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

[Bryan Quigley]

Built a eoan package with xnox's updated patch in my ppa:
https://launchpad.net/~bryanquigley/+archive/ubuntu/1796501/+packages

1. Confirm failure with DNSSEC=yes, DNS server 1.1.1.1
$ resolvectl query people.com.cn
people.com.cn: resolve call failed: DNSSEC validation failed: failed-auxiliary

2. Add PPA and upgrade systemd to PPA version.

3. Confirm success:
resolvectl query people.com.cn
people.com.cn: 106.48.12.140                   -- link: ens2
               106.48.12.141                   -- link: ens2
               (hpcc-download-foreign.chinacache.net)

** Patch added: "eoan debdiff"
   https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1796501/+attachment/5288416/+files/systemd_241-7ubuntu2.debdiff

-- 
You received this bug notification because you are a member of Ubuntu
Foundations Bugs, which is subscribed to systemd in Ubuntu.
https://bugs.launchpad.net/bugs/1796501

Title:
  systemd-resolved tries to mitigate DVE-2018-0001 even if DNSSEC=yes

Status in systemd package in Ubuntu:
  In Progress
Status in systemd source package in Bionic:
  In Progress
Status in systemd source package in Cosmic:
  Won't Fix
Status in systemd source package in Disco:
  In Progress

Bug description:
  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.

To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1796501/+subscriptions