From 77e43bcb7ec177e293a5c3f1b91a2c5aebfb6c68 Mon Sep 17 00:00:00 2001 From: Florian Westphal Date: Wed, 1 Jul 2026 07:44:17 +0200 Subject: [PATCH] netfilter: nf_nat_sip: reload possible stale data pointer quoting sashiko: ------------------------------------------------------------------------ [..] noticed a potential memory bug and header corruption involving the SIP NAT helper. In net/netfilter/nf_nat_sip.c:nf_nat_sip(): if (skb_ensure_writable(skb, skb->len)) { nf_ct_helper_log(skb, ct, "cannot mangle packet"); return NF_DROP; } uh = (void *)skb->data + protoff; uh->dest = ct_sip_info->forced_dport; if (!nf_nat_mangle_udp_packet(skb, ct, ctinfo, protoff, 0, 0, NULL, 0)) { If a cloned or fragmented SKB is reallocated by skb_ensure_writable(), the old data buffer is freed. However, nf_nat_sip() fails to update *dptr to point to the new buffer. It also appears to use nf_nat_mangle_udp_packet() on what could be a TCP packet, which would overwrite the sequence number with a checksum update. ------------------------------------------------------------------------ nf_conntrack_sip linerizes skbs, hence no fragmented skb can be seen. But clones are possible, so rebuild dptr. Disable nf_nat_mangle_udp_packet() branch for TCP streams. It doesn't look like this can ever happen, else we should have received bug reports about this, so just check the conntrack is UDP and drop otherwise. The calling conntrack_sip set ->forced_dport for SIP_HDR_VIA_UDP messages, so I don't think this is ever expected to be true for a TCP stream. Fixes: 7266507d8999 ("netfilter: nf_ct_sip: support Cisco 7941/7945 IP phones") Cc: stable@vger.kernel.org Assisted-by: Claude:claude-sonnet-4-6 Signed-off-by: Florian Westphal --- net/netfilter/nf_nat_sip.c | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/net/netfilter/nf_nat_sip.c b/net/netfilter/nf_nat_sip.c index 67c04d8143ab..aea02f6aff09 100644 --- a/net/netfilter/nf_nat_sip.c +++ b/net/netfilter/nf_nat_sip.c @@ -289,13 +289,24 @@ static unsigned int nf_nat_sip(struct sk_buff *skb, unsigned int protoff, /* Mangle destination port for Cisco phones, then fix up checksums */ if (dir == IP_CT_DIR_REPLY && ct_sip_info->forced_dport) { + int doff = *dptr - (const char *)skb->data; struct udphdr *uh; + if (doff <= 0) { + DEBUG_NET_WARN_ON_ONCE(1); + return NF_DROP; + } + + /* ct_sip_info->forced_dport only expected with UDP */ + if (nf_ct_protonum(ct) != IPPROTO_UDP) + return NF_DROP; + if (skb_ensure_writable(skb, skb->len)) { nf_ct_helper_log(skb, ct, "cannot mangle packet"); return NF_DROP; } + *dptr = skb->data + doff; uh = (void *)skb->data + protoff; uh->dest = ct_sip_info->forced_dport;