git: 856230b94a54 - stable/12 - Fortuna: Fix false negatives in is_random_seeded()

The branch stable/12 has been updated by obrien:

URL: https://cgit.FreeBSD.org/src/commit/?id=856230b94a544161dbb71406471bf15d48473cfd

commit 856230b94a544161dbb71406471bf15d48473cfd
Author:     Conrad Meyer <cem@FreeBSD.org>
AuthorDate: 2019-05-13 19:35:35 +0000
Commit:     David E. O'Brien <obrien@FreeBSD.org>
CommitDate: 2022-02-13 06:39:46 +0000

    Fortuna: Fix false negatives in is_random_seeded()
    
    (1) We may have had sufficient entropy to consider Fortuna seeded, but the
    random_fortuna_seeded() function would produce a false negative if
    fs_counter was still zero.  This condition could arise after
    random_harvestq_prime() processed the /boot/entropy file and before any
    read-type operation invoked "pre_read()."  Fortuna's fs_counter variable is
    only incremented (if certain conditions are met) by reseeding, which is
    invoked by random_fortuna_pre_read().
    
    is_random_seeded(9) was introduced in r346282, but the function was unused
    prior to r346358, which introduced this regression.  The regression broke
    initial seeding of arc4random(9) and broke periodic reseeding[A], until something
    other than arc4random(9) invoked read_random(9) or read_random_uio(9) directly.
    (Such as userspace getrandom(2) or read(2) of /dev/random.  By default,
    /etc/rc.d/random does this during multiuser start-up.)
    
    (2) The conditions under which Fortuna will reseed (including initial seeding)
    are: (a) sufficient "entropy" (by sheer byte count; default 64) is collected
    in the zeroth pool (of 32 pools), and (b) it has been at least 100ms since
    the last reseed (to prevent trivial DoS; part of FS&K design).  Prior to
    this revision, initial seeding might have been prevented if the reseed
    function was invoked during the first 100ms of boot.
    
    This revision addresses both of these issues.  If random_fortuna_seeded()
    observes a zero fs_counter, it invokes random_fortuna_pre_read() and checks
    again.  This addresses the problem where entropy actually was sufficient,
    but nothing had attempted a read -> pre_read yet.
    
    The second change is to disable the 100ms reseed guard when Fortuna has
    never been seeded yet (fs_lasttime == 0).  The guard is intended to prevent
    gratuitous subsequent reseeds, not initial seeding!
    
    Machines running CURRENT between r346358 and this revision are encouraged to
    refresh when possible.  Keys generated by userspace with /dev/random or
    getrandom(9) during this timeframe are safe, but any long-term session keys
    generated by kernel arc4random consumers are potentially suspect.
    
    [A]: Broken in the sense that is_random_seeded(9) false negatives would cause
    arc4random(9) to (re-)seed with weak entropy (SHA256(cyclecount ||
    FreeBSD_version)).
    
    (cherry picked from commit e8e1f0b4204dc766c4c5af5f259af11f9b0e0452)
---
 sys/dev/random/fortuna.c | 30 +++++++++++++++++++++++++-----
 1 file changed, 25 insertions(+), 5 deletions(-)

diff --git a/sys/dev/random/fortuna.c b/sys/dev/random/fortuna.c
index a6cfccaa3a1a..e0991d411df3 100644
--- a/sys/dev/random/fortuna.c
+++ b/sys/dev/random/fortuna.c
@@ -130,6 +130,7 @@ static uint8_t zero_region[RANDOM_ZERO_BLOCKSIZE];
 static void random_fortuna_pre_read(void);
 static void random_fortuna_read(uint8_t *, u_int);
 static bool random_fortuna_seeded(void);
+static bool random_fortuna_seeded_internal(void);
 static void random_fortuna_process_event(struct harvest_event *);
 static void random_fortuna_init_alg(void *);
 static void random_fortuna_deinit_alg(void *);
@@ -277,7 +278,7 @@ random_fortuna_reseed_internal(uint32_t *entropy_data, u_int blockcount)
 
 	RANDOM_RESEED_ASSERT_LOCK_OWNED();
 
-	seeded = random_fortuna_seeded();
+	seeded = random_fortuna_seeded_internal();
 	if (seeded) {
 		randomdev_getkey(&fortuna_state.fs_key, &keymaterial, &keysz);
 		KASSERT(keysz == RANDOM_KEYSIZE, ("%s: key size %zu not %u",
@@ -377,8 +378,12 @@ random_fortuna_pre_read(void)
 
 	if (fortuna_state.fs_pool[0].fsp_length < fortuna_state.fs_minpoolsize
 #ifdef _KERNEL
-	    /* FS&K - Use 'getsbinuptime()' to prevent reseed-spamming. */
-	    || (now - fortuna_state.fs_lasttime <= SBT_1S/10)
+	    /*
+	     * FS&K - Use 'getsbinuptime()' to prevent reseed-spamming, but do
+	     * not block initial seeding (fs_lasttime == 0).
+	     */
+	    || (__predict_true(fortuna_state.fs_lasttime != 0) &&
+		now - fortuna_state.fs_lasttime <= SBT_1S/10)
 #endif
 	) {
 		RANDOM_RESEED_UNLOCK();
@@ -460,7 +465,13 @@ SYSCTL_BOOL(_kern_random, OID_AUTO, block_seeded_status, CTLFLAG_RWTUN,
     "unavailable.");
 #endif
 
-bool
+static bool
+random_fortuna_seeded_internal(void)
+{
+	return (!uint128_is_zero(fortuna_state.fs_counter));
+}
+
+static bool
 random_fortuna_seeded(void)
 {
 
@@ -469,5 +480,14 @@ random_fortuna_seeded(void)
 		return (false);
 #endif
 
-	return (!uint128_is_zero(fortuna_state.fs_counter));
+	if (__predict_true(random_fortuna_seeded_internal()))
+		return (true);
+
+	/*
+	 * Maybe we have enough entropy in the zeroth pool but just haven't
+	 * kicked the initial seed step.  Do so now.
+	 */
+	random_fortuna_pre_read();
+
+	return (random_fortuna_seeded_internal());
 }