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

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Message-ID: <aLBQLVEAqLOHl15T@strlen.de>
Date: Thu, 28 Aug 2025 14:48:45 +0200
From: Florian Westphal <fw@...len.de>
To: Fabian Bläse <fabian@...ese.de>
Cc: 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

Fabian Bläse <fabian@...ese.de> 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`.

Reviewed-by: Florian Westphal <fw@...len.de>