From: "Jason A. Donenfeld" Jason@zx2c4.com
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: Sebastian Andrzej Siewior bigeasy@linutronix.de Reviewed-by: Dominik Brodowski linux@dominikbrodowski.net Signed-off-by: Jason A. Donenfeld Jason@zx2c4.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/char/random.c | 65 ++++++++++++++------------------------------------ 1 file changed, 19 insertions(+), 46 deletions(-)
--- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -442,10 +442,6 @@ static void crng_make_state(u32 chacha_s * 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 @@ -454,19 +450,15 @@ static void crng_make_state(u32 chacha_s * 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; @@ -475,21 +467,9 @@ static size_t crng_pre_init_inject(const 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; @@ -1030,7 +1010,7 @@ void add_device_randomness(const void *b 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)); @@ -1151,7 +1131,7 @@ void add_hwgenerator_randomness(const vo 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; @@ -1291,8 +1271,14 @@ static void mix_interrupt_randomness(str fast_pool->last = jiffies; 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); + mix_pool_bytes(pool, sizeof(pool)); + } else { + mix_pool_bytes(pool, sizeof(pool)); + credit_entropy_bits(1); + } + memzero_explicit(pool, sizeof(pool)); }
@@ -1325,24 +1311,11 @@ 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))