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Date: Wed, 15 Apr 2026 11:02:15 +0200
From: Florian Westphal <fw@strlen.de>
To: "Kito Xu (veritas501)" <hxzene@gmail.com>
Cc: pablo@netfilter.org, phil@nwl.cc, davem@davemloft.net,
	edumazet@google.com, kuba@kernel.org, pabeni@redhat.com,
	horms@kernel.org, jengelh@medozas.de, kaber@trash.net,
	netfilter-devel@vger.kernel.org, coreteam@netfilter.org,
	netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] netfilter: xt_realm: fix null-ptr-deref in realm_mt()
Message-ID: <ad9UF5Cr12YGJnbi@strlen.de>
References: <20260415034343.107920-1-hxzene@gmail.com>
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In-Reply-To: <20260415034343.107920-1-hxzene@gmail.com>

Kito Xu (veritas501) <hxzene@gmail.com> wrote:
> realm_mt() unconditionally dereferences skb_dst(skb) without a NULL
> check. The xt_realm match registers with .family = NFPROTO_UNSPEC,
> making it available to all netfilter protocol families. Through the
> nftables compat layer (nft_compat), an unprivileged user inside a
> user/net namespace can load this match into a bridge-family chain.

I do not think this bug is related to nft_compat.
You can also use ebtables setsockopt api to request xt_realm, no?

> Fixes: ab4f21e6fb1c ("netfilter: xtables: use NFPROTO_UNSPEC in more extensions")

Looks correct.  Alternatively we could revert the xt_realm.c change.
But I don't have a strong opinion here, patch looks correct.