svn commit: r339492 - head/sys/dev/random

[Conrad Meyer]

Author: cem
Date: Sat Oct 20 21:09:12 2018
New Revision: 339492
URL: https://svnweb.freebsd.org/changeset/base/339492

Log:
  Fortuna: Fix a race to prevent reseed spamming
  
  If multiple threads enter fortuna_pre_read contemporaneously, such as via
  read(2) or getrandom(2), they could race to check how long it has been since
  the last update due to a TOCTOU problem with 'now'.
  
  Here is an example problematic execution:
  
  Thread A:                       Thread B:
  now_A = getsbinuptime();
                                  now_B = getsbinuptime();  // now_B > now_A
                                  RANDOM_RESEED_LOCK();
                                  if (now - fs_lasttime > SBT_1S/10) {
                                          fs_lasttime = now;
                                          ... // reseed
                                  }
                                  RANDOM_RESEED_UNLOCK();
  RANDOM_RESEED_LOCK();
  if (now_A - fs_lasttime > SBT_1S/10)  // now_A - fs_lasttime underflows
          fs_lasttime = now_A;
          ... // reseed again, despite less than 100ms elapsing
  }
  RANDOM_RESEED_UNLOCK();
  
  To resolve the race, simply check the current time after we win the lock
  race.
  
  If getsbinuptime is perceived to be expensive, another option might be to
  just accept the race and validate that fs_lasttime isn't "in the future."
  (It should be within the last ~2^31 seconds out of ~2^32 seconds
  representable duration.)
  
  Reviewed by:	delphij, markm
  Approved by:	secteam (delphij)
  Sponsored by:	Dell EMC Isilon
  Differential Revision:	https://reviews.freebsd.org/D16984

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

Modified: head/sys/dev/random/fortuna.c
==============================================================================
--- head/sys/dev/random/fortuna.c	Sat Oct 20 21:06:15 2018	(r339491)
+++ head/sys/dev/random/fortuna.c	Sat Oct 20 21:09:12 2018	(r339492)
@@ -368,11 +368,11 @@ random_fortuna_pre_read(void)
 	u_int i;
 
 	KASSERT(fortuna_state.fs_minpoolsize > 0, ("random: Fortuna threshold must be > 0"));
+	RANDOM_RESEED_LOCK();
 #ifdef _KERNEL
 	/* FS&K - Use 'getsbinuptime()' to prevent reseed-spamming. */
 	now = getsbinuptime();
 #endif
-	RANDOM_RESEED_LOCK();
 
 	if (fortuna_state.fs_pool[0].fsp_length >= fortuna_state.fs_minpoolsize
 #ifdef _KERNEL