[PATCH] cipso: harden use of skb_cow() in cipso_v4_skbuff_setattr()

[Paul Moore]

On Tue, Jan 20, 2026 at 10:57 AM Will Rosenberg <whrosenb at asu.edu> wrote:
>
> If skb_cow() is passed a headroom <= -NET_SKB_PAD, it will trigger a
> BUG. As a result, use cases should avoid calling with a headroom that
> is negative to prevent triggering this issue.
>
> This is the same code pattern fixed in Commit 58fc7342b529 ("ipv6:
> BUG() in pskb_expand_head() as part of calipso_skbuff_setattr()").
>
> In cipso_v4_skbuff_setattr(), len_delta can become negative, leading to
> a negative headroom passed to skb_cow(). However, the BUG is not
> triggerable because the condition headroom <= -NET_SKB_PAD cannot be
> satisfied due to limits on the IPv4 options header size.
>
> Avoid potential problems in the future by only using skb_cow() to grow
> the skb headroom.
>
> Signed-off-by: Will Rosenberg <whrosenb at asu.edu>
> ---
>
> Notes:
>     Given that IPv4 option length should not change,
>     this may not be a worthwhile patch.
>
>     Apologies in advance if this ends up being a waste
>     of time.
>
>  net/ipv4/cipso_ipv4.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)

I think it's a reasonable thing to do in an effort to avoid future
problems.  Thanks Will.

Acked-by: Paul Moore <paul at paul-moore.com>

> diff --git a/net/ipv4/cipso_ipv4.c b/net/ipv4/cipso_ipv4.c
> index 709021197e1c..32b951ebc0c2 100644
> --- a/net/ipv4/cipso_ipv4.c
> +++ b/net/ipv4/cipso_ipv4.c
> @@ -2196,7 +2196,8 @@ int cipso_v4_skbuff_setattr(struct sk_buff *skb,
>         /* if we don't ensure enough headroom we could panic on the skb_push()
>          * call below so make sure we have enough, we are also "mangling" the
>          * packet so we should probably do a copy-on-write call anyway */
> -       ret_val = skb_cow(skb, skb_headroom(skb) + len_delta);
> +       ret_val = skb_cow(skb,
> +                         skb_headroom(skb) + (len_delta > 0 ? len_delta : 0));
>         if (ret_val < 0)
>                 return ret_val;
>
>
> base-commit: 58bae918d73e3b6cd57d1e39fcf7c75c7dd1a8fe
> --
> 2.34.1

-- 
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