linux-kernel - Re: ipv4: cipso potential BUG()

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Message-ID: <CAHC9VhR4d7WXOVR7Y9ee2+=-t2nThzOo-ySMB+5x_87LfBJbZw@mail.gmail.com>
Date: Mon, 19 Jan 2026 18:31:44 -0500
From: Paul Moore <paul@...l-moore.com>
To: Will Rosenberg <whrosenb@....edu>
Cc: "David S. Miller" <davem@...emloft.net>, David Ahern <dsahern@...nel.org>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Simon Horman <horms@...nel.org>, Huw Davies <huw@...eweavers.com>, netdev@...r.kernel.org, 
	linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: ipv4: cipso potential BUG()

On Mon, Jan 19, 2026 at 3:46 PM Will Rosenberg <whrosenb@....edu> wrote:
>
> Previously, it was discussed that skb_cow() has a bug due to implicit
> integer casting that can lead to a BUG when headroom < -NET_SKB_PAD. We
> concluded that it was not worthwhile to fix the root cause and to
> instead fix the symptom found in calipso. The thread for this issue can
> be found here:
>
> https://lore.kernel.org/netdev/CAHC9VhQmR8A2vz0W-VrrhYNQ2wgCYxHbAmdgmM2yTL-uh4qiOg@mail.gmail.com/
>
> I recently reviewed the use cases of skb_cow() throughout the kernel and
> found that cipso_v4_skbuff_setattr() comes very close to triggering the
> same BUG. However, I concluded this was not triggerable. Even though
> len_delta can become negative, leading to a negative headroom passed to
> skb_cow(), we do not satisfy the condition headroom < -NET_SKB_PAD.
>
> Nonetheless, I believe cipso is using skb_cow() dangerously, but since
> the issue is not triggerable, would it still make sense to patch it?
> I figured I would throw out a quick email. Please let me know and I can
> make a similar patch for cipso if necessary.

Sometimes the easiest way to get an answer to questions like this is
to send a patch; since I would expect this particular patch to be of
limited scope and very small, I think this advice holds true here.

-- 
paul-moore.com