[PATCH net v2 0/2] xfrm: fix IPv6 atomic fragment policy handling

[PATCH net v2 0/2] xfrm: fix IPv6 atomic fragment policy handling

[Jérémy Jean]

This series of two patches fixes two IPv6 atomic fragment issues in
XFRM session decoding.

Patch 1 restores upper-layer protocol decoding for IPv6 first
fragments.

Patch 2 builds on Patch 1 and restores selector fields for IPv6 atomic
fragments, while leaving them unset for real fragments.

Changes in v2 after a review from Sashiko:
- Patch 1: Leave ports unset for fragments with BPF flow dissectors.
- Patch 2: Include the atomic fragment selector fix.

v1: https://lore.kernel.org/all/20261101230745.2043474-2-Jeremy.Jean@oss.cyber.gouv.fr/
Sashiko: https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20261101230745.2043474-2-Jeremy.Jean%40oss.cyber.gouv.fr

Jérémy Jean (2):
  xfrm: fix IPv6 first-fragment protocol decode
  xfrm: fix selector matching for IPv6 atomic fragments

 net/xfrm/xfrm_policy.c | 53 +++++++++++++++++++++++++++++++-----------
 1 file changed, 39 insertions(+), 14 deletions(-)

-- 
2.47.3

[PATCH net v2 1/2] xfrm: fix IPv6 first-fragment protocol decode

[Jérémy Jean]

Commit 7a0207094f1b ("xfrm: policy: replace session decode with flow
dissector") replaced XFRM session decoding with the flow dissector.
While the old IPv6 decoder kept walking past an IPv6 Fragment
extension header, the flow dissector stops unless the caller sets
FLOW_DISSECTOR_F_PARSE_1ST_FRAG. For an atomic fragment followed by
extension headers, inbound policy lookup can therefore see the header
after Fragment instead of the actual upper-layer protocol and miss
protocol selectors.

Make the dissector continue through first IPv6 fragments. Keep the port,
ICMPv6 and GRE selector fields zero for all fragments, as the old decoder
did. This also keeps the result the same when a BPF flow dissector fills
ports for a first fragment.

Fixes: 7a0207094f1b ("xfrm: policy: replace session decode with flow dissector")
Cc: stable@vger.kernel.org
Signed-off-by: Jérémy Jean <Jeremy.Jean@oss.cyber.gouv.fr>
---
 net/xfrm/xfrm_policy.c | 35 ++++++++++++++++++++++-------------
 1 file changed, 22 insertions(+), 13 deletions(-)

diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
index 9ea60c7717ff8..bda5100c9fdbf 100644
--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c
@@ -3480,23 +3480,29 @@ decode_session6(const struct xfrm_flow_keys *flkeys, struct flowi *fl, bool reve
 	if (reverse) {
 		fl6->saddr = flkeys->addrs.ipv6.dst;
 		fl6->daddr = flkeys->addrs.ipv6.src;
-		fl6->fl6_sport = flkeys->ports.dst;
-		fl6->fl6_dport = flkeys->ports.src;
 	} else {
 		fl6->saddr = flkeys->addrs.ipv6.src;
 		fl6->daddr = flkeys->addrs.ipv6.dst;
-		fl6->fl6_sport = flkeys->ports.src;
-		fl6->fl6_dport = flkeys->ports.dst;
 	}
 
-	switch (flkeys->basic.ip_proto) {
-	case IPPROTO_GRE:
-		fl6->fl6_gre_key = flkeys->gre.keyid;
-		break;
-	case IPPROTO_ICMPV6:
-		fl6->fl6_icmp_type = flkeys->icmp.type;
-		fl6->fl6_icmp_code = flkeys->icmp.code;
-		break;
+	if (!(flkeys->control.flags & FLOW_DIS_IS_FRAGMENT)) {
+		if (reverse) {
+			fl6->fl6_sport = flkeys->ports.dst;
+			fl6->fl6_dport = flkeys->ports.src;
+		} else {
+			fl6->fl6_sport = flkeys->ports.src;
+			fl6->fl6_dport = flkeys->ports.dst;
+		}
+
+		switch (flkeys->basic.ip_proto) {
+		case IPPROTO_GRE:
+			fl6->fl6_gre_key = flkeys->gre.keyid;
+			break;
+		case IPPROTO_ICMPV6:
+			fl6->fl6_icmp_type = flkeys->icmp.type;
+			fl6->fl6_icmp_code = flkeys->icmp.code;
+			break;
+		}
 	}
 
