Date:   Wed, 17 Nov 2021 10:46:33 -0800
From:   Kees Cook <keescook@chromium.org>
To:     Jordy Zomer <jordy@pwning.systems>
Cc:     linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org,
        Steffen Klassert <steffen.klassert@secunet.com>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        "David S. Miller" <davem@davemloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        David Ahern <dsahern@kernel.org>,
        Jakub Kicinski <kuba@kernel.org>, netdev@vger.kernel.org
Subject: Re: [PATCH] ipv6: check return value of ipv6_skip_exthdr
Message-ID: <202111171046.A3390D8A4A@keescook>
References: <20211117181610.2731938-1-jordy@pwning.systems>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20211117181610.2731938-1-jordy@pwning.systems>
Precedence: bulk

On Wed, Nov 17, 2021 at 07:16:10PM +0100, Jordy Zomer wrote:
> The offset value is used in pointer math on skb->data.
> Since ipv6_skip_exthdr may return -1 the pointer to uh and th
> may not point to the actual udp and tcp headers and potentially
> overwrite other stuff. This is why I think this should be checked.
> 
> Signed-off-by: Jordy Zomer <jordy@pwning.systems>
> ---
>  net/ipv6/esp6.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/net/ipv6/esp6.c b/net/ipv6/esp6.c
> index ed2f061b8768..dc4251655df9 100644
> --- a/net/ipv6/esp6.c
> +++ b/net/ipv6/esp6.c
> @@ -808,6 +808,11 @@ int esp6_input_done2(struct sk_buff *skb, int err)
>  		struct tcphdr *th;
>  
>  		offset = ipv6_skip_exthdr(skb, offset, &nexthdr, &frag_off);
> +
> +		if (offset < 0)
> +			err = -EINVAL;
> +			goto out;
> +

Oh, and, this needs { }s. The CI noticed too:
https://patchwork.kernel.org/project/netdevbpf/patch/20211117181610.2731938-1-jordy@pwning.systems/

>  		uh = (void *)(skb->data + offset);
>  		th = (void *)(skb->data + offset);
>  		hdr_len += offset;
> -- 
> 2.27.0
> 

-- 
Kees Cook