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

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Message-ID: <20210809091918.68ae21e8@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Mon, 9 Aug 2021 09:19:18 -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 Sun, 8 Aug 2021 18:16:25 +0200 Guillaume Nault wrote:
> 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).

Right, the advantage would be in the "fast" case of skb_header_pointer()
being inlined.

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