[Bug 1347147] Re: krb5 database operations enter infinite loop

[Martin Pitt]

** Also affects: krb5 (Ubuntu Trusty)
   Importance: Undecided
       Status: New

** Also affects: gcc-4.8 (Ubuntu Trusty)
   Importance: Undecided
       Status: New

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https://bugs.launchpad.net/bugs/1347147

Title:
  krb5 database operations enter infinite loop

Status in The GNU Compiler Collection:
  Unknown
Status in Network Authentication System:
  Unknown
Status in “gcc-4.8” package in Ubuntu:
  Confirmed
Status in “krb5” package in Ubuntu:
  Triaged
Status in “gcc-4.8” source package in Trusty:
  New
Status in “krb5” source package in Trusty:
  New

Bug description:
  [Impact]

  On krb5 KDC databases with more than a few hundred principals,
  operations can enter an infinite loop in the database library.  This
  affects both read and write operations.  If operators are fortunate,
  they will encounter this bug while testing a migration.  If they are
  not so fortunate, they will encounter this bug in a production KDC
  when the number of principals crosses the threshold where this bug
  manifests, resulting in a service outage and possible database
  corruption.  Probably the only way to restore service in that
  situation is to install a patched KDC or to downgrade to an unaffected
  version.

  Both Trusty and Utopic amd64 have been verified to have this issue.

  One concrete reported example is an invocation of kdb5_util load (as
  part of a slave KDC propagation) spinning:

  http://mailman.mit.edu/pipermail/kerberos/2014-July/020007.html

  Additional failure modes are likely

  A branch is linked including the upstream work around for this bug,
  along with two other patches to bugs already nominated for trusty
  applied to the krb5 in trusty.

  For utopic, the simplest fix is to rebuild krb5 with the compiler
  currently in utopic.  An alternative is to request that the Debian
  maintainers (both monitoring this bug for such a request) upload the
  upstream work around to Debian and sync that.  You could do an ubuntu-
  specific upload but it seems undesirable to introduce a change between
  Ubuntu and Debian when all the right parties are happy to avoid it.

  The upstream patch works around a compiler optimizer bug in the
  gcc-4.8 series, which incorrectly deduces that a strict aliasing
  violation has occurred and miscompiles part of the bundled libdb2
  library that the KDC database back end depends upon.  The
  miscompilation causes a data structure to contain an inappropriate
  cycle, which leads to an infinite loop when the structure is
  traversed.

  [Test Case]

  apt-get install krb5-kdc krb5-admin-server
  kdb5_util -W -r T create -s
  awk 'BEGIN{ for (i = 0; i < 1024; i++) { printf("ank -randkey a%06d\n", i) } }' /dev/null | kadmin.local -r T

  (Enter any password for the master key when requested.)

  On platforms with this issue, kadmin.local spins consuming 100% CPU
  after a few hundred principals have been created.  (This is "a000762"
  on two examples.)

  To clean up,

  rm /etc/krb5kdc/principal*

  or

  krb5kdc -r T destroy

  but the latter can possibly enter the same infinite loop.

  [Regression Potential]

  Negligible.

  It is theoretically possible that our upstream workaround, which
  involves using TAILQ macros instead of CIRCLEQ macros in the bundled
  libdb2 that backs the KDC database, will have some as-yet undiscovered
  bugs or compiler interactions with consequences worse than this
  current issue.  I think this is rather unlikely.

  The patched libdb2 passes both the extensive libdb2 test suite and the
  rest of the krb5 test suite.  Prior to patching, compiling krb5 with
  an affected gcc would cause the krb5 test suite to stall when it
  reached the libdb2 test suite.  (The test suite stall is how we became
  aware of the gcc optimizer bug.)

  The BSD TAILQ macros are generally considered to be safer than the
  CIRCLEQ macros, and the various open-source BSD derivatives have made
  the corresponding change to their libdb sources years ago, with no
  reported ill effects that I can see.

  Original report from Ben Kaduk:

  ==========

  In some conditions, propagating a kerberos database to a slave KDC server can stall.
  This is due to a misoptimization by gcc 4.8 of the CIRCLEQ famliy of macros, apparently due to overzealous strict aliasing deductions.

  One case of this stall is reported at
  http://mailman.mit.edu/pipermail/kerberos/2014-July/020007.html (and
  the rest of the thread), and there is an entry in the upstream
  bugtracker at http://krbdev.mit.edu/rt/Ticket/Display.html?id=7860 .

  gcc 4.9 (as used in Debian unstable at present) is not believed to
  induce this problem.  Upstream has patched their code to use the TAILQ
  family of macros instead, as a workaround, but that workaround has not
  yet appeared in an upstream release:
  https://github.com/krb5/krb5/commit/26d8744129

  Because of the different compiler versions used on Debian and Ubuntu,
  I am filing this as an Ubuntu-specific bug.

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