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

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Message-ID: <20210808161625.GA2912@pc-32.home>
Date:   Sun, 8 Aug 2021 18:16:25 +0200
From:   Guillaume Nault <gnault@...hat.com>
To:     Jakub Kicinski <kuba@...nel.org>
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, Aug 06, 2021 at 04:22:34PM -0700, Jakub Kicinski wrote:
> 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?

I have no preference either. I just used skb_copy_bits() because it
didn't seem useful to get a pointer to the buffer (just to read one
byte of data).

But I don't mind reposting with skb_header_pointer() if anyone prefers
that solution.