[PATCH nf 2/2] ipv6: update *offset for non-first fragments in ipv6_find_hdr()

[Axel Mierczuk <axel.mierczuk@1password.com>]

The non-first fragment early return is the only successful exit of
ipv6_find_hdr() that leaves *offset untouched. IPVS used the initial
offset as if it had been updated.

For non-first fragments with target < 0, set *offset immediately
after the Fragment header, at the start of the fragment payload.
Callers must still account for the nonzero fragment offset. A
NEXTHDR_FRAGMENT search continues to return the Fragment header's
offset. Other target < 0 callers check the returned fragment offset
or ignore *offset here.

Suggested-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Axel Mierczuk <axel.mierczuk@1password.com>
---
 net/ipv6/exthdrs_core.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/exthdrs_core.c b/net/ipv6/exthdrs_core.c
index 4a9748338cf4..e27f5b8cc154 100644
--- a/net/ipv6/exthdrs_core.c
+++ b/net/ipv6/exthdrs_core.c
@@ -179,7 +179,10 @@ EXPORT_SYMBOL_GPL(ipv6_find_tlv);
  *
  * Note that non-1st fragment is special case that "the protocol number
  * of last header" is "next header" field in Fragment header. In this case,
- * *offset is meaningless and fragment offset is stored in *fragoff if fragoff
+ * for target < 0, *offset points immediately after the Fragment header,
+ * at the start of the fragment payload. Callers must still account for
+ * the nonzero fragment offset before interpreting the payload. The
+ * fragment offset is stored in *fragoff if fragoff
  * isn't NULL.
  *
  * if flags is not NULL and it's a fragment, then the frag flag
@@ -261,6 +264,7 @@ int ipv6_find_hdr(const struct sk_buff *skb, unsigned int *offset,
 				     hp->nexthdr == NEXTHDR_NONE)) {
 					if (fragoff)
 						*fragoff = _frag_off;
+					*offset = start + sizeof(struct frag_hdr);
 					return hp->nexthdr;
 				}
 				if (!found)
-- 
2.43.0