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Restore stricter SSH hostkeys permissions
This document represents a proposed Change. As part of the Changes process, proposals are publicly announced in order to receive community feedback. This proposal will only be implemented if approved by the Fedora Engineering Steering Committee.
Summary
We want to - drop a downstream-only patch to ssh permitting group-readable ssh host keys - drop a ssh_keys group - restore suid bit instead of sgid on a helper utility ssh-keysign
Owner
- Name: Dmitry Belyavskiy
- Email: dbelyavs@redhat.com
Current status
- Targeted release: Fedora Linux 38
- Last updated: 2022-12-08
- FESCo issue: <will be assigned by the Wrangler>
- Tracker bug: <will be assigned by the Wrangler>
- Release notes tracker: <will be assigned by the Wrangler>
Detailed Description
Many years ago we implemented the patch https://src.fedoraproject.org/rpms/openssh/c/1ddd0ee5 Unfortunately, as it was 11 years ago, we can't find the exact explanation where did the requirement come from. We think that we intended to increase security, but it probably caused more confusion than gain of the security over the years.
The patch allows have more relaxed permissions for the private keys than upstream OpenSSH permits - 0640 instead of 0600, and the key file must belong to the ssh_keys group. The ssh_keysign utility was simultaneously changed from suid root to sgid ssh_keys.
The side effect of this solution is that ssh with hostbased auth (HBA) started to fail after changing group ID ( with newgrp, etc.). In case of HBA ssh invokes ssh-keysign that does a lot of sanity checks including groups checks. The workaround is returning setuid bit instead of sgid, and we recommend it to our clients.
Some more information is available in https://bugzilla.redhat.com/show_bug.cgi?id=1498845
As this problem affects several clients, and it is a deviation from upstream (the similar patch was rejected by upstream), we want to drop this downstream patch in Fedora. We also can get rid of a designated ssh_keys group
The proposed changes are available https://src.fedoraproject.org/rpms/openssh/pull-request/37
Feedback
Benefit to Fedora
We reduce deviation from upstream and reduce maintenance cost for customers.
Scope
- Proposal owners:
- Other developers:
Developers managing SSH host keys beyond the standard scenarios will be affected.
- Release engineering: #Releng issue number
Not affected
- Policies and guidelines: N/A (not needed for this Change)
Not affected
- Trademark approval: N/A (not needed for this Change)
- Alignment with Objectives:
Upgrade/compatibility impact
The problem we expect is that after implementing the change we can lose the remote access to the hosts because sshd will reject starting because of group reading permissions. This should be covered by upgrade script, though we still may come across some issues, especially if you use host keys in non-standard location.
There is possible risk with config mgmt tools like puppet/ansible, that might be managing SSH host keys and their permissions/ownership.
How To Test
sshd successfully starts on the freshly installed systems and systems remain remotely accessable via SSH. sshd successfully restarts on the upgraded systems and systems remain remotely accessable via SSH.
User Experience
This change shouldn't be noticeable by users.
Dependencies
No other changes may affect this change.
Contingency Plan
Revert the patch
- Contingency mechanism: (What to do? Who will do it?) N/A (not a System Wide Change)
- Contingency deadline: N/A (not a System Wide Change)
- Blocks release? N/A (not a System Wide Change), Yes/No
Documentation
https://src.fedoraproject.org/rpms/openssh/pull-request/37 is a patch, and there should be a some RN item describing the change in details.
Release Notes
TBD