It appears that under some circumstances, like virtualisiation, the

'rdrand' instruction may occasionally not return random numbers, in
spite of looping attempts to do so. The reusult is a KASSERT/panic.

Reluctantly accept this state-of-affairs, but make a noise about it.
if this 'noise' spams the console, it may be time to discontinue
using that source.

This is written in a general way to account for /any/ source that
might not supply random numbers when required.

Submitted by:	jkh (report and slightly different fix)
Approved by:	so (/dev/random blanket)

1 files changed, 10 insertions, 1 deletions

diff --git a/sys/dev/random/random_harvestq.c b/sys/dev/random/random_harvestq.c
index aad6763..9993bbe 100644
--- a/sys/dev/random/random_harvestq.c
+++ b/sys/dev/random/random_harvestq.c
@@ -211,7 +211,16 @@ random_sources_feed(void)
 	LIST_FOREACH(rrs, &source_list, rrs_entries) {
 		for (i = 0; i < p_random_alg_context->ra_poolcount*(local_read_rate + 1); i++) {
 			n = rrs->rrs_source->rs_read(entropy, sizeof(entropy));
-			KASSERT((n > 0 && n <= sizeof(entropy)), ("very bad return from rs_read (= %d) in %s", n, __func__));
+			KASSERT((n <= sizeof(entropy)), ("%s: rs_read returned too much data (%d > %d) in %s", __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?
+			 */
+			if (n == 0) {
+				printf("%s: rs_read for hardware device '%s' returned no entropy.\n", __func__, rrs->rrs_source->rs_ident);
+				continue;
+			}
 			random_harvest_direct(entropy, n, (n*8)/2, rrs->rrs_source->rs_source);
 		}
 	}