netdev - Re: [PATCH net] bareudp: Fix invalid read beyond skb's linear data

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Message-ID: <20210806162234.69334902@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Fri, 6 Aug 2021 16:22:34 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Guillaume Nault <gnault@...hat.com>
Cc:     David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        Martin Varghese <martin.varghese@...ia.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net] bareudp: Fix invalid read beyond skb's linear data

On Fri, 6 Aug 2021 17:52:06 +0200 Guillaume Nault wrote:
> Data beyond the UDP header might not be part of the skb's linear data.
> Use skb_copy_bits() instead of direct access to skb->data+X, so that
> we read the correct bytes even on a fragmented skb.
> 
> Fixes: 4b5f67232d95 ("net: Special handling for IP & MPLS.")
> Signed-off-by: Guillaume Nault <gnault@...hat.com>
> ---
>  drivers/net/bareudp.c | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/net/bareudp.c b/drivers/net/bareudp.c
> index a7ee0af1af90..54e321a695ce 100644
> --- a/drivers/net/bareudp.c
> +++ b/drivers/net/bareudp.c
> @@ -71,12 +71,18 @@ static int bareudp_udp_encap_recv(struct sock *sk, struct sk_buff *skb)
>  		family = AF_INET6;
>  
>  	if (bareudp->ethertype == htons(ETH_P_IP)) {
> -		struct iphdr *iphdr;
> +		__u8 ipversion;
>  
> -		iphdr = (struct iphdr *)(skb->data + BAREUDP_BASE_HLEN);
> -		if (iphdr->version == 4) {
> -			proto = bareudp->ethertype;
> -		} else if (bareudp->multi_proto_mode && (iphdr->version == 6)) {
> +		if (skb_copy_bits(skb, BAREUDP_BASE_HLEN, &ipversion,
> +				  sizeof(ipversion))) {

No preference just curious - could skb_header_pointer() be better suited?

> +			bareudp->dev->stats.rx_dropped++;
> +			goto drop;
> +		}
> +		ipversion >>= 4;
> +
> +		if (ipversion == 4) {
> +			proto = htons(ETH_P_IP);
> +		} else if (ipversion == 6 && bareudp->multi_proto_mode) {
>  			proto = htons(ETH_P_IPV6);
>  		} else {
>  			bareudp->dev->stats.rx_dropped++;