DHCPv6: Fix a potential read overflow with D6_OPTION_PD_EXCLUDE

dhcpcd only checks that the prefix length of the exclusion
matches the prefix length of the ia and equals the length of the
data in the option.
This could potentially overrun the in6_addr structure.

This is fixed by enforcing RFC 6603 section 4.2 option limits
more clearly.

Thanks to Maxime Villard <max@m00nbsd.net> for finding this.

diff --git a/src/dhcp6.c b/src/dhcp6.c
index dee8d4b66e5c702463315c7143949e97caa6e218..583f3b3f523f1d85d082dfec8f838b00cb65f861 100644 (file)
--- a/src/dhcp6.c
+++ b/src/dhcp6.c
@@ -2166,40 +2166,38 @@ dhcp6_findpd(struct interface *ifp, const uint8_t *iaid,
                        state->expire = a->prefix_vltime;
                i++;
 
-               o = dhcp6_findoption(o, ol, D6_OPTION_PD_EXCLUDE, &ol);
                a->prefix_exclude_len = 0;
                memset(&a->prefix_exclude, 0, sizeof(a->prefix_exclude));
-#if 0
-               if (ex == NULL) {
-                       struct dhcp6_option *w;
-                       uint8_t *wp;
-
-                       w = calloc(1, 128);
-                       w->len = htons(2);
-                       wp = D6_OPTION_DATA(w);
-                       *wp++ = 64;
-                       *wp++ = 0x78;
-                       ex = w;
-               }
-#endif
+               o = dhcp6_findoption(o, ol, D6_OPTION_PD_EXCLUDE, &ol);
                if (o == NULL)
                        continue;
-               if (ol < 2) {
-                       logerrx("%s: truncated PD Exclude", ifp->name);
+
+               /* RFC 6603 4.2 says option length MUST be between 2 and 17.
+                * This allows 1 octet for prefix length and 16 for the
+                * subnet ID. */
+               if (ol < 2 || ol > 17) {
+                       logerrx("%s: invalid PD Exclude option", ifp->name);
                        continue;
                }
-               a->prefix_exclude_len = *o++;
-               ol--;
-               if (((a->prefix_exclude_len - a->prefix_len - 1) / NBBY) + 1
-                   != ol)
-               {
+
+               /* RFC 6603 4.2 says prefix length MUST be between the
+                * length of the IAPREFIX prefix length + 1 and 128. */
+               if (*o < a->prefix_len + 1 || *o > 128) {
+                       logerrx("%s: invalid PD Exclude length", ifp->name);
+                       continue;
+               }
+
+               /* Check option length matches prefix length. */
+               if (((*o - a->prefix_len - 1) / NBBY) + 1 != ol) {
                        logerrx("%s: PD Exclude length mismatch", ifp->name);
-                       a->prefix_exclude_len = 0;
                        continue;
                }
-               nb = a->prefix_len % NBBY;
+
+               a->prefix_exclude_len = *o++;
+               ol--;
                memcpy(&a->prefix_exclude, &a->prefix,
                    sizeof(a->prefix_exclude));
+               nb = a->prefix_len % NBBY;
                if (nb)
                        ol--;
                pw = a->prefix_exclude.s6_addr +