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net: qualcomm: rmnet: return earlier for bad checksum

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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>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Alex Elder 2021-06-12 09:37:32 -05:00 committed by David S. Miller
parent 9d0407bc4c
commit fab01a6f3a
1 changed files with 27 additions and 9 deletions
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36
drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c
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@ -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)) {