On Thu, Aug 28, 2025 at 11:14:35AM +0200, Fabian Bläse wrote:
> The icmp_ndo_send function was originally introduced to ensure proper
> rate limiting when icmp_send is called by a network device driver,
> where the packet's source address may have already been transformed
> by SNAT.
>
> However, the original implementation only considers the
> IP_CT_DIR_ORIGINAL direction for SNAT and always replaced the packet's
> source address with that of the original-direction tuple. This causes
> two problems:
>
> 1. For SNAT:
> Reply-direction packets were incorrectly translated using the source
> address of the CT original direction, even though no translation is
> required.
>
> 2. For DNAT:
> Reply-direction packets were not handled at all. In DNAT, the original
> direction's destination is translated. Therefore, in the reply
> direction the source address must be set to the reply-direction
> source, so rate limiting works as intended.
>
> Fix this by using the connection direction to select the correct tuple
> for source address translation, and adjust the pre-checks to handle
> reply-direction packets in case of DNAT.
>
> Additionally, wrap the `ct->status` access in READ_ONCE(). This avoids
> possible KCSAN reports about concurrent updates to `ct->status`.
I think such concurrent update cannot not happen, NAT bits are only
set for the first packet of a connection, which sets up the nat
configuration, so READ_ONCE() can go away.
Florian?
> Fixes: 0b41713b6066 ("icmp: introduce helper for nat'd source address in network device context")
>
> Signed-off-by: Fabian Bläse <fabian@xxxxxxxxx>
> Cc: Jason A. Donenfeld <Jason@xxxxxxxxx>
> Cc: Florian Westphal <fw@xxxxxxxxx>
> ---
> Changes v1->v2:
> - Implement fix for ICMPv6 as well
>
> Changes v2->v3:
> - Collapse conditional tuple selection into a single direction lookup [Florian]
> - Always apply source address translation if IPS_NAT_MASK is set [Florian]
> - Wrap ct->status in READ_ONCE()
> - Add a clearer explanation of the behaviour change for DNAT
> ---
> net/ipv4/icmp.c | 6 ++++--
> net/ipv6/ip6_icmp.c | 6 ++++--
> 2 files changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
> index 2ffe73ea644f..c48c572f024d 100644
> --- a/net/ipv4/icmp.c
> +++ b/net/ipv4/icmp.c
> @@ -799,11 +799,12 @@ void icmp_ndo_send(struct sk_buff *skb_in, int type, int code, __be32 info)
> struct sk_buff *cloned_skb = NULL;
> struct ip_options opts = { 0 };
> enum ip_conntrack_info ctinfo;
> + enum ip_conntrack_dir dir;
> struct nf_conn *ct;
> __be32 orig_ip;
>
> ct = nf_ct_get(skb_in, &ctinfo);
> - if (!ct || !(ct->status & IPS_SRC_NAT)) {
> + if (!ct || !(READ_ONCE(ct->status) & IPS_NAT_MASK)) {
> __icmp_send(skb_in, type, code, info, &opts);
> return;
> }
> @@ -818,7 +819,8 @@ void icmp_ndo_send(struct sk_buff *skb_in, int type, int code, __be32 info)
> goto out;
>
> orig_ip = ip_hdr(skb_in)->saddr;
> - ip_hdr(skb_in)->saddr = ct->tuplehash[0].tuple.src.u3.ip;
> + dir = CTINFO2DIR(ctinfo);
> + ip_hdr(skb_in)->saddr = ct->tuplehash[dir].tuple.src.u3.ip;
> __icmp_send(skb_in, type, code, info, &opts);
> ip_hdr(skb_in)->saddr = orig_ip;
> out:
> diff --git a/net/ipv6/ip6_icmp.c b/net/ipv6/ip6_icmp.c
> index 9e3574880cb0..233914b63bdb 100644
> --- a/net/ipv6/ip6_icmp.c
> +++ b/net/ipv6/ip6_icmp.c
> @@ -54,11 +54,12 @@ void icmpv6_ndo_send(struct sk_buff *skb_in, u8 type, u8 code, __u32 info)
> struct inet6_skb_parm parm = { 0 };
> struct sk_buff *cloned_skb = NULL;
> enum ip_conntrack_info ctinfo;
> + enum ip_conntrack_dir dir;
> struct in6_addr orig_ip;
> struct nf_conn *ct;
>
> ct = nf_ct_get(skb_in, &ctinfo);
> - if (!ct || !(ct->status & IPS_SRC_NAT)) {
> + if (!ct || !(READ_ONCE(ct->status) & IPS_NAT_MASK)) {
> __icmpv6_send(skb_in, type, code, info, &parm);
> return;
> }
> @@ -73,7 +74,8 @@ void icmpv6_ndo_send(struct sk_buff *skb_in, u8 type, u8 code, __u32 info)
> goto out;
>
> orig_ip = ipv6_hdr(skb_in)->saddr;
> - ipv6_hdr(skb_in)->saddr = ct->tuplehash[0].tuple.src.u3.in6;
> + dir = CTINFO2DIR(ctinfo);
> + ipv6_hdr(skb_in)->saddr = ct->tuplehash[dir].tuple.src.u3.in6;
> __icmpv6_send(skb_in, type, code, info, &parm);
> ipv6_hdr(skb_in)->saddr = orig_ip;
> out:
> --
> 2.51.0
>
>
