The VM uses SIGUSR2 (by default, can be overridden via _JAVA_SR_SIGNUM) to implement suspend/resume on java threads. It sends, via pthread_kill, SIGUSR2 to targeted threads to interrupt them. It knows the target thread, and the target thread is always a VM-attached thread.

However, if SIGUSR2 gets sent from outside, any thread may receive the signal, and if the target thread is not attached to the VM (e.g. primordial), it is unable to handle it. The result is an assert (debug VM) or a crash (release VM). On my box, this can be reliably reproduced by sending SIGUSR2 to any VM.

This has been discussed here: https://mail.openjdk.org/pipermail/core-libs-dev/2022-June/091450.html

The proposed solutions range from "works as designed" (on the ground that sending arbitrary signals to the JVM is an error in itself, and we should rather crash hard and fast) to "lets catch and ignore the signal".