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Date: Thu, 24 Feb 2022 12:31:22 +0100
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: LKML <linux-kernel@...r.kernel.org>,
Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
Sebastian Siewior <bigeasy@...utronix.de>
Cc: Dominik Brodowski <linux@...inikbrodowski.net>,
Sultan Alsawaf <sultan@...neltoast.com>,
Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <peterz@...radead.org>,
"Theodore Ts'o" <tytso@....edu>, Eric Biggers <ebiggers@...nel.org>
Subject: Re: [PATCH] random: do crng pre-init loading in worker rather than irq
CC +Eric
On Wed, Feb 23, 2022 at 7:55 PM Jason A. Donenfeld <Jason@...c4.com> wrote:
>
> Taking spinlocks from IRQ context is problematic for PREEMPT_RT. That
> is, in part, why we take trylocks instead. But apparently this still
> trips up various lock dependency analyzers. That seems like a bug in the
> analyzers that should be fixed, rather than having to change things
> here.
>
> But maybe there's another reason to change things up: 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: Dominik Brodowski <linux@...inikbrodowski.net>
> Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Cc: Sultan Alsawaf <sultan@...neltoast.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Theodore Ts'o <tytso@....edu>
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> ---
> drivers/char/random.c | 62 ++++++++++++-------------------------------
> 1 file changed, 17 insertions(+), 45 deletions(-)
>
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 536237a0f073..9fb06fc298d3 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -443,10 +443,6 @@ static void crng_make_state(u32 chacha_state[CHACHA_STATE_WORDS],
> * 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
> @@ -455,19 +451,15 @@ static void crng_make_state(u32 chacha_state[CHACHA_STATE_WORDS],
> * 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;
> @@ -476,21 +468,9 @@ 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);
> - }
> + 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;
> @@ -1040,7 +1020,7 @@ void add_device_randomness(const void *buf, size_t size)
> unsigned long flags;
>
> 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(buf, size);
> @@ -1157,7 +1137,7 @@ 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;
> @@ -1298,7 +1278,12 @@ static void mix_interrupt_randomness(struct work_struct *work)
> local_irq_enable();
>
> mix_pool_bytes(pool, sizeof(pool));
> - credit_entropy_bits(1);
> +
> + if (unlikely(crng_init == 0))
> + crng_pre_init_inject(pool, sizeof(pool), true);
> + else
> + credit_entropy_bits(1);
> +
> memzero_explicit(pool, sizeof(pool));
> }
>
> @@ -1331,24 +1316,11 @@ 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))
> --
> 2.35.1
>
FYI, I think you were concerned about those trylocks too. This should
make that go away.
Jason
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