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seg6: fix HMAC validation when an extension header precedes the SRH

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seg6_hmac_validate_skb() derived the SRH from skb_transport_header().
That only holds while the two coincide, which is not true on the
seg6_local input path.

ip6_rcv_core() leaves the transport header just past the IPv6 header.
A Hop-by-Hop options header is consumed before the route lookup and
advances it, but a Destination Options header is not: the seg6_local
lwtunnel is entered through an input redirect from the route lookup,
which bypasses the extension header handlers. The transport header
then still points at the Destination Options header while
seg6_get_srh() has located the real SRH further down the chain.

The HMAC is therefore computed over the Destination Options header,
and a packet carrying a valid HMAC TLV is dropped when
seg6_require_hmac is set. Such a packet is legitimate: RFC 8200 allows
Destination Options before a routing header, and get_srh() has walked
the header chain since commit 5829d70b0b ("ipv6: sr: fix get_srh() to
comply with IPv6 standard "RFC 8200"").

With a Fragment or an Authentication header in front of the SRH, the
same mistake also reads past the data pulled by seg6_get_srh(). This
happens before seg6_require_hmac is read, so the default configuration
is affected.

Reproduce by giving a node a seg6local End SID with
net.ipv6.conf.<dev>.seg6_require_hmac=1 and a key installed with
"ip sr hmac set <keyid> sha1", then sending

  IPv6 -> Destination Options -> SRH (carrying a valid HMAC TLV) -> payload

to that SID: it is dropped, while the same packet without the
Destination Options header passes.

Fixes: 5829d70b0b ("ipv6: sr: fix get_srh() to comply with IPv6 standard "RFC 8200"")
Assisted-by: LLM
Signed-off-by: Yuya Kusakabe <yuya.kusakabe@gmail.com>
Reviewed-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Link: https://patch.msgid.link/20260926-b4-seg6-hmac-transport-header-v2-1-8087b76f6375@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
This commit is contained in:
Yuya Kusakabe 2026-09-26 20:19:27 +09:00 • committed by Paolo Abeni
commit f9cb4d296d
4 changed files with 7 additions and 9 deletions
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