svn commit: r347329 - head/sys/dev/random

[Conrad Meyer]

Author: cem
Date: Wed May  8 14:54:32 2019
New Revision: 347329
URL: https://svnweb.freebsd.org/changeset/base/347329

Log:
  random(4): Don't complain noisily when an entropy source is slow
  
  Mjg@ reports that RDSEED (r347239) causes a lot of logspam from this printf,
  and I don't feel that it is especially useful (even ratelimited).  There are
  many other quality/quantity checks we're not performing on entropy sources;
  lack of high frequency availability does not disqualify a good entropy
  source.
  
  There is some discussion in the linked Differential about what logging might
  be appropriate and/or polling policy for slower TRNG sources.  Please feel
  free to chime in if you have opinions.
  
  Reported by:	mjg
  Reviewed by:	markm, delphij
  Approved by:	secteam(delphij)
  X-MFC-With:	r347239
  Sponsored by:	Dell EMC Isilon
  Differential Revision:	https://reviews.freebsd.org/D20195

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

Modified: head/sys/dev/random/random_harvestq.c
==============================================================================
--- head/sys/dev/random/random_harvestq.c	Wed May  8 13:35:51 2019	(r347328)
+++ head/sys/dev/random/random_harvestq.c	Wed May  8 14:54:32 2019	(r347329)
@@ -235,15 +235,19 @@ random_sources_feed(void)
 		for (i = 0; i < p_random_alg_context->ra_poolcount*local_read_rate; i++) {
 			n = rrs->rrs_source->rs_read(entropy, sizeof(entropy));
 			KASSERT((n <= sizeof(entropy)), ("%s: rs_read returned too much data (%u > %zu)", __func__, n, sizeof(entropy)));
-			/* It would appear that in some circumstances (e.g. virtualisation),
-			 * the underlying hardware entropy source might not always return
-			 * random numbers. Accept this but make a noise. If too much happens,
-			 * can that source be trusted?
+			/*
+			 * Sometimes the HW entropy source doesn't have anything
+			 * ready for us.  This isn't necessarily untrustworthy.
+			 * We don't perform any other verification of an entropy
+			 * source (i.e., length is allowed to be anywhere from 1
+			 * to sizeof(entropy), quality is unchecked, etc), so
+			 * don't balk verbosely at slow random sources either.
+			 * There are reports that RDSEED on x86 metal falls
+			 * behind the rate at which we query it, for example.
+			 * But it's still a better entropy source than RDRAND.
 			 */
-			if (n == 0) {
-				printf("%s: rs_read for hardware device '%s' returned no entropy.\n", __func__, rrs->rrs_source->rs_ident);
+			if (n == 0)
 				continue;
-			}
 			random_harvest_direct(entropy, n, rrs->rrs_source->rs_source);
 		}
 	}