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[Launchpad Bug Tracker <1846787@xxxxxxxxxxxxxxxxxx>]

This bug was fixed in the package dbus - 1.10.6-1ubuntu3.5

---------------
dbus (1.10.6-1ubuntu3.5) xenial; urgency=medium

  * Prevent logind from leaking session files (LP: #1846787). Fixed by
    upstream patches:
    - d/p/Only-read-one-message-at-a-time-if-there-are-fds-pen.patch
    - d/p/bus-Fix-timeout-restarts.patch
    - d/p/DBusMainLoop-ensure-all-required-timeouts-are-restar.patch

 -- Heitor Alves de Siqueira <halves@xxxxxxxxxxxxx>  Mon, 07 Oct 2019
08:29:04 -0300

** Changed in: dbus (Ubuntu Xenial)
       Status: Fix Committed => Fix Released

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https://bugs.launchpad.net/bugs/1846787

Title:
  systemd-logind leaves leftover sessions and scope files

Status in dbus package in Ubuntu:
  Fix Released
Status in systemd package in Ubuntu:
  Fix Released
Status in dbus source package in Xenial:
  Fix Released
Status in systemd source package in Xenial:
  Fix Released

Bug description:
  [Impact]
  Scope file leakage can cause SSH delays and reduce performance in systemd

  [Description]
  The current systemd-logind version present in Xenial can leave abandoned SSH
  sessions and scope files in cases where the host sees a lot of concurrent SSH
  connections. These leftover sessions can slow down systemd performance
  greatly, and can have an impact on sshd handling a great number of concurrent
  connections.

  To fix this issue, patches are needed in both dbus and systemd. These improve the
  performance of the communication between dbus and systemd, so that they can
  handle a better volume of events (e.g. SSH logins). All of those patches are
  already present from Bionic onwards, so we only need those fixes for Xenial.

  == Systemd ==
  Upstream patches:
  - core: use an AF_UNIX/SOCK_DGRAM socket for cgroup agent notification (d8fdc62037b5)

  $ git describe --contains d8fdc62037b5
  v230~71^2~2

  $ rmadison systemd
   systemd | 229-4ubuntu4     | xenial          | source, ...
   systemd | 229-4ubuntu21.21 | xenial-security | source, ...
   systemd | 229-4ubuntu21.22 | xenial-updates  | source, ... <--------
   systemd | 237-3ubuntu10    | bionic          | source, ...
   systemd | 237-3ubuntu10.29 | bionic-security | source, ...
   systemd | 237-3ubuntu10.29 | bionic-updates  | source, ...
   systemd | 237-3ubuntu10.31 | bionic-proposed | source, ...

  == DBus ==
  Upstream patches:
  - Only read one message at a time if there are fds pending (892f084eeda0)
  - bus: Fix timeout restarts  (529600397bca)
  - DBusMainLoop: ensure all required timeouts are restarted (446b0d9ac75a)

  $ git describe --contains 892f084eeda0 529600397bca 446b0d9ac75a
  dbus-1.11.10~44
  dbus-1.11.10~45
  dbus-1.11.16~2

  $ rmadison dbus
   dbus | 1.10.6-1ubuntu3    | xenial           | source, ...
   dbus | 1.10.6-1ubuntu3.4  | xenial-security  | source, ...
   dbus | 1.10.6-1ubuntu3.4  | xenial-updates   | source, ... <--------
   dbus | 1.12.2-1ubuntu1    | bionic           | source, ...
   dbus | 1.12.2-1ubuntu1.1  | bionic-security  | source, ...
   dbus | 1.12.2-1ubuntu1.1  | bionic-updates   | source, ...

  [Test Case]
  1) Simulate a lot of concurrent SSH connections with e.g. a for loop:
  multipass@xenial-logind:~$ for i in {1..1000}; do sleep 0.1; ssh localhost sleep 1 & done

  2) Check for leaked sessions in /run/systemd/system/:
  multipass@xenial-logind:~$ ls -ld /run/systemd/system/session-*.scope*
  drwxr-xr-x 2 root root 160 Oct 4 15:34 /run/systemd/system/session-103.scope.d
  drwxr-xr-x 2 root root 160 Oct 4 15:34 /run/systemd/system/session-104.scope.d
  drwxr-xr-x 2 root root 160 Oct 4 15:34 /run/systemd/system/session-105.scope.d
  drwxr-xr-x 2 root root 160 Oct 4 15:34 /run/systemd/system/session-106.scope.d
  drwxr-xr-x 2 root root 160 Oct 4 15:34 /run/systemd/system/session-110.scope.d
  drwxr-xr-x 2 root root 160 Oct 4 15:34 /run/systemd/system/session-111.scope.d
  drwxr-xr-x 2 root root 160 Oct 4 15:34 /run/systemd/system/session-112.scope.d
  drwxr-xr-x 2 root root 160 Oct 4 15:34 /run/systemd/system/session-113.scope.d
  drwxr-xr-x 2 root root 160 Oct 4 15:34 /run/systemd/system/session-114.scope.d
  drwxr-xr-x 2 root root 160 Oct 4 15:34 /run/systemd/system/session-115.scope.d
  drwxr-xr-x 2 root root 160 Oct 4 15:34 /run/systemd/system/session-116.scope.d
  drwxr-xr-x 2 root root 160 Oct 4 15:34 /run/systemd/system/session-117.scope.d
  drwxr-xr-x 2 root root 160 Oct 4 15:34 /run/systemd/system/session-118.scope.d
  drwxr-xr-x 2 root root 160 Oct 4 15:34 /run/systemd/system/session-119.scope.d
  drwxr-xr-x 2 root root 160 Oct 4 15:34 /run/systemd/system/session-120.scope.d
  drwxr-xr-x 2 root root 160 Oct 4 15:34 /run/systemd/system/session-121.scope.d
  drwxr-xr-x 2 root root 160 Oct 4 15:34 /run/systemd/system/session-122.scope.d
  drwxr-xr-x 2 root root 160 Oct 4 15:34 /run/systemd/system/session-123.scope.d
  drwxr-xr-x 2 root root 160 Oct 4 15:34 /run/systemd/system/session-126.scope.d
  drwxr-xr-x 2 root root 160 Oct 4 15:34 /run/systemd/system/session-131.scope.d
  drwxr-xr-x 2 root root 160 Oct 4 15:34 /run/systemd/system/session-134.scope.d
  ...

  [Regression Potential]
  As the patches change the communication socket between dbus and systemd, possible regressions could cause systemd to not be notified of dbus events and vice-versa. We could see units not getting started properly, and communication between different services break down (e.g. between systemd-logind and other processes).

  In this case, the regression potential should be low as these patches
  have seen extensive testing both upstream and in more recent releases
  of Ubuntu. Nonetheless, these new packages will be rigorously tested
  through autopkgtest to avoid any possible Xenial-specific regressions.

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