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

[Launchpad Bug Tracker]

This bug was fixed in the package systemd - 242-7ubuntu2

---------------
systemd (242-7ubuntu2) eoan; urgency=medium

  [ Bryan Quigley ]
  * Update patch for resolved: Mitigate DVE-2018-0001, by retrying NXDOMAIN
    without EDNS0. This disables the workaround if DNSSEC=yes.
    Falls back directly to simple UDP instead of trying an intermediate.
    (LP: #1796501)
    Author: Bryan Quigley
    File: debian/patches/resolved-Mitigate-DVE-2018-0001-by-retrying-NXDOMAIN-with.patch
    https://git.launchpad.net/~ubuntu-core-dev/ubuntu/+source/systemd/commit/?id=2974114ed9b89ea922a23893e8eff70d5cac77fe

  [ Balint Reczey ]
  * Pass personality test even when i386 userland runs on amd64 kernel
    File: debian/patches/debian/UBUNTU-test-Pass-personality-test-even-when-i386-userland-runs-o.patch
    https://git.launchpad.net/~ubuntu-core-dev/ubuntu/+source/systemd/commit/?id=42e0bfc426f19430f6768ef4922a9531a345765f
  * Refresh patches
    Files:
    - debian/patches/Revert-namespace-be-more-careful-when-handling-namespacin.patch
    - debian/patches/debian/Ubuntu-core-in-execute-soft-fail-setting-Nice-priority-when.patch
    - debian/patches/test-execute-Filter-dev-.lxc-in-exec-dynamicuser-statedir.patch
    https://git.launchpad.net/~ubuntu-core-dev/ubuntu/+source/systemd/commit/?id=ff8387be07322230e9afe87f1c767ee241e9a0e1

 -- Balint Reczey <rbalint at ubuntu.com>  Tue, 08 Oct 2019 22:31:17 +0200

** Changed in: systemd (Ubuntu)
       Status: In Progress => Fix Released

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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:
  Fix Released
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:
  [impact]

  an NXDOMAIN response from a dns server when systemd-resolved is
  configured as DNSSEC=yes breaks dns resolution as it downgrades from
  DNSSEC.

  [test case]

  see comment 9

  [regression potential]

  as with the original patch that introduced this problem, this has the
  potential to break dns resolution.

  [other info]

  original 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.

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