net: ethtool: prevent flow steering to RSS contexts which don't exist

Since commit 42dc431 ("ethtool: rss: prevent rss ctx deletion
when in use") we prevent removal of RSS contexts pointed to by
existing flow rules. Core should also prevent creation of rules
which point to RSS context which don't exist in the first place.

Reviewed-by: Joe Damato <jdamato@fastly.com>
Link: https://patch.msgid.link/20250206235334.1425329-2-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ethtool/ioctl.c

@@ -993,10 +993,14 @@ static noinline_for_stack int ethtool_set_rxnfc(struct net_device *dev,
 		return rc;
 
 	/* Nonzero ring with RSS only makes sense if NIC adds them together */
-	if (cmd == ETHTOOL_SRXCLSRLINS && info.fs.flow_type & FLOW_RSS &&
-	    !ops->cap_rss_rxnfc_adds &&
-	    ethtool_get_flow_spec_ring(info.fs.ring_cookie))
-		return -EINVAL;
+	if (cmd == ETHTOOL_SRXCLSRLINS && info.fs.flow_type & FLOW_RSS) {
+		if (!ops->cap_rss_rxnfc_adds &&
+		    ethtool_get_flow_spec_ring(info.fs.ring_cookie))
+			return -EINVAL;
+
+		if (!xa_load(&dev->ethtool->rss_ctx, info.rss_context))
+			return -EINVAL;
+	}
 
 	if (cmd == ETHTOOL_SRXFH && ops->get_rxfh) {
 		struct ethtool_rxfh_param rxfh = {};