[PATCH 02/12] random: make 'add_interrupt_randomness()' do something sane

Theodore Ts'o
2012-07-06T22:52:18Z

We've been moving away from add_interrupt_randomness() for various
reasons: it's too expensive to do on every interrupt, and flooding the
CPU with interrupts could theoretically cause bogus floods of entropy
from a somewhat externally controllable source.

This solves both problems by limiting the actual randomness addition
to just once a second or after 128 interrupts, whicever comes first.
During that time, the interrupt cycle data is buffered up in a per-cpu
pool. Also, we make sure the the nonblocking pool used by urandom is
initialized before we start feeding the normal input pool. This
assures that /dev/urandom is returning unpredictable data as soon as
possible.

(Based on an original patch by Linus, but significantly modified by
tytso.)

Tested-by: Eric Wustrow <ewust@umich.edu>
Reported-by: Eric Wustrow <ewust@umich.edu>
Reported-by: Nadia Heninger <nadiah@cs.ucsd.edu>
Reported-by: Zakir Durumeric <zakir@umich.edu>
Reported-by: J. Alex Halderman <jhalderm@umich.edu>.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@vger.kernel.org
---
drivers/char/random.c | 103 ++++++++++++++++++++++++++++++++++++++--------
drivers/mfd/ab3100-core.c | 2 -
include/linux/random.h | 2 +-
kernel/irq/handle.c | 7 ++--
4 files changed, 90 insertions(+), 24 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index cb541b9..556088b 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -127,19 +127,15 @@
*
* void add_input_randomness(unsigned int type, unsigned int code,
* unsigned int value);
- * void add_interrupt_randomness(int irq);
+ * void add_interrupt_randomness(int irq, int irq_flags);
* void add_disk_randomness(struct gendisk *disk);
*
* add_input_randomness() uses the input layer interrupt timing, as well as
* the event type information from the hardware.
*
- * add_interrupt_randomness() uses the inter-interrupt timing as random
- * inputs to the entropy pool. Note that not all interrupts are good
- * sources of randomness! For example, the timer interrupts is not a
- * good choice, because the periodicity of the interrupts is too
- * regular, and hence predictable to an attacker. Network Interface
- * Controller interrupts are a better measure, since the timing of the
- * NIC interrupts are more unpredictable.
+ * add_interrupt_randomness() uses the interrupt timing as random
+ * inputs to the entropy pool. Using the cycle counters and the irq source
+ * as inputs, it feeds the randomness roughly once a second.
*
* add_disk_randomness() uses what amounts to the seek time of block
* layer request events, on a per-disk_devt basis, as input to the
@@ -248,6 +244,7 @@
#include <linux/percpu.h>
#include <linux/cryptohash.h>
#include <linux/fips.h>
+#include <linux/ptrace.h>

#ifdef CONFIG_GENERIC_HARDIRQS
# include <linux/irq.h>
@@ -256,6 +253,7 @@
#include <asm/processor.h>
#include <asm/uaccess.h>
#include <asm/irq.h>
+#include <asm/irq_regs.h>
#include <asm/io.h>

/*
@@ -421,7 +419,9 @@ struct entropy_store {
spinlock_t lock;
unsigned add_ptr;
int entropy_count;
+ int entropy_total;
int input_rotate;
+ unsigned int initialized:1;
__u8 last_data[EXTRACT_SIZE];
};

@@ -454,6 +454,10 @@ static struct entropy_store nonblocking_pool = {
.pool = nonblocking_pool_data
};

+static __u32 const twist_table[8] = {
+ 0x00000000, 0x3b6e20c8, 0x76dc4190, 0x4db26158,
+ 0xedb88320, 0xd6d6a3e8, 0x9b64c2b0, 0xa00ae278 };
+
/*
* This function adds bytes into the entropy "pool". It does not
* update the entropy estimate. The caller should call
@@ -467,9 +471,6 @@ static struct entropy_store nonblocking_pool = {
static void mix_pool_bytes_extract(struct entropy_store *r, const void *in,
int nbytes, __u8 out[64])
{
- static __u32 const twist_table[8] = {
- 0x00000000, 0x3b6e20c8, 0x76dc4190, 0x4db26158,
- 0xedb88320, 0xd6d6a3e8, 0x9b64c2b0, 0xa00ae278 };
unsigned long i, j, tap1, tap2, tap3, tap4, tap5;
int input_rotate;
int wordmask = r->poolinfo->poolwords - 1;
@@ -528,6 +529,36 @@ static void mix_pool_bytes(struct entropy_store *r, const void *in, int bytes)
mix_pool_bytes_extract(r, in, bytes, NULL);
}

+struct fast_pool {
+ __u32 pool[4];
+ unsigned long last;
+ unsigned short count;
+ unsigned char rotate;
+ unsigned char last_timer_intr;
+};
+
+/*
+ * This is a fast mixing routine used by the interrupt randomness
+ * collector. It's hardcoded for an 128 bit pool and assumes that any
+ * locks that might be needed are taken by the caller.
+ */
+static void fast_mix(struct fast_pool *f, const void *in, int nbytes)
+{
+ const char *bytes = in;
+ __u32 w;
+ unsigned i = f->count;
+ unsigned input_rotate = f->rotate;
+
+ while (nbytes--) {
+ w = rol32(*bytes++, input_rotate & 31) ^ f->pool[i & 3] ^
+ f->pool[(i + 1) & 3];
+ f->pool[i & 3] = (w >> 3) ^ twist_table[w & 7];
+ input_rotate += (i++ & 3) ? 7 : 14;
+ }
+ f->count = i;
+ f->rotate = input_rotate;
+}
+
/*
* Credit (or debit) the entropy store with n bits of entropy
*/
@@ -551,6 +582,12 @@ static void credit_entropy_bits(struct entropy_store *r, int nbits)
entropy_count = r->poolinfo->POOLBITS;
r->entropy_count = entropy_count;

