netdev - Re: [PATCH v3] icmp: fix icmp_ndo_send address translation for reply direction

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Message-ID: <aLBIeS4_x7dbrL-j@strlen.de>
Date: Thu, 28 Aug 2025 14:15:53 +0200
From: Florian Westphal <fw@...len.de>
To: Pablo Neira Ayuso <pablo@...filter.org>
Cc: Fabian Bläse <fabian@...ese.de>,
	netdev@...r.kernel.org, netfilter-devel@...r.kernel.org,
	"Jason A. Donenfeld" <Jason@...c4.com>
Subject: Re: [PATCH v3] icmp: fix icmp_ndo_send address translation for reply
 direction

Pablo Neira Ayuso <pablo@...filter.org> wrote:
> 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.

Yes, the NAT bits stay in place but not other flags in ->status, e.g.
DYING, ASSURED, etc.

So I believe its needed, concurrent update of ->status is possible and
KCSAN would warn.  Other spots either use READ_ONCE or use test_bit().