[Bug 1966591] Re: ssh-keygen -R changes known_hosts file permissions (mode)

[Sergio Durigan Junior]

Verification for Focal:

First, verifying that the bug manifests with the current package:

# apt policy openssh-client
openssh-client:
  Installed: 1:8.2p1-4ubuntu0.4
  Candidate: 1:8.2p1-4ubuntu0.4
  Version table:
 *** 1:8.2p1-4ubuntu0.4 500
        500 http://archive.ubuntu.com/ubuntu focal-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     1:8.2p1-4ubuntu0.2 500
        500 http://security.ubuntu.com/ubuntu focal-security/main amd64 Packages
     1:8.2p1-4 500
        500 http://archive.ubuntu.com/ubuntu focal/main amd64 Packages
# ssh-keygen -R github.com -f test_known_hosts
# Host github.com found: line 1
# Host github.com found: line 2
# Host github.com found: line 3
test_known_hosts updated.
Original contents retained as test_known_hosts.old
# stat test_known_hosts | grep ^Access
Access: (0600/-rw-------)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2022-04-13 21:54:40.698183232 +0000

Now, enabling -proposed and verifying that the new package fixes the
bug:

# apt policy openssh-client
openssh-client:
  Installed: 1:8.2p1-4ubuntu0.5
  Candidate: 1:8.2p1-4ubuntu0.5
  Version table:
 *** 1:8.2p1-4ubuntu0.5 500
        500 http://archive.ubuntu.com/ubuntu focal-proposed/main amd64 Packages
        100 /var/lib/dpkg/status
     1:8.2p1-4ubuntu0.4 500
        500 http://archive.ubuntu.com/ubuntu focal-updates/main amd64 Packages
     1:8.2p1-4ubuntu0.2 500
        500 http://security.ubuntu.com/ubuntu focal-security/main amd64 Packages
     1:8.2p1-4 500
        500 http://archive.ubuntu.com/ubuntu focal/main amd64 Packages
# ssh-keygen -R github.com -f test_known_hosts
# Host github.com found: line 1
# Host github.com found: line 2
# Host github.com found: line 3
test_known_hosts updated.
Original contents retained as test_known_hosts.old
# stat test_known_hosts | grep ^Access
Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2022-04-13 21:57:25.676987718 +0000


The bug has been fixed.

** Tags removed: server-todo verification-needed verification-needed-focal
** Tags added: verification-done-focal

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https://bugs.launchpad.net/bugs/1966591

Title:
  ssh-keygen -R changes known_hosts file permissions (mode)

Status in portable OpenSSH:
  Unknown
Status in openssh package in Ubuntu:
  Fix Released
Status in openssh source package in Bionic:
  Fix Committed
Status in openssh source package in Focal:
  Fix Committed
Status in openssh source package in Impish:
  Fix Released
Status in openssh source package in Jammy:
  Fix Released

Bug description:
  [Impact]

  When using "ssh-keygen -R" to remove a host from "known_hosts" the
  command changes permissions on the file.  This can cause problems
  particularly when used on the global "known_hosts" file
  (/etc/ssh/ssh_known_hosts), because then only root can read it.
  Programs running non-interactively as non-root users suddenly fail to
  SSH and it's not immediately obvious why.

  [Test Plan]

  The problem happens on Bionic and Focal.

  $ lxc launch ubuntu-daily:focal openssh-bug1966591
  $ lxc shell openssh-bug1966591
  # ssh-keyscan github.com > test_known_hosts
  # chmod 644 test_known_hosts
  # ssh-keygen -R github.com -f test_known_hosts
  # stat test_known_hosts
  ...
  Access: (0600/-rw-------) ...
  ...

  [Where problems could occur]

  The upstream patch is very simple and it is unlikely that it will
  cause any regressions.  An indirect problem that could occur is that
  users might expect to see a more strict set of permissions on a
  "known_hosts" file after using "ssh-keygen -R", but arguably this is
  not defined behaviour and should not be relied upon.  Of course, there
  is always a (very) small risk of introducing problems when rebuilding
  packages using newer versions of its dependencies (especially on
  Bionic, because it's older).

  [Original Description]

  When I use ssh-keygen -R to remove a host from known_hosts it changes
  permissions on the file. This causes problems particularly when used
  on the global known hosts file (/etc/ssh/ssh_known_hosts), because
  then only root can read it. Programs running non-interactively as non-
  root users suddenly fail to SSH and it's not immediately obvious why.

  To reproduce:

  $ ssh-keyscan github.com >test_known_hosts
  $ chmod 741 test_known_hosts
  $ ssh-keygen -R github.com -f test_known_hosts
  $ stat test_known_hosts
  ...
  Access: (0600/-rw-------) ...

  Expected behavior: file permissions remain unchanged (mode 0741 in
  this example).

  $ lsb_release -rd
  Description:	Ubuntu 18.04.6 LTS
  Release:	18.04

  $ apt-cache policy openssh-client
  openssh-client:
    Installed: 1:7.6p1-4ubuntu0.6

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