From: Xiang Mei <xmei5@asu.edu>
To: Pablo Neira Ayuso <pablo@netfilter.org>,
Florian Westphal <fw@strlen.de>, Phil Sutter <phil@nwl.cc>
Cc: "David S . Miller" <davem@davemloft.net>,
Eric Dumazet <edumazet@google.com>,
Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
Simon Horman <horms@kernel.org>,
netfilter-devel@vger.kernel.org, coreteam@netfilter.org,
netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
Xiang Mei <xmei5@asu.edu>, Weiming Shi <bestswngs@gmail.com>
Subject: [PATCH nf] netfilter: nf_conntrack_reasm: guard mac_header adjustment after IPv6 defrag
Date: Sun, 5 Jul 2026 16:36:29 -0700 [thread overview]
Message-ID: <20260705233629.2287813-1-xmei5@asu.edu> (raw)
nf_ct_frag6_reasm() slides the packet head forward to drop the IPv6
fragment header and then unconditionally advances skb->mac_header:
skb->mac_header += sizeof(struct frag_hdr);
On the NF_INET_LOCAL_OUT defrag path the skb has no link-layer header
yet, so skb->mac_header is still the "not set" sentinel (u16)~0U. Adding
sizeof(struct frag_hdr) wraps it to a small value (0xffff + 8 == 7),
after which skb_mac_header_was_set() wrongly reports a MAC header is
present and skb_mac_header() points into the headroom.
The reassembler has done this unconditional add since it was introduced;
it was harmless while mac_header was a bare pointer, but wrong once
mac_header became a u16 offset whose unset state is the ~0U sentinel
tested by skb_mac_header_was_set(). The sibling net/ipv6/reassembly.c
does the same relocation and does guard the adjustment; mirror the
guard here.
Fixes: 9fb9cbb1082d ("[NETFILTER]: Add nf_conntrack subsystem.")
Reported-by: Weiming Shi <bestswngs@gmail.com>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Xiang Mei <xmei5@asu.edu>
---
net/ipv6/netfilter/nf_conntrack_reasm.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/net/ipv6/netfilter/nf_conntrack_reasm.c b/net/ipv6/netfilter/nf_conntrack_reasm.c
index 64ab23ff559b..3637b20d3fa4 100644
--- a/net/ipv6/netfilter/nf_conntrack_reasm.c
+++ b/net/ipv6/netfilter/nf_conntrack_reasm.c
@@ -348,7 +348,8 @@ static int nf_ct_frag6_reasm(struct frag_queue *fq, struct sk_buff *skb,
skb_network_header(skb)[fq->nhoffset] = skb_transport_header(skb)[0];
memmove(skb->head + sizeof(struct frag_hdr), skb->head,
(skb->data - skb->head) - sizeof(struct frag_hdr));
- skb->mac_header += sizeof(struct frag_hdr);
+ if (skb_mac_header_was_set(skb))
+ skb->mac_header += sizeof(struct frag_hdr);
skb->network_header += sizeof(struct frag_hdr);
skb_reset_transport_header(skb);
--
2.43.0
reply other threads:[~2026-07-05 23:36 UTC|newest]
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