Summary

When debug.rdrand is non-zero rdrand_rng_harvest kprintf-s first 4 bytes of raw RDRAND output to kernel message buffer (rdrand.c:175-183). Those 4 bytes are same bytes simultaneously fed into CSPRNG entropy pool via add_buffer_randomness_src at rdrand.c:171. Kernel message buffer readable by all users by default (subr_prf.c:126 static int unprivileged_read_msgbuf=1; sysctl_kern_msgbuf only enforces priv when 0). Debug flag set via SYSCTL_INT (rdrand.c:75 debug.rdrand CTLFLAG_RW) requires root but LOGGED bytes then readable by any user. Impact: partial view of raw entropy source input; practical negligible because only 4 of 16 bytes per harvest exposed CSPRNG mixes RDRAND with interrupt/timing/thread/seeding entropy. Requires root to enable debug.rdrand (privileged) default unprivileged_read_msgbuf=1 for read side. Defense-in-depth concern: raw entropy-source inputs should not be observable.