udhcpc: check that 4-byte options are indeed 4-byte, closes 11506

function                                             old     new   delta
udhcp_get_option32                                     -      27     +27
udhcp_get_option                                     231     248     +17
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 1/0 up/down: 44/0)               Total: 44 bytes

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

1 files changed, 19 insertions, 0 deletions

diff --git a/networking/udhcp/common.c b/networking/udhcp/common.c
index e5fd74f91..41b05b855 100644
--- a/networking/udhcp/common.c
+++ b/networking/udhcp/common.c
@@ -272,6 +272,15 @@ uint8_t* FAST_FUNC udhcp_get_option(struct dhcp_packet *packet, int code)
                         goto complain; /* complain and return NULL */
 
                 if (optionptr[OPT_CODE] == code) {
+                        if (optionptr[OPT_LEN] == 0) {
+                                /* So far no valid option with length 0 known.
+                                 * Having this check means that searching
+                                 * for DHCP_MESSAGE_TYPE need not worry
+                                 * that returned pointer might be unsafe
+                                 * to dereference.
+                                 */
+                                goto complain; /* complain and return NULL */
+                        }
                         log_option("option found", optionptr);
                         return optionptr + OPT_DATA;
                 }
@@ -289,6 +298,16 @@ uint8_t* FAST_FUNC udhcp_get_option(struct dhcp_packet *packet, int code)
         return NULL;
 }
 
+uint8_t* FAST_FUNC udhcp_get_option32(struct dhcp_packet *packet, int code)
+{
+        uint8_t *r = udhcp_get_option(packet, code);
+        if (r) {
+                if (r[-1] != 4)
+                        r = NULL;
+        }
+        return r;
+}
+
 /* Return the position of the 'end' option (no bounds checking) */
 int FAST_FUNC udhcp_end_option(uint8_t *optionptr)
 {