netdev - [PATCH net-next 2/2] xen-netfront: re-order error checks in xennet_get

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Date:   Wed, 13 Jul 2022 11:19:55 +0200
From:   Jan Beulich <jbeulich@...e.com>
To:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>
Cc:     Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <stefano@...bellini.net>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
Subject: [PATCH net-next 2/2] xen-netfront: re-order error checks in
 xennet_get_responses()

Check the retrieved grant reference first; there's no point trying to
have xennet_move_rx_slot() move invalid data (and further defer
recognition of the issue, likely making diagnosis yet more difficult).

Signed-off-by: Jan Beulich <jbeulich@...e.com>
---
I question the log message claiming a bad ID (which is how I read its
wording): rx->id isn't involved in determining ref. I don't see what
else to usefully log, though, yet making the message just "Bad rx
response" also doesn't look very useful.

--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -1043,16 +1043,6 @@ static int xennet_get_responses(struct n
 	}
 
 	for (;;) {
-		if (unlikely(rx->status < 0 ||
-			     rx->offset + rx->status > XEN_PAGE_SIZE)) {
-			if (net_ratelimit())
-				dev_warn(dev, "rx->offset: %u, size: %d\n",
-					 rx->offset, rx->status);
-			xennet_move_rx_slot(queue, skb, ref);
-			err = -EINVAL;
-			goto next;
-		}
-
 		/*
 		 * This definitely indicates a bug, either in this driver or in
 		 * the backend driver. In future this should flag the bad
@@ -1065,6 +1055,16 @@ static int xennet_get_responses(struct n
 			err = -EINVAL;
 			goto next;
 		}
+
+		if (unlikely(rx->status < 0 ||
+			     rx->offset + rx->status > XEN_PAGE_SIZE)) {
+			if (net_ratelimit())
+				dev_warn(dev, "rx->offset: %u, size: %d\n",
+					 rx->offset, rx->status);
+			xennet_move_rx_slot(queue, skb, ref);
+			err = -EINVAL;
+			goto next;
+		}
 
 		if (!gnttab_end_foreign_access_ref(ref)) {
 			dev_alert(dev,