net: warn about attempts to register negative ifindex

Since the xarray changes we mix returning valid ifindex and negative
errno in a single int returned from dev_index_reserve(). This depends
on the fact that ifindexes can't be negative. Otherwise we may insert
into the xarray and return a very large negative value. This in turn
may break ERR_PTR().

OvS is susceptible to this problem and lacking validation (fix posted
separately for net).

Reject negative ifindex explicitly. Add a warning because the input
validation is better handled by the caller.

Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Link: https://lore.kernel.org/r/20230814205627.2914583-2-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/dev.c

@@ -9589,6 +9589,11 @@ static int dev_index_reserve(struct net *net, u32 ifindex)
 {
 	int err;
 
+	if (ifindex > INT_MAX) {
+		DEBUG_NET_WARN_ON_ONCE(1);
+		return -EINVAL;
+	}
+
 	if (!ifindex)
 		err = xa_alloc_cyclic(&net->dev_by_index, &ifindex, NULL,
 				      xa_limit_31b, &net->ifindex, GFP_KERNEL);