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Fortuna: Fix false negatives in is_random_seeded()

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(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)).

PR:		237869
Reported by:	delphij, dim
Reviewed by:	delphij
Approved by:	secteam(delphij)
X-MFC-With:	r346282, r346358 (if ever)
Security:	yes
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D20239
This commit is contained in:
Conrad Meyer 2019-05-13 19:35:35 +00:00
parent aa0a893384
commit e8e1f0b420
1 changed files with 25 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

30
sys/dev/random/fortuna.c
View File

@ -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());
}