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

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Message-ID: <aLBMktniIkqsfWQY@calendula>
Date: Thu, 28 Aug 2025 14:33:22 +0200
From: Pablo Neira Ayuso <pablo@...filter.org>
To: Florian Westphal <fw@...len.de>
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

On Thu, Aug 28, 2025 at 02:15:53PM +0200, Florian Westphal wrote:
> 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().

There are a more checks for ct->status & NAT_MASK in the tree that I
can see, if you are correct, then maybe a new helper function to check
for NAT_MASK is needed.

Anyway, as for this patch, READ_ONCE should not harm.