I have set up a small samba server for 5 local users, it seems to work
fine, when I use smbclient from localhost with my user, everything
seems in order, but they report to me that after some time, they lose
write permissions of their files.

When i check their files as root, I see that they all have only r-x on
their files, and no w.

This was with the default setup of a centos smb.conf.

I later added createmask 0600 and direcotry mask 0700, but did not
change the behaviour.

What could cause this? Perhaps it's some obscure centos service that
does this. Or could it be any client applications that change
permissions of the files on the server?