 	fl6->flowi6_proto = flkeys->basic.ip_proto;
@@ -3507,10 +3513,13 @@ int __xfrm_decode_session(struct net *net, struct sk_buff *skb, struct flowi *fl
 			  unsigned int family, int reverse)
 {
 	struct xfrm_flow_keys flkeys;
+	unsigned int flags = FLOW_DISSECTOR_F_STOP_AT_ENCAP;
 
 	memset(&flkeys, 0, sizeof(flkeys));
+	if (family == AF_INET6)
+		flags |= FLOW_DISSECTOR_F_PARSE_1ST_FRAG;
 	__skb_flow_dissect(net, skb, &xfrm_session_dissector, &flkeys,
-			   NULL, 0, 0, 0, FLOW_DISSECTOR_F_STOP_AT_ENCAP);
+			   NULL, 0, 0, 0, flags);
 
 	switch (family) {
 	case AF_INET:
-- 
2.47.3

[PATCH net v2 2/2] xfrm: fix selector matching for IPv6 atomic fragments

[Jérémy Jean]

Commit 7e3a42a12c4b ("xfrm6: handling fragment") stopped decoding IPv6
selector fields after a Fragment header. That is correct for real
fragments because later fragments do not carry those fields, yet an
atomic fragment is different because it contains the complete datagram,
which allows XFRM to read its ports, ICMPv6 type and code, and GRE key.
Leaving those fields zero lets a plaintext atomic fragment miss a policy
that matches on one of them.

Leave ports, ICMPv6 type and code, and GRE key unset for real
fragments. For atomic fragments, treat the packet as a complete datagram
during IPv6 session decoding and recover the ports at the transport
offset found by the flow dissector.

Fixes: 7e3a42a12c4b ("xfrm6: handling fragment")
Cc: stable@vger.kernel.org
Signed-off-by: Jérémy Jean <Jeremy.Jean@oss.cyber.gouv.fr>
---
 net/xfrm/xfrm_policy.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
index bda5100c9fdbf..24470dbd82639 100644
--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c
@@ -3526,9 +3526,25 @@ int __xfrm_decode_session(struct net *net, struct sk_buff *skb, struct flowi *fl
 		decode_session4(&flkeys, fl, reverse);
 		break;
 #if IS_ENABLED(CONFIG_IPV6)
-	case AF_INET6:
+	case AF_INET6: {
+		struct frag_hdr _fh, *fh;
+		unsigned int offset = 0;
+
+		if ((flkeys.control.flags & FLOW_DIS_IS_FRAGMENT) &&
+		    ipv6_find_hdr(skb, &offset, NEXTHDR_FRAGMENT,
+				  NULL, NULL) == NEXTHDR_FRAGMENT) {
+			fh = skb_header_pointer(skb, offset, sizeof(_fh), &_fh);
+			if (fh && !(fh->frag_off & htons(IP6_OFFSET | IP6_MF))) {
+				flkeys.control.flags &= ~FLOW_DIS_IS_FRAGMENT;
+				flkeys.ports.ports = skb_flow_get_ports(skb,
+									flkeys.control.thoff,
+									flkeys.basic.ip_proto,
+									NULL, 0);
+			}
+		}
 		decode_session6(&flkeys, fl, reverse);
 		break;
+	}
 #endif
 	default:
 		return -EAFNOSUPPORT;
-- 
2.47.3

Re: [PATCH net v2 0/2] xfrm: fix IPv6 atomic fragment policy handling

[netdev-bot+sinfo]

Hi!

This is an automated message. This series looks like a fix, but its
commit messages seem to be missing some information:

 - How the issue was discovered, e.g. hit in production, hit during
   development, syzbot report, manual code inspection, LLM or static
   analysis tool scan.

 - Whether the issue was actually triggered, or is only theoretical
   (e.g. found by code inspection). If it was triggered please include
   the symptoms, like the stack trace or error messages.

Please do not repost the series just to address the above. Instead,
reply to this email with the missing information, so that reviewers
can take it into account. If the series needs another revision for
other reasons, please include the information in the commit messages
then.

The evaluation is done by an LLM so it may be wrong, if you think
that is the case please reply and explain.