udhcpc: sanitize hostnames in incoming packets. Closes 3979.

The following options are replaced with string "bad" if they
contain malformed hostname:
HOST_NAME, DOMAIN_NAME, NIS_DOMAIN, TFTP_SERVER_NAME

function                                             old     new   delta
xmalloc_optname_optval                               850     888     +38
attach_option                                        440     443      +3
len_of_option_as_string                               13      14      +1
dhcp_option_lengths                                   13      14      +1
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 4/0 up/down: 43/0)               Total: 43 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 61 insertions, 0 deletions

diff --git a/networking/udhcp/dhcpc.c b/networking/udhcp/dhcpc.c
index 945600c6b..2f2016cd5 100644
--- a/networking/udhcp/dhcpc.c
+++ b/networking/udhcp/dhcpc.c
@@ -135,6 +135,63 @@ static int mton(uint32_t mask)
         return i;
 }
 
+/* Check if a given label represents a valid DNS label
+ * Return pointer to the first character after the label upon success,
+ * NULL otherwise.
+ * See RFC1035, 2.3.1
+ */
+/* We don't need to be particularly anal. For example, allowing _, hyphen
+ * at the end, or leading and trailing dots would be ok, since it
+ * can't be used for attacks. (Leading hyphen can be, if someone uses
+ * cmd "$hostname"
+ * in the script: then hostname may be treated as an option)
+ */
+static const char *valid_domain_label(const char *label)
+{
+        unsigned char ch;
+        unsigned pos = 0;
+
+        for (;;) {
+                ch = *label;
+                if ((ch|0x20) < 'a' || (ch|0x20) > 'z') {
+                        if (pos == 0) {
+                                /* label must begin with letter */
+                                return NULL;
+                        }
+                        if (ch < '0' || ch > '9') {
+                                if (ch == '\0' || ch == '.')
+                                        return label;
+                                /* DNS allows only '-', but we are more permissive */
+                                if (ch != '-' && ch != '_')
+                                        return NULL;
+                        }
+                }
+                label++;
+                pos++;
+                //Do we want this?
+                //if (pos > 63) /* NS_MAXLABEL; labels must be 63 chars or less */
+                //      return NULL;
+        }
+}
+
+/* Check if a given name represents a valid DNS name */
+/* See RFC1035, 2.3.1 */
+static int good_hostname(const char *name)
+{
+        //const char *start = name;
+
+        for (;;) {
+                name = valid_domain_label(name);
+                if (!name)
+                        return 0;
+                if (!name[0])
+                        return 1;
+                        //Do we want this?
+                        //return ((name - start) < 1025); /* NS_MAXDNAME */
+                name++;
+        }
+}
+
 /* Create "opt_name=opt_value" string */
 static NOINLINE char *xmalloc_optname_optval(uint8_t *option, const struct dhcp_optflag *optflag, const char *opt_name)
 {
@@ -187,8 +244,11 @@ static NOINLINE char *xmalloc_optname_optval(uint8_t *option, const struct dhcp_
                  * the case of list of options.
                  */
                 case OPTION_STRING:
+                case OPTION_STRING_HOST:
                         memcpy(dest, option, len);
                         dest[len] = '\0';
+                        if (type == OPTION_STRING_HOST && !good_hostname(dest))
+                                safe_strncpy(dest, "bad", len);
                         return ret;
                 case OPTION_STATIC_ROUTES: {
                         /* Option binary format:
@@ -368,6 +428,7 @@ static char **fill_envp(struct dhcp_packet *packet)
         /* +1 element for each option, +2 for subnet option: */
         if (packet) {
                 /* note: do not search for "pad" (0) and "end" (255) options */
+//TODO: change logic to scan packet _once_
                 for (i = 1; i < 255; i++) {
                         temp = udhcp_get_option(packet, i);
                         if (temp) {