svn commit: r355014 - head/sys/dev/random

[Conrad Meyer]

Author: cem
Date: Fri Nov 22 19:30:31 2019
New Revision: 355014
URL: https://svnweb.freebsd.org/changeset/base/355014

Log:
  random/ivy: Provide mechanism to read independent seed values from rdrand
  
  On x86 platforms with the intrinsic, rdrand is a deterministic bit generator
  (AES-CTR) seeded from an entropic source.  On x86 platforms with rdseed, it
  is something closer to the upstream entropic source.  (There is more nuance;
  a block diagram is provided in [1].)
  
  On devices with rdrand and without rdseed, there is no good intrinsic for
  acecssing the good entropic soure directly.  However, the DRBG is guaranteed
  to reseed every 8 kB on these platforms.  As a conservative option, on such
  hardware we can read an extra 7.99kB samples every time we want a sample
  from an independent seed.
  
  As one can imagine, this drastically slows the effective read rate of
  RDRAND (a factor of 1024 on amd64 and 2048 on ia32).  Microbenchmarks on AMD
  Zen (has RDSEED) show an RDRAND rate of 25 MB/s and Intel Haswell (no
  RDSEED) show RDRAND of 170 MB/s.  This would reduce the read rate on Haswell
  to ~170 kB/s (at 100% CPU).  random(4)'s harvestq thread periodically
  "feeds" from pure sources in amounts of 128-1024 bytes.  On Haswell,
  enabling this feature increases the CPU time of RDRAND in each "feed" from
  approximately 0.7-6 µs to 0.7-6 ms.
  
  Because there is some performance penalty to this more conservative option,
  a knob is provided to enable the change.  The change does not affect
  platforms with RDSEED.
  
  [1]: https://software.intel.com/en-us/articles/intel-digital-random-number-generator-drng-software-implementation-guide#inpage-nav-4-2
  
  Approved by:	csprng(delphij, markm)
  Differential Revision:	https://reviews.freebsd.org/D22455

Modified:
  head/sys/dev/random/ivy.c

Modified: head/sys/dev/random/ivy.c
==============================================================================
--- head/sys/dev/random/ivy.c	Fri Nov 22 18:55:27 2019	(r355013)
+++ head/sys/dev/random/ivy.c	Fri Nov 22 19:30:31 2019	(r355014)
@@ -40,6 +40,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/malloc.h>
 #include <sys/module.h>
 #include <sys/random.h>
+#include <sys/sysctl.h>
 #include <sys/systm.h>
 
 #include <machine/md_var.h>
@@ -59,23 +60,46 @@ static struct random_source random_ivy = {
 	.rs_read = random_ivy_read
 };
 
+SYSCTL_NODE(_kern_random, OID_AUTO, rdrand, CTLFLAG_RW, 0,
+    "rdrand (ivy) entropy source");
+static bool acquire_independent_seed_samples = false;
+SYSCTL_BOOL(_kern_random_rdrand, OID_AUTO, rdrand_independent_seed,
+    CTLFLAG_RWTUN, &acquire_independent_seed_samples, 0,
+    "If non-zero, use more expensive and slow, but safer, seeded samples "
+    "where RDSEED is not present.");
+
 static bool
 x86_rdrand_store(u_long *buf)
 {
-	u_long rndval;
+	u_long rndval, seed_iterations, i;
 	int retry;
 
-	retry = RETRY_COUNT;
-	__asm __volatile(
-	    "1:\n\t"
-	    "rdrand	%1\n\t"	/* read randomness into rndval */
-	    "jc		2f\n\t" /* CF is set on success, exit retry loop */
-	    "dec	%0\n\t" /* otherwise, retry-- */
-	    "jne	1b\n\t" /* and loop if retries are not exhausted */
-	    "2:"
-	    : "+r" (retry), "=r" (rndval) : : "cc");
+	/* Per [1], "§ 5.2.6 Generating Seeds from RDRAND,"
+	 * machines lacking RDSEED will guarantee RDRAND is reseeded every 8kB
+	 * of generated output.
+	 *
+	 * [1]: https://software.intel.com/en-us/articles/intel-digital-random-number-generator-drng-software-implementation-guide#inpage-nav-6-8
+	 */
+	if (acquire_independent_seed_samples)
+		seed_iterations = 8 * 1024 / sizeof(*buf);
+	else
+		seed_iterations = 1;
+
+	for (i = 0; i < seed_iterations; i++) {
+		retry = RETRY_COUNT;
+		__asm __volatile(
+		    "1:\n\t"
+		    "rdrand	%1\n\t"	/* read randomness into rndval */
+		    "jc		2f\n\t" /* CF is set on success, exit retry loop */
+		    "dec	%0\n\t" /* otherwise, retry-- */
+		    "jne	1b\n\t" /* and loop if retries are not exhausted */
+		    "2:"
+		    : "+r" (retry), "=r" (rndval) : : "cc");
+		if (retry == 0)
+			return (false);
+	}
 	*buf = rndval;
-	return (retry != 0);
+	return (true);
 }
 
 static bool