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Subject[PATCH 02/10] random: use lockless techniques when mixing entropy pools
Date
The real-time Linux folks didn't like add_interrupt_randomness()
taking a spinlock since it is called in the low-level interrupt
routine. Using atomic_t's and cmpxchg is also too expensive on some
of the older architectures. So we'll bite the bullet and use
ACCESS_ONCE() and smp_rmb()/smp_wmb() to minimize the race windows
when mixing in the entropy pool.

Also, we will use a trylock when trying to increase then entropy
accounting during the interrupt path to avoid taking a spinlock there;
if there is contention, we will simply not credit the entropy count,
thus failing safe. Thanks to Dan Carpenter for suggesting this
approach.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
---
drivers/char/random.c | 41 ++++++++++++++++++++++++-----------------
1 file changed, 24 insertions(+), 17 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 53b3e85..789709e 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -418,9 +418,9 @@ struct entropy_store {
/* read-write data: */
spinlock_t lock;
unsigned add_ptr;
+ unsigned input_rotate;
int entropy_count;
int entropy_total;
- int input_rotate;
unsigned int initialized:1;
__u8 last_data[EXTRACT_SIZE];
};
@@ -478,16 +478,16 @@ static void mix_pool_bytes_extract(struct entropy_store *r, const void *in,
__u32 w;
unsigned long flags;

- /* Taps are constant, so we can load them without holding r->lock. */
tap1 = r->poolinfo->tap1;
tap2 = r->poolinfo->tap2;
tap3 = r->poolinfo->tap3;
tap4 = r->poolinfo->tap4;
tap5 = r->poolinfo->tap5;

- spin_lock_irqsave(&r->lock, flags);
- input_rotate = r->input_rotate;
- i = r->add_ptr;
+ local_irq_save(flags);
+ smp_rmb();
+ input_rotate = ACCESS_ONCE(r->input_rotate);
+ i = ACCESS_ONCE(r->add_ptr);

/* mix one byte at a time to simplify size handling and churn faster */
while (nbytes--) {
@@ -514,19 +514,19 @@ static void mix_pool_bytes_extract(struct entropy_store *r, const void *in,
input_rotate += i ? 7 : 14;
}

- r->input_rotate = input_rotate;
- r->add_ptr = i;
+ ACCESS_ONCE(r->input_rotate) = input_rotate;
+ ACCESS_ONCE(r->add_ptr) = i;
+ local_irq_restore(flags);
+ smp_wmb();

if (out)
for (j = 0; j < 16; j++)
((__u32 *)out)[j] = r->pool[(i - j) & wordmask];
-
- spin_unlock_irqrestore(&r->lock, flags);
}

static void mix_pool_bytes(struct entropy_store *r, const void *in, int bytes)
{
- mix_pool_bytes_extract(r, in, bytes, NULL);
+ mix_pool_bytes_extract(r, in, bytes, NULL);
}

struct fast_pool {
@@ -558,10 +558,12 @@ static void fast_mix(struct fast_pool *f, const void *in, int nbytes)
f->rotate = input_rotate;
}

+#define CREDIT_ENTROPY_BITS_NOWAIT 0x01
+
/*
* Credit (or debit) the entropy store with n bits of entropy
*/
-static void credit_entropy_bits(struct entropy_store *r, int nbits)
+static void credit_entropy_bits(struct entropy_store *r, int nbits, int fl)
{
unsigned long flags;
int entropy_count;
@@ -569,7 +571,11 @@ static void credit_entropy_bits(struct entropy_store *r, int nbits)
if (!nbits)
return;

- spin_lock_irqsave(&r->lock, flags);
+ if (fl & CREDIT_ENTROPY_BITS_NOWAIT) {
+ if (!spin_trylock_irqsave(&r->lock, flags))
+ return;
+ } else
+ spin_lock_irqsave(&r->lock, flags);

DEBUG_ENT("added %d entropy credits to %s\n", nbits, r->name);
entropy_count = r->entropy_count;
@@ -592,6 +598,7 @@ static void credit_entropy_bits(struct entropy_store *r, int nbits)
wake_up_interruptible(&random_read_wait);
kill_fasync(&fasync, SIGIO, POLL_IN);
}
+
spin_unlock_irqrestore(&r->lock, flags);
}

@@ -714,7 +721,7 @@ static void add_timer_randomness(struct timer_rand_state *state, unsigned num)
* and limit entropy entimate to 12 bits.
*/
credit_entropy_bits(&input_pool,
- min_t(int, fls(delta>>1), 11));
+ min_t(int, fls(delta>>1), 11), 0);
}
out:
preempt_enable();
@@ -764,7 +771,7 @@ void add_interrupt_randomness(int irq)

r = nonblocking_pool.initialized ? &input_pool : &nonblocking_pool;
mix_pool_bytes(r, &fast_pool->pool, sizeof(fast_pool->pool));
- credit_entropy_bits(r, 1);
+ credit_entropy_bits(r, 1, CREDIT_ENTROPY_BITS_NOWAIT);
}

#ifdef CONFIG_BLOCK
@@ -816,7 +823,7 @@ static void xfer_secondary_pool(struct entropy_store *r, size_t nbytes)
bytes = extract_entropy(r->pull, tmp, bytes,
random_read_wakeup_thresh / 8, rsvd);
mix_pool_bytes(r, tmp, bytes);
- credit_entropy_bits(r, bytes*8);
+ credit_entropy_bits(r, bytes*8, 0);
}
}

@@ -1211,7 +1218,7 @@ static long random_ioctl(struct file *f, unsigned int cmd, unsigned long arg)
return -EPERM;
if (get_user(ent_count, p))
return -EFAULT;
- credit_entropy_bits(&input_pool, ent_count);
+ credit_entropy_bits(&input_pool, ent_count, 0);
return 0;
case RNDADDENTROPY:
if (!capable(CAP_SYS_ADMIN))
@@ -1226,7 +1233,7 @@ static long random_ioctl(struct file *f, unsigned int cmd, unsigned long arg)
size);
if (retval < 0)
return retval;
- credit_entropy_bits(&input_pool, ent_count);
+ credit_entropy_bits(&input_pool, ent_count, 0);
return 0;
case RNDZAPENTCNT:
case RNDCLEARPOOL:
--
1.7.11.1.108.gb129051


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