Commit 7b2ea97e46 introduced a logic bug
in resume_dslookup(): that function now only conditionally checks
whether DS chasing can still make progress. Specifically, that check is
only performed when the previous resume_dslookup() call invokes
dns_resolver_createfetch() with the 'nameservers' argument set to
something else than NULL, which may not always be the case. Failing to
perform that check may trigger assertion failures as a result of
dns_resolver_createfetch() attempting to resolve an invalid name.
Example scenario that leads to such outcome:
1. A validating resolver is configured to forward all queries to
another resolver. The latter returns broken DS responses that
trigger DS chasing.
2. rctx_chaseds() calls dns_resolver_createfetch() with the
'nameservers' argument set to NULL.
3. The fetch fails, so resume_dslookup() is called. Due to
fevent->result being set to e.g. DNS_R_SERVFAIL, the default branch
is taken in the switch statement.
4. Since 'nameservers' was set to NULL for the fetch which caused the
resume_dslookup() callback to be invoked
(fctx->nsfetch->private->nameservers), resume_dslookup() chops off
one label off fctx->nsname and calls dns_resolver_createfetch()
again, for a name containing one label less than before.
5. Steps 3-4 are repeated (i.e. all attempts to find the name servers
authoritative for the DS RRset being chased fail) until fctx->nsname
becomes stripped down the the root name.
6. Since resume_dslookup() does not check whether DS chasing can still
make progress, it strips off a label off the root name and continues
its attempts at finding the name servers authoritative for the DS
RRset being chased, passing an invalid name to
dns_resolver_createfetch().
Fix by ensuring resume_dslookup() always checks whether DS chasing can
still make progress when a name server fetch fails. Update code
comments to ensure the purpose of the relevant dns_name_equal() check is
clear.
1a79aeab44