@@ -76,6 +76,17 @@ rmnet_map_ipv4_dl_csum_trailer(struct sk_buff *skb,
* We verified above that the IP header contributes zero to the
* trailer checksum. Therefore the checksum in the trailer is
* just the checksum computed over the IP payload.
+
+ * If the IP payload arrives intact, adding the pseudo header
+ * checksum to the IP payload checksum will yield 0xffff (negative
+ * zero). This means the trailer checksum and the pseudo checksum
+ * are additive inverses of each other. Put another way, the
+ * message passes the checksum test if the trailer checksum value
+ * is the negated pseudo header checksum.
+ *
+ * Knowing this, we don't even need to examine the transport
+ * header checksum value; it is already accounted for in the
+ * checksum value found in the trailer.
*/
ip_payload_csum = (__force __sum16)~csum_trailer->csum_value;
@@ -84,11 +95,11 @@ rmnet_map_ipv4_dl_csum_trailer(struct sk_buff *skb,
ip4h->protocol, 0);
pseudo_csum = csum16_add(ip_payload_csum, (__force __be16)pseudo_csum);
- /* The trailer checksum *includes* the checksum in the transport
- * header. Adding that to the pseudo checksum will yield 0xffff
- * ("negative 0") if the message arrived intact.
- */
- WARN_ON((__sum16)~pseudo_csum);
+ /* The cast is required to ensure only the low 16 bits are examined */
+ if ((__sum16)~pseudo_csum) {
+ priv->stats.csum_validation_failed++;
+ return -EINVAL;
+ }
csum_value_final = ~csum16_sub(pseudo_csum, (__force __be16)*csum_field);
if (unlikely(!csum_value_final)) {
@@ -143,6 +154,11 @@ rmnet_map_ipv6_dl_csum_trailer(struct sk_buff *skb,
* transport checksum from this, we first subract the contribution
* of the IP header from the trailer checksum. We then add the
* checksum computed over the pseudo header.
+ *
+ * It's sufficient to compare the IP payload checksum with the
+ * negated pseudo checksum to determine whether the packet
+ * checksum was good. (See further explanation in comments
+ * in rmnet_map_ipv4_dl_csum_trailer()).
*/
ip_header_csum = (__force __be16)ip_fast_csum(ip6h, sizeof(*ip6h) / 4);
ip6_payload_csum = ~csum16_sub((__force __sum16)csum_trailer->csum_value,
@@ -155,10 +171,12 @@ rmnet_map_ipv6_dl_csum_trailer(struct sk_buff *skb,
length, ip6h->nexthdr, 0);
pseudo_csum = csum16_add(ip6_payload_csum, (__force __be16)pseudo_csum);
- /* Adding the payload checksum to the pseudo checksum yields 0xffff
- * ("negative 0") if the message arrived intact.
- */
- WARN_ON((__sum16)~pseudo_csum);
+ /* The cast is required to ensure only the low 16 bits are examined */
+ if ((__sum16)~pseudo_csum) {
+ priv->stats.csum_validation_failed++;
+ return -EINVAL;
+ }
+
csum_value_final = ~csum16_sub(pseudo_csum, (__force __be16)*csum_field);
if (unlikely(csum_value_final == 0)) {
In rmnet_map_ipv4_dl_csum_trailer(), if the sum of the trailer checksum and the pseudo checksum is non-zero, checksum validation has failed. We can return an error as soon as we know that. We can do the same thing in rmnet_map_ipv6_dl_csum_trailer(). Add some comments that explain where we're headed. Signed-off-by: Alex Elder <elder@linaro.org> --- .../ethernet/qualcomm/rmnet/rmnet_map_data.c | 36 ++++++++++++++----- 1 file changed, 27 insertions(+), 9 deletions(-)