ip6_randomflowlabel: Avoid blocking if random(4) is not available

If kern.random.initial_seeding.bypass_before_seeding is disabled, random(4)
and arc4random(9) will block indefinitely until enough entropy is available
to initially seed Fortuna.

It seems that zero flowids are perfectly valid, so avoid blocking on random
until initial seeding takes place.

Discussed with:	bz (earlier revision)
Reviewed by:	thj
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D20011

1 files changed, 11 insertions, 0 deletions

diff --git a/sys/netinet6/ip6_id.c b/sys/netinet6/ip6_id.c
index f41df2a2e5a9..1d6c2c7d9668 100644
--- a/sys/netinet6/ip6_id.c
+++ b/sys/netinet6/ip6_id.c
@@ -89,6 +89,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/types.h>
 #include <sys/param.h>
 #include <sys/kernel.h>
+#include <sys/random.h>
 #include <sys/socket.h>
 #include <sys/libkern.h>
 
@@ -258,5 +259,15 @@ u_int32_t
 ip6_randomflowlabel(void)
 {
 
+	/*
+	 * It's ok to emit zero flow labels early, before random is available
+	 * (seeded).  RFC 6437:
+	 *
+	 * "A Flow Label of zero is used to indicate packets that have not been
+	 * labeled."
+	 */
+	if (__predict_false(!is_random_seeded()))
+		return (0);
+
 	return randomid(&randomtab_20) & 0xfffff;
 }