net: core: improve sanity checking in __dev_alloc_name

__dev_alloc_name is called from the public (and exported)
dev_alloc_name(), so we don't have a guarantee that strlen(name) is at
most IFNAMSIZ. If somebody manages to get __dev_alloc_name called with a
% char beyond the 31st character, we'd be making a snprintf() call that
will very easily crash the kernel (using an appropriate %p extension,
we'll likely dereference some completely bogus pointer).

In the normal case where strlen() is sane, we don't even save anything
by limiting to IFNAMSIZ, so just use strchr().
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -1064,7 +1064,7 @@ static int __dev_alloc_name(struct net *net, const char *name, char *buf)
 	unsigned long *inuse;
 	struct net_device *d;
 
-	p = strnchr(name, IFNAMSIZ-1, '%');
+	p = strchr(name, '%');
 	if (p) {
 		/*
 		 * Verify the string as this thing may have come from