Previously we'd used the existance of a specific AF_UNIX socket in the
abstract namespace as lock for disabling lookup recursions. (for
breaking out of the loop: userdb synthesized from nss → nss synthesized
from userdb → userdb synthesized from nss → …)
I did it like that because it promised to work the same both in static
and in dynmically linked environments and is accessible easily from any
programming language.
However, it has a weakness regarding reuse attacks: the socket is
securely hashed (siphash) from the thread ID in combination with the
AT_RANDOM secret. Thus it should not be guessable from an attacker in
advance. That's only true if a thread takes the lock only once and
keeps it forever. However, if a thread takes and releases it multiple
times an attacker might monitor that and quickly take the lock
after the first iteration for follow-up iterations.
It's not a big issue given that userdb (as the primary user for this)
never released the lock and we never made the concept a public
interface, and it was only included in one release so far, but it's
something that deserves fixing. (moreover it's a local DoS only, only
permitting to disable native userdb lookups)
With this rework the libnss_systemd.so.2 module will now export two
additional symbols. These symbols are not used by glibc, but can be used
by arbitrary programs: one can be used to disable nss-systemd, the other
to check if it is currently disabled.
The lock is per-thread. It's slightly less pretty, since it requires
people to manually link against C code via dlopen()/dlsym(), but it
should work safely without the aforementioned weakness.
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