Bug 1610541

Summary:	systemd-sysusers creates system users without honoring login.defs limits
Product:	[Fedora] Fedora	Reporter:	Scott Shambarger <scott-fedora>
Component:	systemd	Assignee:	systemd-maint
Status:	CLOSED EOL	QA Contact:	Fedora Extras Quality Assurance <extras-qa>
Severity:	unspecified	Docs Contact:
Priority:	unspecified
Version:	28	CC:	lnykryn, msekleta, s, systemd-maint, zbyszek
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Last Closed:	2019-05-28 18:57:02 UTC	Type:	Bug
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Description Scott Shambarger 2018-07-31 21:28:52 UTC

Description of problem:
system-sysusers is creating system users with UIDs in regular user range. This allows those users to potentially bypass PAM restrictions and other limits based on UID range.

Version-Release number of selected component (if applicable):
systemd-238-9.git0e0aa59

How reproducible:
Anytime systemd-sysuser is used to create users, for example during systemd upgrade (systemd-coredump user) or install of dnsmasq (dnsmasq user)

Steps to Reproduce:
1. Edit login.defs so that UID_MIN is != 1000, eg "UID_MIN 500" (as was the default in earlier Fedora releases)... you could also update SYS_UID_MAX if that's defined.

2. Create sysuser conf file /run/sysusers.d/test-sysusers.conf containing

u test-sysusers - "Test sysusers"

3. Run: systemd-sysusers /run/sysusers.d/test-sysusers.conf

Actual results:

Creating group test-sysusers with gid 998.
Creating user test-sysusers (Test sysusers) with uid 998 and gid 998.

Expected results:

System user created with UID < 500, as useradd -r does and almost all RPM packages that create system users.

Additional info:

From looking at the systemd source, it appears to be setting a build config option SYSTEM_UID_MAX from the build machines /etc/login.defs SYS_UID_MAX value (with default 999 if missing) -- so the max is being set at compile time.

Even this is incorrect, as SYS_UID_MAX defaults to UID_MIN-1, not 999...

src/sysusers/sysusers.c should be modified to use the logic documented in login.defs(5): SYS_UID_MAX, or UID_MIN-1, or 999, in that order... and probably also honoring SYS_UID_MIN etc...

Comment 1 Scott Shambarger 2018-08-01 09:10:33 UTC

Submitted upstream: https://github.com/systemd/systemd/issues/9769

Comment 2 Scott Shambarger 2018-08-01 19:48:02 UTC

Lennart closed the upstream bug referring to https://github.com/systemd/systemd/blob/f4ea7552c109942b49cc1a3c37e959716fb8c453/doc/UIDS-GIDS.md - it states that systemd systems have their SYS_UID_MAX fixed at compile time (distro chooses it), and are not user configurable.

This pretty much makes systemd-sysusers incompatible with Fedora since Fedora systems do have configurable SYS_UID_MAX, and has changed it over the years (100, later 500, now 1000) - and so there are many, many upgraded systems that cannot easily re-number all their existing users...

So it appears that there are two options here:

a) create a patch for Fedora that make systemd-sysusers reference login.defs knowing the patch will not be accepted upstream

or,
b) fix all rpm.spec files that currently user sysusers to pre-create system users using useradd -r so that any installed sysusers.d/*.conf files will be a no-op.

Of course, there's always c) do nothing, and break security on older systems.

Personally, I'd suggest (b) since it's simple, and doesn't involve patching systemd, and is therefore easier to maintain... but does involve patching a number of spec files (and hence creating a number of new bugs :P)

Comment 3 Ben Cotton 2019-05-02 21:13:12 UTC

This message is a reminder that Fedora 28 is nearing its end of life.
On 2019-May-28 Fedora will stop maintaining and issuing updates for
Fedora 28. It is Fedora's policy to close all bug reports from releases
that are no longer maintained. At that time this bug will be closed as
EOL if it remains open with a Fedora 'version' of '28'.

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Comment 4 Ben Cotton 2019-05-28 18:57:02 UTC

Fedora 28 changed to end-of-life (EOL) status on 2019-05-28. Fedora 28 is
no longer maintained, which means that it will not receive any further
security or bug fix updates. As a result we are closing this bug.

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