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Commit 75d95c1b authored by Jason A. Donenfeld's avatar Jason A. Donenfeld Committed by Greg Kroah-Hartman
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random: do crng pre-init loading in worker rather than irq 



commit c2a7de4feb6e09f23af7accc0f882a8fa92e7ae5 upstream.

Taking spinlocks from IRQ context is generally problematic for
PREEMPT_RT. That is, in part, why we take trylocks instead. However, a
spin_try_lock() is also problematic since another spin_lock() invocation
can potentially PI-boost the wrong task, as the spin_try_lock() is
invoked from an IRQ-context, so the task on CPU (random task or idle) is
not the actual owner.

Additionally, by deferring the crng pre-init loading to the worker, we
can use the cryptographic hash function rather than xor, which is
perhaps a meaningful difference when considering this data has only been
through the relatively weak fast_mix() function.

The biggest downside of this approach is that the pre-init loading is
now deferred until later, which means things that need random numbers
after interrupts are enabled, but before workqueues are running -- or
before this particular worker manages to run -- are going to get into
trouble. Hopefully in the real world, this window is rather small,
especially since this code won't run until 64 interrupts had occurred.

Cc: Sultan Alsawaf <sultan@kerneltoast.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Eric Biggers <ebiggers@kernel.org>
Cc: Theodore Ts'o <tytso@mit.edu>
Acked-by: default avatarSebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: default avatarDominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: default avatarJason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 219c84fe

drivers/char/random.c

+19 −46
Original line number Diff line number Diff line
@@ -441,10 +441,6 @@ static void crng_make_state(u32 chacha_state[CHACHA_BLOCK_SIZE / sizeof(u32)], 
 * boot time when it's better to have something there rather than

 * nothing.

 *

 * There are two paths, a slow one and a fast one. The slow one

 * hashes the input along with the current key. The fast one simply

 * xors it in, and should only be used from interrupt context.

 *

 * If account is set, then the crng_init_cnt counter is incremented.

 * This shouldn't be set by functions like add_device_randomness(),

 * where we can't trust the buffer passed to it is guaranteed to be
@@ -453,19 +449,15 @@ static void crng_make_state(u32 chacha_state[CHACHA_BLOCK_SIZE / sizeof(u32)], 
 * Returns the number of bytes processed from input, which is bounded

 * by CRNG_INIT_CNT_THRESH if account is true.

 */

static size_t crng_pre_init_inject(const void *input, size_t len,

				   bool fast, bool account)

static size_t crng_pre_init_inject(const void *input, size_t len, bool account)

{

	static int crng_init_cnt = 0;

	struct blake2s_state hash;

	unsigned long flags;



	if (fast) {

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

			return 0;

	} else {

		spin_lock_irqsave(&base_crng.lock, flags);

	}

	blake2s_init(&hash, sizeof(base_crng.key));



	spin_lock_irqsave(&base_crng.lock, flags);

	if (crng_init != 0) {

		spin_unlock_irqrestore(&base_crng.lock, flags);

		return 0;
@@ -474,21 +466,9 @@ static size_t crng_pre_init_inject(const void *input, size_t len, 
	if (account)

		len = min_t(size_t, len, CRNG_INIT_CNT_THRESH - crng_init_cnt);



	if (fast) {

		const u8 *src = input;

		size_t i;



		for (i = 0; i < len; ++i)

			base_crng.key[(crng_init_cnt + i) %

				      sizeof(base_crng.key)] ^= src[i];

	} else {

		struct blake2s_state hash;



		blake2s_init(&hash, sizeof(base_crng.key));

	blake2s_update(&hash, base_crng.key, sizeof(base_crng.key));

	blake2s_update(&hash, input, len);

	blake2s_final(&hash, base_crng.key);

	}



	if (account) {

		crng_init_cnt += len;
@@ -1029,7 +1009,7 @@ void add_device_randomness(const void *buf, size_t size) 
	unsigned long flags, now = jiffies;



	if (crng_init == 0 && size)

		crng_pre_init_inject(buf, size, false, false);

		crng_pre_init_inject(buf, size, false);



	spin_lock_irqsave(&input_pool.lock, flags);

	_mix_pool_bytes(&cycles, sizeof(cycles));
@@ -1150,7 +1130,7 @@ void add_hwgenerator_randomness(const void *buffer, size_t count, 
				size_t entropy)

{

	if (unlikely(crng_init == 0)) {

		size_t ret = crng_pre_init_inject(buffer, count, false, true);

		size_t ret = crng_pre_init_inject(buffer, count, true);

		mix_pool_bytes(buffer, ret);

		count -= ret;

		buffer += ret;
@@ -1290,8 +1270,14 @@ static void mix_interrupt_randomness(struct work_struct *work) 
	fast_pool->last = jiffies;

	local_irq_enable();



	if (unlikely(crng_init == 0)) {

		crng_pre_init_inject(pool, sizeof(pool), true);

		mix_pool_bytes(pool, sizeof(pool));

	} else {

		mix_pool_bytes(pool, sizeof(pool));

		credit_entropy_bits(1);

	}



	memzero_explicit(pool, sizeof(pool));

}
@@ -1324,24 +1310,11 @@ void add_interrupt_randomness(int irq) 
	fast_mix(fast_pool->pool32);

	new_count = ++fast_pool->count;



	if (unlikely(crng_init == 0)) {

		if (new_count >= 64 &&

		    crng_pre_init_inject(fast_pool->pool32, sizeof(fast_pool->pool32),

					 true, true) > 0) {

			fast_pool->count = 0;

			fast_pool->last = now;

			if (spin_trylock(&input_pool.lock)) {

				_mix_pool_bytes(&fast_pool->pool32, sizeof(fast_pool->pool32));

				spin_unlock(&input_pool.lock);

			}

		}

		return;

	}



	if (new_count & MIX_INFLIGHT)

		return;



	if (new_count < 64 && !time_after(now, fast_pool->last + HZ))

	if (new_count < 64 && (!time_after(now, fast_pool->last + HZ) ||

			       unlikely(crng_init == 0)))

		return;



	if (unlikely(!fast_pool->mix.func))