useradd: race conditions with --create-home

[snejok]

When creating user with --create-home, for example this way:

# useradd -m test1 ; useradd -m test1
useradd: user 'test1' already exists

# grep test1 /etc/passwd
test1:x:1097:1097::/home/test1:/bin/sh

# stat -c '%u %U' /home/test1
1097 test1

second useradd failed, ownership is OK

but if for some reason two processes try to create the same user simultanuosly, there is a race condition:

# useradd -m test2 & useradd -m test2
useradd: warning: the home directory /home/test2 already exists.
useradd: Not copying any file from skel directory into it.

# grep test2 /etc/passwd
test2:x:1099:1099::/home/test2:/bin/sh

# stat -c '%U' /home/test2
1098 UNKNOWN

ownership is bad, because uid is wrong

Tested on:
ubuntu 22.04
ubuntu 24.04
ubuntu 26.04

I have local only installation, no ldap or something like that

egrep '(passwd|group|shadow)' /etc/nsswitch.conf

passwd: files systemd
group: files systemd
shadow: files systemd
gshadow: files systemd

[alejandro-colomar]

Thanks!

I haven't looked at the code, but I suspect it's due to some issues in the locking code.

The PR #1590, which is scoped to just usermod, should improve that for usermod. The plan is to later do the same thing for all the other programs, and that might (or might not) fix this bug.

If you have any other information about this bug please let us know, and if you want to work on a fix, feel free to open a PR.