file: /proc/sys/kernel/dmesg_restrict
variable: kernel.dmesg_restrict
Official reference

This toggle indicates whether unprivileged users are prevented from using dmesg(8) to view messages from the kernel’s log buffer. When dmesg_restrict is set to (0) there are no restrictions. When dmesg_restrict is set set to (1), users must have CAP_SYSLOG to use dmesg(8).

The kernel config option CONFIG_SECURITY_DMESG_RESTRICT sets the default value of dmesg_restrict.