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

[Guillaume Nault <gnault@redhat.com>]

On Mon, Aug 09, 2021 at 09:19:18AM -0700, Jakub Kicinski wrote:
> 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@redhat.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.

Yes indeed. The problem was found because of some automated functionnal
tests, not because of any practical use cases. So I didn't consider the
possible performance differences.

I see that you've applied the patch as is already. I can switch to
skb_header_pointer() in the future, if anyone sees any practical
benefit for it.

Thanks for the review.