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Commit dc12baac authored by Theodore Ts'o's avatar Theodore Ts'o
Browse files

random: use a different mixing algorithm for add_device_randomness() 



add_device_randomness() use of crng_fast_load() was highly
problematic.  Some callers of add_device_randomness() can pass in a
large amount of static information.  This would immediately promote
the crng_init state from 0 to 1, without really doing much to
initialize the primary_crng's internal state with something even
vaguely unpredictable.

Since we don't have the speed constraints of add_interrupt_randomness(),
we can do a better job mixing in the what unpredictability a device
driver or architecture maintainer might see fit to give us, and do it
in a way which does not bump the crng_init_cnt variable.

Also, since add_device_randomness() doesn't bump any entropy
accounting in crng_init state 0, mix the device randomness into the
input_pool entropy pool as well.  This is related to CVE-2018-1108.

Reported-by: default avatarJann Horn <jannh@google.com>
Fixes: ee7998c5 ("random: do not ignore early device randomness")
Cc: stable@kernel.org # 4.13+
Signed-off-by: default avatarTheodore Ts'o <tytso@mit.edu>
parent 43838a23

drivers/char/random.c

+51 −4
Original line number Diff line number Diff line
@@ -787,6 +787,10 @@ static void crng_initialize(struct crng_state *crng) 
	crng->init_time = jiffies - CRNG_RESEED_INTERVAL - 1;

}



/*

 * crng_fast_load() can be called by code in the interrupt service

 * path.  So we can't afford to dilly-dally.

 */

static int crng_fast_load(const char *cp, size_t len)

{

	unsigned long flags;
@@ -813,6 +817,51 @@ static int crng_fast_load(const char *cp, size_t len) 
	return 1;

}



/*

 * crng_slow_load() is called by add_device_randomness, which has two

 * attributes.  (1) We can't trust the buffer passed to it is

 * guaranteed to be unpredictable (so it might not have any entropy at

 * all), and (2) it doesn't have the performance constraints of

 * crng_fast_load().

 *

 * So we do something more comprehensive which is guaranteed to touch

 * all of the primary_crng's state, and which uses a LFSR with a

 * period of 255 as part of the mixing algorithm.  Finally, we do

 * *not* advance crng_init_cnt since buffer we may get may be something

 * like a fixed DMI table (for example), which might very well be

 * unique to the machine, but is otherwise unvarying.

 */

static int crng_slow_load(const char *cp, size_t len)

{

	unsigned long		flags;

	static unsigned char	lfsr = 1;

	unsigned char		tmp;

	unsigned		i, max = CHACHA20_KEY_SIZE;

	const char *		src_buf = cp;

	char *			dest_buf = (char *) &primary_crng.state[4];



	if (!spin_trylock_irqsave(&primary_crng.lock, flags))

		return 0;

	if (crng_init != 0) {

		spin_unlock_irqrestore(&primary_crng.lock, flags);

		return 0;

	}

	if (len > max)

		max = len;



	for (i = 0; i < max ; i++) {

		tmp = lfsr;

		lfsr >>= 1;

		if (tmp & 1)

			lfsr ^= 0xE1;

		tmp = dest_buf[i % CHACHA20_KEY_SIZE];

		dest_buf[i % CHACHA20_KEY_SIZE] ^= src_buf[i % len] ^ lfsr;

		lfsr += (tmp << 3) | (tmp >> 5);

	}

	spin_unlock_irqrestore(&primary_crng.lock, flags);

	return 1;

}



static void crng_reseed(struct crng_state *crng, struct entropy_store *r)

{

	unsigned long	flags;
@@ -981,10 +1030,8 @@ void add_device_randomness(const void *buf, unsigned int size) 
	unsigned long time = random_get_entropy() ^ jiffies;

	unsigned long flags;



	if (!crng_ready()) {

		crng_fast_load(buf, size);

		return;

	}

	if (!crng_ready() && size)

		crng_slow_load(buf, size);



	trace_add_device_randomness(size, _RET_IP_);

	spin_lock_irqsave(&input_pool.lock, flags);