[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 - 240-6ubuntu5.8

---------------
systemd (240-6ubuntu5.8) disco; urgency=medium

  [ Victor Tapia ]
  * d/p/resolved_disable-connection-downgrade-when-DNSSEC-yes.patch
    Fix regression introduced by
    resolved-Mitigate-DVE-2018-0001-by-retrying-NXDOMAIN-with.patch when
    DNSSEC=yes (LP: #1796501)

  [ Dan Streetman ]
  * d/p/lp1840640-shared-seccomp-add-sync_file_range2.patch:
    allow sync_file_range2 in nspawn container (LP: #1840640)
  * d/p/lp1847527-journal-remote-do-not-request-Content-Length-if-Tran.patch:
    do not request Content-Length if Transfer-Encoding is chunked
    (LP: #1847527)
  * d/t/storage: fix flaky test
    (LP: #1847815)
  * d/p/lp1843381-dell_passthrough_skip_rename_retry.patch,
    debian/extra/rules/73-usb-net-by-mac.rules:
    fix rename delay for systems using "Dell MAC passthrough"
    (LP: #1843381)
  * d/p/lp1849733/0001-resolved-if-we-can-t-append-EDNS-OPT-RR-then-indicat.patch,
    d/p/lp1849733/0002-resolved-don-t-let-EDNS0-OPT-dgram-size-affect-TCP.patch:
    ignore EDNS0 payload limit when responding over TCP (LP: #1849733)
  * d/p/lp1849658-resolved-set-stream-type-during-DnsStream-creation.patch:
    - Fix bug in refcounting TCP stream types (LP: #1849658)
  * d/extra/dhclient-enter-resolved-hook:
    - only restart resolved if dhclient conf changed (LP: #1805183)

  [ Balint Reczey ]
  * d/p/test-execute-Filter-dev-.lxc-in-exec-dynamicuser-statedir.patch:
    fix test breakage due to running in nested lxd container
    (LP: #1845337)

 -- Dan Streetman <ddstreet at canonical.com>  Fri, 04 Oct 2019 09:06:58
-0400

** Changed in: systemd (Ubuntu Disco)
       Status: Fix Committed => 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:
  Fix Committed
Status in systemd source package in Cosmic:
  Won't Fix
Status in systemd source package in Disco:
  Fix Released

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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