[Bug 1796501] [NEW] systemd-resolved tries to mitigate DVE-2018-0001 even if DNSSEC=yes

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I ask systemd-resolved through dig to resolve the SOA of test.asdf. (doesn't exist) but it returns SERVFAIL instead of NXDOMAIN. It seems to do the following steps:
1. Ask upstream for SOA of test.asdf. with EDNS0, DO-bit and 4k size.
2. Ask upstream for SOA of test.asdf. with EDNS0 and DO-bit.
3. Ask upstream for SOA of test.asdf. with EDNS0.
4. Ask upstream for SOA of test.asdf. without EDNS0.
5. Repeat 1-4 for DS of test.asdf.
6. Repeat 1-5 for asdf.
7. Ask upstream for SOA of . with EDNS0, DO-bit and 4k size.
8. Ask upstream for DNSKEY of . with EDNS0, DO-bit and 4k size.

The upstream returns an unfragmented NXDOMAIN response for steps 1-6, an
unfragmented NOERROR response for step 7 and a fragmented NOERROR
response for step 8 which is the correct behaviour. DNSSEC records are
included in the response if the DO-bit in the request was set.

systemd-resolved should take the response from step 1 and start with
validation instead of starting useless retries with reduced feture set.
Step 3 and 4 are completely useless and probably lead to the SERVFAIL
because I have configured it with DNSSEC=yes to prevent downgrade
attacks.

This regression seems to be caused by the patch resolved-Mitigate-
DVE-2018-0001-by-retrying-NXDOMAIN-with.patch. The downgrade logic
should only be executed if it is configured as DNSSEC=allow-downgrade or
DNSSEC=no. See also
https://github.com/systemd/systemd/pull/8608#issuecomment-396927885.

** Affects: systemd (Ubuntu)
     Importance: Medium
     Assignee: Dimitri John Ledkov (xnox)
         Status: In Progress

** Affects: systemd (Ubuntu Bionic)
     Importance: Medium
     Assignee: Dimitri John Ledkov (xnox)
         Status: In Progress

** Affects: systemd (Ubuntu Cosmic)
     Importance: Medium
     Assignee: Dimitri John Ledkov (xnox)
         Status: Won't Fix

** Affects: systemd (Ubuntu Disco)
     Importance: Medium
     Assignee: Dimitri John Ledkov (xnox)
         Status: In Progress


** Tags: patch sts
-- 
systemd-resolved tries to mitigate DVE-2018-0001 even if DNSSEC=yes
https://bugs.launchpad.net/bugs/1796501
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