
@@ -155,10 +155,10 @@ static int rxrpc_bind(struct socket *sock, struct sockaddr *saddr, int len)
 
 		if (service_id) {
 			write_lock(&local->services_lock);
-			if (rcu_access_pointer(local->service))
+			if (local->service)
 				goto service_in_use;
 			rx->local = local;
-			rcu_assign_pointer(local->service, rx);
+			local->service = rx;
 			write_unlock(&local->services_lock);
 
 			rx->sk.sk_state = RXRPC_SERVER_BOUND;
@@ -875,9 +875,9 @@ static int rxrpc_release_sock(struct sock *sk)
 
 	sk->sk_state = RXRPC_CLOSE;
 
-	if (rx->local && rcu_access_pointer(rx->local->service) == rx) {
+	if (rx->local && rx->local->service == rx) {
 		write_lock(&rx->local->services_lock);
-		rcu_assign_pointer(rx->local->service, NULL);
+		rx->local->service = NULL;
 		write_unlock(&rx->local->services_lock);
 	}