netinet6: Don't return non-IPv6 enabled interfaces from in6_getlinkifnet()

[kp]

Summary

There are scenarios where we can end up looking up an interface by its scope and
turn up an interface that doesn't have IPv6 enabled on it. If that happens we
could end up dereferencing a NULL pointer accessing ifp->if_afdata[AF_INET6].
Check for this.

One such scenario is if a firewall rewrites a destination address to a
link-local address, with an embedded scope for such an interface. Attach a test
case which provokes this.

PR: 288263
Reported by: Robert Morris <[e-mail]>
Sponsored by: Rubicon Communications, LLC ("Netgate")

[kp]

Whoops

[zlei]

The change to in6_getlinkifnet() looks good to me.

sys/netinet6/scope6.c

511

I'd like to have a description about the NULL check for if_afdata[AF_INET6], say An interface may not be IPv6 capable .

I think it may deserve a log entry about this rare case. This is probably by mis-configuration, say firewall rewriting etc.

[kp]

Log when we end up selecting an interface without IPv6 support.