[PATCH net v2 2/3] net/iucv: claim the receive credit atomically

afiucv_hs_send() samples msg_recv, advertises it to the peer as the window,
and subtracts it once dev_queue_xmit() has returned. Nothing owns the
counter across the two: iucv_sock_sendmsg() reaches it under lock_sock()
and iucv_sock_recvmsg() reaches it under no socket lock, so an unprivileged
process running both on one socket can have them subtract the same value.
msg_recv goes negative and trips the WARN_ON(); the same interleaving puts
that credit on the wire twice, and the peer's afiucv_hs_callback_win()
subtracts the wire value from msg_sent unchecked.

Claim it with atomic_xchg(), after the last error exit so the counter reads
zero only while the transmit is in flight, and hand it back if that fails.
Nothing subtracts now, so the WARN_ON() goes too.

Fixes: 3881ac441f64 ("af_iucv: add HiperSockets transport")
Cc: [email protected]
Signed-off-by: Bryam Vargas <[email protected]>
---
 net/iucv/af_iucv.c | 21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/net/iucv/af_iucv.c b/net/iucv/af_iucv.c
index 0bc4a15f4b56..492a45bb2bba 100644
--- a/net/iucv/af_iucv.c
+++ b/net/iucv/af_iucv.c
@@ -210,12 +210,6 @@ static int afiucv_hs_send(struct iucv_message *imsg, struct sock *sock,
 	phs_hdr->flags = flags;
 	if (flags == AF_IUCV_FLAG_SYN)
 		phs_hdr->window = iucv->msglimit;
-	else if ((flags == AF_IUCV_FLAG_WIN) || !flags) {
-		confirm_recv = atomic_read(&iucv->msg_recv);
-		phs_hdr->window = confirm_recv;
-		if (confirm_recv)
-			phs_hdr->flags = phs_hdr->flags | AF_IUCV_FLAG_WIN;
-	}
 	memcpy(phs_hdr->destUserID, iucv->dst_user_id, 8);
 	memcpy(phs_hdr->destAppName, iucv->dst_name, 8);
 	memcpy(phs_hdr->srcUserID, iucv->src_user_id, 8);
@@ -250,13 +244,22 @@ static int afiucv_hs_send(struct iucv_message *imsg, struct sock *sock,
 	}
 	skb->protocol = cpu_to_be16(ETH_P_AF_IUCV);
 
+	/* Claim the receive credit here, not while building the header: every
+	 * way this frame can be dropped has now been ruled out, so the window
+	 * is zeroed only for as long as the transmit itself takes.
+	 */
+	if (flags == AF_IUCV_FLAG_WIN || !flags) {
+		confirm_recv = atomic_xchg(&iucv->msg_recv, 0);
+		phs_hdr->window = confirm_recv;
+		if (confirm_recv)
+			phs_hdr->flags = phs_hdr->flags | AF_IUCV_FLAG_WIN;
+	}
+
 	atomic_inc(&iucv->skbs_in_xmit);
 	err = dev_queue_xmit(skb);
 	if (net_xmit_eval(err)) {
 		atomic_dec(&iucv->skbs_in_xmit);
-	} else {
-		atomic_sub(confirm_recv, &iucv->msg_recv);
-		WARN_ON(atomic_read(&iucv->msg_recv) < 0);
+		atomic_add(confirm_recv, &iucv->msg_recv);
 	}
 	return net_xmit_eval(err);
 

-- 
2.55.0