[rng] Add RTC-based entropy source

The RTC-based entropy source uses the nanosecond-scale CPU TSC to
measure the time between two 1kHz interrupts generated by the CMOS
RTC.  In a physical machine these clocks are driven from independent
crystals, resulting in some observable clock drift.  In a virtual
machine, the CMOS RTC is typically emulated using host-OS
constructions such as SIGALRM.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

2 files changed, 64 insertions, 0 deletions

diff --git a/src/arch/i386/include/bits/entropy.h b/src/arch/i386/include/bits/entropy.h
index db8ba18e0..6dcceec6d 100644
--- a/src/arch/i386/include/bits/entropy.h
+++ b/src/arch/i386/include/bits/entropy.h
@@ -9,4 +9,6 @@
 
 FILE_LICENCE ( GPL2_OR_LATER );
 
+#include <ipxe/rtc_entropy.h>
+
 #endif /* _BITS_ENTROPY_H */
diff --git a/src/arch/i386/include/ipxe/rtc_entropy.h b/src/arch/i386/include/ipxe/rtc_entropy.h
new file mode 100644
index 000000000..6c3cf2104
--- /dev/null
+++ b/src/arch/i386/include/ipxe/rtc_entropy.h
@@ -0,0 +1,62 @@
+#ifndef _IPXE_RTC_ENTROPY_H
+#define _IPXE_RTC_ENTROPY_H
+
+/** @file
+ *
+ * RTC-based entropy source
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER );
+
+#include <stdint.h>
+
+#ifdef ENTROPY_RTC
+#define ENTROPY_PREFIX_rtc
+#else
+#define ENTROPY_PREFIX_rtc __rtc_
+#endif
+
+/**
+ * min-entropy per sample
+ *
+ * @ret min_entropy	min-entropy of each sample
+ */
+static inline __always_inline double
+ENTROPY_INLINE ( rtc, min_entropy_per_sample ) ( void ) {
+
+	/* The min-entropy has been measured on several platforms
+	 * using the entropy_sample test code.  Modelling the samples
+	 * as independent, and using a confidence level of 99.99%, the
+	 * measurements were as follows:
+	 *
+	 *    qemu-kvm		: 7.38 bits
+	 *    VMware		: 7.46 bits
+	 *    Physical hardware	: 2.67 bits
+	 *
+	 * We choose the lowest of these (2.67 bits) and apply a 50%
+	 * safety margin to allow for some potential non-independence
+	 * of samples.
+	 */
+	return 1.3;
+}
+
+extern uint8_t rtc_sample ( void );
+
+/**
+ * Get noise sample
+ *
+ * @ret noise		Noise sample
+ * @ret rc		Return status code
+ */
+static inline __always_inline int
+ENTROPY_INLINE ( rtc, get_noise ) ( noise_sample_t *noise ) {
+
+	/* Get sample */
+	*noise = rtc_sample();
+
+	/* Always successful */
+	return 0;
+}
+
+#endif /* _IPXE_RTC_ENTROPY_H */