netdev - Re: [PATCH net-next v4] ipv6: Fix signed integer overflow in __ip6_append

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Date:   Fri, 03 Jun 2022 10:58:06 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Wang Yufen <wangyufen@...wei.com>, davem@...emloft.net,
        yoshfuji@...ux-ipv6.org, dsahern@...nel.org, edumazet@...gle.com,
        ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        kafai@...com, songliubraving@...com, yhs@...com,
        john.fastabend@...il.com, kpsingh@...nel.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH net-next v4] ipv6: Fix signed integer overflow in
 __ip6_append_data

On Thu, 2022-06-02 at 09:02 -0700, Jakub Kicinski wrote:
> On Thu, 02 Jun 2022 12:38:10 +0200 Paolo Abeni wrote:
> > I'm sorry for the multiple incremental feedback on this patch. It's
> > somewhat tricky.
> > 
> > AFAICS Jakub mentioned only udpv6_sendmsg(). In l2tp_ip6_sendmsg() we
> > can have an overflow:
> > 
> >         int transhdrlen = 4; /* zero session-id */
> >         int ulen = len + transhdrlen;
> > 
> > when len >= INT_MAX - 4. That will be harmless, but I guess it could
> > still trigger a noisy UBSAN splat. 
> 
> Good point, I wonder if that's a separate issue. Should we
> follow what UDP does and subtract the transhdr from the max?
> My gut feeling is that stricter checks are cleaner than just 
> bumping variable sizes.
> 
> diff --git a/net/l2tp/l2tp_ip6.c b/net/l2tp/l2tp_ip6.c
> index c6ff8bf9b55f..9dbd801ddb98 100644
> --- a/net/l2tp/l2tp_ip6.c
> +++ b/net/l2tp/l2tp_ip6.c
> @@ -504,14 +504,15 @@ static int l2tp_ip6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
>         struct ipcm6_cookie ipc6;
>         int addr_len = msg->msg_namelen;
>         int transhdrlen = 4; /* zero session-id */
> -       int ulen = len + transhdrlen;
> +       int ulen;
>         int err;
>  
>         /* Rough check on arithmetic overflow,
>          * better check is made in ip6_append_data().
>          */
> -       if (len > INT_MAX)
> +       if (len > INT_MAX - transhdrlen)
>                 return -EMSGSIZE;
> +       ulen = len + transhdrlen;
>  
>         /* Mirror BSD error message compatibility */
>         if (msg->msg_flags & MSG_OOB)
> 
LGTM. Imho this can even land in a separated patch (whatever is easier)

Thanks!

Paolo