rxrpc: Fix use-after-free in rxrpc_receive_data()

The subpacket scanning loop in rxrpc_receive_data() references the
subpacket count in the private data part of the sk_buff in the loop
termination condition.  However, when the final subpacket is pasted into
the ring buffer, the function is no longer has a ref on the sk_buff and
should not be looking at sp->* any more.  This point is actually marked in
the code when skb is cleared (but sp is not - which is an error).

Fix this by caching sp->nr_subpackets in a local variable and using that
instead.

Also clear 'sp' to catch accesses after that point.

This can show up as an oops in rxrpc_get_skb() if sp->nr_subpackets gets
trashed by the sk_buff getting freed and reused in the meantime.

Fixes: e2de6c40 ("rxrpc: Use info in skbuff instead of reparsing a jumbo packet")
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/rxrpc/input.c

@@ -413,7 +413,7 @@ static void rxrpc_input_data(struct rxrpc_call *call, struct sk_buff *skb)
 {
 	struct rxrpc_skb_priv *sp = rxrpc_skb(skb);
 	enum rxrpc_call_state state;
-	unsigned int j;
+	unsigned int j, nr_subpackets;
 	rxrpc_serial_t serial = sp->hdr.serial, ack_serial = 0;
 	rxrpc_seq_t seq0 = sp->hdr.seq, hard_ack;
 	bool immediate_ack = false, jumbo_bad = false;
@@ -457,7 +457,8 @@ static void rxrpc_input_data(struct rxrpc_call *call, struct sk_buff *skb)
 	call->ackr_prev_seq = seq0;
 	hard_ack = READ_ONCE(call->rx_hard_ack);
 
-	if (sp->nr_subpackets > 1) {
+	nr_subpackets = sp->nr_subpackets;
+	if (nr_subpackets > 1) {
 		if (call->nr_jumbo_bad > 3) {
 			ack = RXRPC_ACK_NOSPACE;
 			ack_serial = serial;
@@ -465,11 +466,11 @@ static void rxrpc_input_data(struct rxrpc_call *call, struct sk_buff *skb)
 		}
 	}
 
-	for (j = 0; j < sp->nr_subpackets; j++) {
+	for (j = 0; j < nr_subpackets; j++) {
 		rxrpc_serial_t serial = sp->hdr.serial + j;
 		rxrpc_seq_t seq = seq0 + j;
 		unsigned int ix = seq & RXRPC_RXTX_BUFF_MASK;
-		bool terminal = (j == sp->nr_subpackets - 1);
+		bool terminal = (j == nr_subpackets - 1);
 		bool last = terminal && (sp->rx_flags & RXRPC_SKB_INCL_LAST);
 		u8 flags, annotation = j;
 
@@ -506,7 +507,7 @@ static void rxrpc_input_data(struct rxrpc_call *call, struct sk_buff *skb)
 		}
 
 		if (call->rxtx_buffer[ix]) {
-			rxrpc_input_dup_data(call, seq, sp->nr_subpackets > 1,
+			rxrpc_input_dup_data(call, seq, nr_subpackets > 1,
 					     &jumbo_bad);
 			if (ack != RXRPC_ACK_DUPLICATE) {
 				ack = RXRPC_ACK_DUPLICATE;
@@ -564,6 +565,7 @@ static void rxrpc_input_data(struct rxrpc_call *call, struct sk_buff *skb)
 			 * ring.
 			 */
 			skb = NULL;
+			sp = NULL;
 		}
 
 		if (last) {