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e712417936
Systemd provides an option for allocating DynamicUsers which we want to use in NixOS to harden service configuration. However, we discovered that the user wasn't allocated properly for services. After some digging this turned out to be, of course, a cache inconsistency problem. When a DynamicUser creation is performed, Systemd check beforehand whether the requested user already exists statically. If it does, it bails out. If it doesn't, systemd continues with allocating the user. However, by checking whether the user exists, nscd will store the fact that the user does not exist in it's negative cache. When the service tries to lookup what user is associated to its uid (By calling whoami, for example), it will try to consult libnss_systemd.so However this will read from the cache and tell report that the user doesn't exist, and thus will return that there is no user associated with the uid. It will continue to do so for the cache duration time. If the service doesn't immediately looks up its username, this bug is not triggered, as the cache will be invalidated around this time. However, if the service is quick enough, it might end up in a situation where it's incorrectly reported that the user doesn't exist. Preferably, we would not be using nscd at all. But we need to use it because glibc reads nss modules from /etc/nsswitch.conf by looking relative to the global LD_LIBRARY_PATH. Because LD_LIBRARY_PATH is not set globally (as that would lead to impurities and ABI issues), glibc will fail to find any nss modules. Instead, as a hack, we start up nscd with LD_LIBRARY_PATH set for only that service. Glibc will forward all nss syscalls to nscd, which will then respect the LD_LIBRARY_PATH and only read from locations specified in the NixOS config. we can load nss modules in a pure fashion. However, I think by accident, we just copied over the default settings of nscd, which actually caches user and group lookups. We already disable this when sssd is enabled, as this interferes with the correct working of libnss_sss.so as it already does its own caching of LDAP requests. (See https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/6/html/deployment_guide/usingnscd-sssd) Because nscd caching is now also interferring with libnss_systemd.so and probably also with other nsss modules, lets just pre-emptively disable caching for now for all options related to users and groups, but keep it for caching hosts ans services lookups. Note that we can not just put in /etc/nscd.conf: enable-cache passwd no As this will actually cause glibc to _not_ forward the call to nscd at all, and thus never reach the nss modules. Instead we set the negative and positive cache ttls to 0 seconds as a workaround. This way, Glibc will always forward requests to nscd, but results will never be cached. Fixes #50273 |
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