+ if (!r->initialized && nbits > 0) {
+ r->entropy_total += nbits;
+ if (r->entropy_total > 128)
+ r->initialized = 1;
+ }
+
/* should we wake readers? */
if (r == &input_pool && entropy_count >= random_read_wakeup_thresh) {
wake_up_interruptible(&random_read_wait);
@@ -700,17 +737,48 @@ void add_input_randomness(unsigned int type, unsigned int code,
}
EXPORT_SYMBOL_GPL(add_input_randomness);

-void add_interrupt_randomness(int irq)
+static DEFINE_PER_CPU(struct fast_pool, irq_randomness);
+
+void add_interrupt_randomness(int irq, int irq_flags)
{
- struct timer_rand_state *state;
+ struct entropy_store *r;
+ struct fast_pool *fast_pool = &__get_cpu_var(irq_randomness);
+ struct pt_regs *regs = get_irq_regs();
+ unsigned long now = jiffies;
+ __u32 input[4], cycles = get_cycles();
+
+ input[0] = cycles ^ jiffies;
+ input[1] = irq;
+ if (regs) {
+ __u64 ip = instruction_pointer(regs);
+ input[2] = ip;
+ input[3] = ip >> 32;
+ }

- state = get_timer_rand_state(irq);
+ fast_mix(fast_pool, input, sizeof(input));

- if (state == NULL)
+ if ((fast_pool->count & 255) &&
+ !time_after(now, fast_pool->last + HZ))
return;

- DEBUG_ENT("irq event %d\n", irq);
- add_timer_randomness(state, 0x100 + irq);
+ fast_pool->last = now;
+
+ r = nonblocking_pool.initialized ? &input_pool : &nonblocking_pool;
+ mix_pool_bytes(r, &fast_pool->pool, sizeof(fast_pool->pool));
+ /*
+ * If we don't have a valid cycle counter, and we see
+ * back-to-back timer interrupts, then skip giving credit for
+ * any entropy.
+ */
+ if (cycles == 0) {
+ if (irq_flags & __IRQF_TIMER) {
+ if (fast_pool->last_timer_intr)
+ return;
+ fast_pool->last_timer_intr = 1;
+ } else
+ fast_pool->last_timer_intr = 0;
+ }
+ credit_entropy_bits(r, 1);
}

#ifdef CONFIG_BLOCK
@@ -971,6 +1039,7 @@ static void init_std_data(struct entropy_store *r)

spin_lock_irqsave(&r->lock, flags);
r->entropy_count = 0;
+ r->entropy_total = 0;
spin_unlock_irqrestore(&r->lock, flags);

now = ktime_get_real();
diff --git a/drivers/mfd/ab3100-core.c b/drivers/mfd/ab3100-core.c
index 1efad20..9522d6b 100644
--- a/drivers/mfd/ab3100-core.c
+++ b/drivers/mfd/ab3100-core.c
@@ -409,8 +409,6 @@ static irqreturn_t ab3100_irq_handler(int irq, void *data)
u32 fatevent;
int err;

- add_interrupt_randomness(irq);
-
err = ab3100_get_register_page_interruptible(ab3100, AB3100_EVENTA1,
event_regs, 3);
if (err)
diff --git a/include/linux/random.h b/include/linux/random.h
index 8f74538..6ef39d7 100644
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -52,7 +52,7 @@ extern void rand_initialize_irq(int irq);

extern void add_input_randomness(unsigned int type, unsigned int code,
unsigned int value);
-extern void add_interrupt_randomness(int irq);
+extern void add_interrupt_randomness(int irq, int irq_flags);

extern void get_random_bytes(void *buf, int nbytes);
void generate_random_uuid(unsigned char uuid_out[16]);
diff --git a/kernel/irq/handle.c b/kernel/irq/handle.c
index bdb1803..131ca17 100644
--- a/kernel/irq/handle.c
+++ b/kernel/irq/handle.c
@@ -133,7 +133,7 @@ irqreturn_t
handle_irq_event_percpu(struct irq_desc *desc, struct irqaction *action)
{
irqreturn_t retval = IRQ_NONE;
- unsigned int random = 0, irq = desc->irq_data.irq;
+ unsigned int flags = 0, irq = desc->irq_data.irq;

do {
irqreturn_t res;
@@ -161,7 +161,7 @@ handle_irq_event_percpu(struct irq_desc *desc, struct irqaction *action)

/* Fall through to add to randomness */
case IRQ_HANDLED:
- random |= action->flags;
+ flags |= action->flags;
break;

default:
@@ -172,8 +172,7 @@ handle_irq_event_percpu(struct irq_desc *desc, struct irqaction *action)
action = action->next;
} while (action);

- if (random & IRQF_SAMPLE_RANDOM)
- add_interrupt_randomness(irq);
+ add_interrupt_randomness(irq, flags);

if (!noirqdebug)
note_interrupt(irq, desc, retval);
--
1.7.11.1.108.gb129051