net: Support for csum_bad in skbuff

This flag indicates that an invalid checksum was detected in the
packet. __skb_mark_checksum_bad helper function was added to set this.

Checksums can be marked bad from a driver or the GRO path (the latter
is implemented in this patch). csum_bad is checked in
__skb_checksum_validate_complete (i.e. calling that when ip_summed ==
CHECKSUM_NONE).

csum_bad works in conjunction with ip_summed value. In the case that
ip_summed is CHECKSUM_NONE and csum_bad is set, this implies that the
first (or next) checksum encountered in the packet is bad. When
ip_summed is CHECKSUM_UNNECESSARY, the first checksum after the last
one validated is bad. For example, if ip_summed == CHECKSUM_UNNECESSARY,
csum_level == 1, and csum_bad is set-- then the third checksum in the
packet is bad. In the normal path, the packet will be dropped when
processing the protocol layer of the bad checksum:
__skb_decr_checksum_unnecessary called twice for the good checksums
changing ip_summed to CHECKSUM_NONE so that
__skb_checksum_validate_complete is called to validate the third
checksum and that will fail since csum_bad is set.

Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -2216,7 +2216,9 @@ static inline void skb_gro_incr_csum_unnecessary(struct sk_buff *skb)
 	if (__skb_gro_checksum_validate_needed(skb, zero_okay, check))	\
 		__ret = __skb_gro_checksum_validate_complete(skb,	\
 				compute_pseudo(skb, proto));		\
-	if (!__ret)							\
+	if (__ret)							\
+		__skb_mark_checksum_bad(skb);				\
+	else								\
 		skb_gro_incr_csum_unnecessary(skb);			\
 	__ret;								\
 })

include/linux/skbuff.h

@@ -617,7 +617,8 @@ struct sk_buff {
 
 	kmemcheck_bitfield_begin(flags3);
 	__u8			csum_level:2;
-	/* 14 bit hole */
+	__u8			csum_bad:1;
+	/* 13 bit hole */
 	kmemcheck_bitfield_end(flags3);
 
 	__be16			inner_protocol;
@@ -2825,6 +2826,21 @@ static inline void __skb_incr_checksum_unnecessary(struct sk_buff *skb)
 	}
 }
 
+static inline void __skb_mark_checksum_bad(struct sk_buff *skb)
+{
+	/* Mark current checksum as bad (typically called from GRO
+	 * path). In the case that ip_summed is CHECKSUM_NONE
+	 * this must be the first checksum encountered in the packet.
+	 * When ip_summed is CHECKSUM_UNNECESSARY, this is the first
+	 * checksum after the last one validated. For UDP, a zero
+	 * checksum can not be marked as bad.
+	 */
+
+	if (skb->ip_summed == CHECKSUM_NONE ||
+	    skb->ip_summed == CHECKSUM_UNNECESSARY)
+		skb->csum_bad = 1;
+}
+
 /* Check if we need to perform checksum complete validation.
  *
  * Returns true if checksum complete is needed, false otherwise
@@ -2866,6 +2882,9 @@ static inline __sum16 __skb_checksum_validate_complete(struct sk_buff *skb,
 			skb->csum_valid = 1;
 			return 0;
 		}
+	} else if (skb->csum_bad) {
+		/* ip_summed == CHECKSUM_NONE in this case */
+		return 1;
 	}
 
 	skb->csum = psum;

net/core/dev.c

@@ -3918,7 +3918,7 @@ static enum gro_result dev_gro_receive(struct napi_struct *napi, struct sk_buff
 	if (!(skb->dev->features & NETIF_F_GRO))
 		goto normal;
 
-	if (skb_is_gso(skb) || skb_has_frag_list(skb))
+	if (skb_is_gso(skb) || skb_has_frag_list(skb) || skb->csum_bad)
 		goto normal;
 
 	gro_list_prepare(napi, skb);