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

[Steve Langasek]

Hello jrb0001, or anyone else affected,

Accepted systemd into bionic-proposed. The package will build now and be
available at
https://launchpad.net/ubuntu/+source/systemd/237-3ubuntu10.33 in a few
hours, and then in the -proposed repository.

Please help us by testing this new package.  See
https://wiki.ubuntu.com/Testing/EnableProposed for documentation on how
to enable and use -proposed.  Your feedback will aid us getting this
update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug,
mentioning the version of the package you tested and change the tag from
verification-needed-bionic to verification-done-bionic. If it does not
fix the bug for you, please add a comment stating that, and change the
tag to verification-failed-bionic. In either case, without details of
your testing we will not be able to proceed.

Further information regarding the verification process can be found at
https://wiki.ubuntu.com/QATeam/PerformingSRUVerification .  Thank you in
advance for helping!

N.B. The updated package will be released to -updates after the bug(s)
fixed by this package have been verified and the package has been in
-proposed for a minimum of 7 days.

** Tags removed: verification-done verification-done-bionic
** Tags added: verification-needed verification-needed-bionic

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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 Committed

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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