Re: [PATCH iproute2] iptunnel: detect protocol mismatch on tunnel change

[Stephen Hemminger <stephen@networkplumber.org>]

On Tue, 11 Apr 2023 09:55:25 -0600
David Ahern <dsahern@kernel.org> wrote:

> That addresses the stack smashing, but ....
> 
> >  
> >  				if (tnl_get_ioctl(*argv, &old_p))
> >  					return -1;
> > -				*p = old_p;
> > +
> > +				if (old_p.ip_tnl.iph.version != 4 ||
> > +				    old_p.ip_tnl.iph.ihl != 5)  
> 
> this field overlays laddr in ip6_tnl_parm2 which means there is a
> collision in valid addresses.

That is why the commit message said this is best effort.
What happens when there is a collision with a valid address is that
the call will fail later as a bogus change to an IP6 tunnel.

 # ip tunnel add gre1 mode ip6gre local 4545:db8::1 remote 2001:db8::2 ttl 255
 # ip tunnel change gre1 mode gre local 192.168.0.0 remote 192.168.0.1 ttl 255
ttl != 0 and nopmtudisc are incompatible

This is off in the non-standard IPv6 address range so not a likely
to bother anyone.