netlink: pad nla_memcpy dest buffer with zeroes

This is especially important in cases where the kernel allocs a new
structure and expects a field to be set from a netlink attribute. If such
attribute is shorter than expected, the rest of the field is left containing
previous data. When such field is read back by the user space, kernel memory
content is leaked.

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

lib/nlattr.c

@@ -279,6 +279,8 @@ int nla_memcpy(void *dest, const struct nlattr *src, int count)
 	int minlen = min_t(int, count, nla_len(src));
 
 	memcpy(dest, nla_data(src), minlen);
+	if (count > minlen)
+		memset(dest + minlen, 0, count - minlen);
 
 	return minlen;
 }