[PATCH net] openvswitch: advance by the real header length in get_ipv6_ext_hdrs()

[Joas Antonio dos Santos <joasantonio108@gmail.com>]

get_ipv6_ext_hdrs() walks the IPv6 extension header chain to fill
key->ipv6.exthdrs, but advances to the next header with ipv6_optlen()
whatever the current header is.  That is only right for Hop-by-Hop,
Destination Options and Routing headers:

 - an Authentication Header is ((hdrlen + 2) << 2) bytes long
   (ipv6_authlen()), so with hdrlen = 4 the walk skips 40 bytes instead
   of 24 and reads the next "header" from the ICV and payload;

 - a Fragment header is always 8 bytes, and the byte read as hdrlen is
   its Reserved field, which the sender controls and receivers ignore,
   so a non-zero value makes the walk skip up to 2048 bytes and miss
   the headers that follow.

In both cases the OFPIEH12_* bits stored in key->ipv6.exthdrs depend on
bytes chosen by the sender rather than on the headers that are actually
present: payload bytes that look like another AH or a Hop-by-Hop header
add OFPIEH12_UNREP/OFPIEH12_UNSEQ, and real headers after a Fragment
header can go unreported.  Flows that match on OVS_KEY_ATTR_IPV6_EXTHDRS
then classify such packets wrongly.  There is no memory safety issue,
skb_header_pointer() bounds every read.

Advance by sizeof(struct frag_hdr) for Fragment headers and by
ipv6_authlen() for AH, as ipv6_find_hdr() does.

Fixes: 28a3f0601727 ("net: openvswitch: IPv6: Add IPv6 extension header support")
Link: https://lore.kernel.org/all/178905555085.219967.6617611783437228045@kernel.org/
Cc: stable@vger.kernel.org
Signed-off-by: Joas Antonio dos Santos <joasantonio108@gmail.com>
Assisted-by: Yaga Agent HackerSec Claude:claude-opus-5-5
---
Found while going through the pre-existing issues the Sashiko review of
"openvswitch: fix wrong flag value in get_ipv6_ext_hdrs()" pointed out
(Link above); Eelco mentioned putting them on his todo list, and I did
not find a fix for this part in net, net-next or on the list.  Happy to
drop this if a fix is already in flight.

Tested by building get_ipv6_ext_hdrs() before and after this patch in a
userspace harness (skb_header_pointer() replaced by a bounds-checked
memcpy from a buffer) and feeding it crafted packets:

  AH(hdrlen=4) + DSTOPTS + TCP, payload mimicking AH at old offset
    before: AUTH DEST UNREP UNSEQ      after: AUTH DEST UNSEQ
  FRAG(Reserved=0xff) + DSTOPTS + ROUTING + TCP
    before: DEST FRAG UNSEQ            after: DEST FRAG ROUTER UNSEQ
  FRAG(Reserved=0) + ROUTING + TCP, HOP + DST + ROUTING + TCP
    unchanged

net/openvswitch/ builds cleanly with W=1 (arm64, net/main 94cea342e891).
I did not test it on a live datapath.

 net/openvswitch/flow.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/openvswitch/flow.c b/net/openvswitch/flow.c
index 1c4f3a079..a1c60d2f5 100644
--- a/net/openvswitch/flow.c
+++ b/net/openvswitch/flow.c
@@ -372,8 +372,13 @@ static void get_ipv6_ext_hdrs(struct sk_buff *skb, struct ipv6hdr *nh,
 		hp = skb_header_pointer(skb, start, sizeof(_hdr), &_hdr);
 		if (!hp)
 			break;
+		if (next_type == IPPROTO_FRAGMENT)
+			start += sizeof(struct frag_hdr);
+		else if (next_type == IPPROTO_AH)
+			start += ipv6_authlen(hp);
+		else
+			start += ipv6_optlen(hp);
 		next_type = hp->nexthdr;
-		start += ipv6_optlen(hp);
 	}
 }
 
-- 
2.39.5