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Subject[PATCH] /dev/random: Insufficient of entropy on many architectures
Date
Hi,

/dev/random uses the get_cycles() function to obtain entropy in addition to jiffies and the event value of hardware events.

Typically the high-resolution timer of get_cycles delivers the majority of entropy, because the event value is quite deterministic and jiffies are very coarse.

However, on the following architectures, get_cycles will return 0:

- MIPS

- User mode Linux

- Sparc 32 bit

- M68K

- M32R

- Hexagon

- H8/300

- FR-V

- CRIS

- AVR32

- ARC

- METAG

- Microblaze

- SCORE

- SH

- Unicore32

That means that on those architectures, /dev/random will not deliver as much entropy as you would hope.

The following patch uses the clocksource clock for a time value in case get_cycles returns 0. As clocksource may not be available during boot time, a flag is introduced which allows random.c to check the availability of clocksource.

Patch tested with disabled call to get_cycles on an x86_64 system to verify that clocksource delivers data.

Ciao
Stephan

Signed-off-by: Stephan Mueller <smueller@chronox.de>

---
diff --git a/drivers/char/random.c b/drivers/char/random.c
index 0d91fe5..d2d14a1 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -255,6 +255,7 @@
#include <linux/fips.h>
#include <linux/ptrace.h>
#include <linux/kmemcheck.h>
+#include <linux/time.h>

#ifdef CONFIG_GENERIC_HARDIRQS
# include <linux/irq.h>
@@ -633,6 +634,23 @@ struct timer_rand_state {
unsigned dont_count_entropy:1;
};

+static inline __u64 get_nstime(void)
+{
+ struct timespec ts;
+ __u64 tmp = 0;
+
+ tmp = get_cycles();
+
+ if((0 == tmp) &&
+ timekeeping_initialized() &&
+ (0 == __getnstimeofday(&ts)))
+ {
+ tmp = ts.tv_sec;
+ tmp = tmp << 32;
+ tmp = tmp | ts.tv_nsec;
+ }
+ return tmp;
+}
/*
* Add device- or boot-specific data to the input and nonblocking
* pools to help initialize them to unique values.
@@ -643,7 +661,7 @@ struct timer_rand_state {
*/
void add_device_randomness(const void *buf, unsigned int size)
{
- unsigned long time = get_cycles() ^ jiffies;
+ unsigned long time = get_nstime() ^ jiffies;

mix_pool_bytes(&input_pool, buf, size, NULL);
mix_pool_bytes(&input_pool, &time, sizeof(time), NULL);
@@ -680,7 +698,7 @@ static void add_timer_randomness(struct timer_rand_state *state, unsigned num)
goto out;

sample.jiffies = jiffies;
- sample.cycles = get_cycles();
+ sample.cycles = get_nstime();
sample.num = num;
mix_pool_bytes(&input_pool, &sample, sizeof(sample), NULL);

@@ -747,7 +765,7 @@ void add_interrupt_randomness(int irq, int irq_flags)
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();
+ __u32 input[4], cycles = get_nstime();

input[0] = cycles ^ jiffies;
input[1] = irq;
@@ -1486,7 +1504,7 @@ unsigned int get_random_int(void)

hash = get_cpu_var(get_random_int_hash);

- hash[0] += current->pid + jiffies + get_cycles();
+ hash[0] += current->pid + jiffies + get_nstime();
md5_transform(hash, random_int_secret);
ret = hash[0];
put_cpu_var(get_random_int_hash);
diff --git a/include/linux/time.h b/include/linux/time.h
index d5d229b..0922661 100644
--- a/include/linux/time.h
+++ b/include/linux/time.h
@@ -180,6 +180,7 @@ extern int timekeeping_inject_offset(struct timespec *ts);
extern s32 timekeeping_get_tai_offset(void);
extern void timekeeping_set_tai_offset(s32 tai_offset);
extern void timekeeping_clocktai(struct timespec *ts);
+extern bool timekeeping_initialized(void);

struct tms;
extern void do_sys_times(struct tms *);
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 48b9fff..75b1613 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -35,6 +35,7 @@ static struct timekeeper timekeeper;
static DEFINE_RAW_SPINLOCK(timekeeper_lock);
static seqcount_t timekeeper_seq;
static struct timekeeper shadow_timekeeper;
+static bool timekeeper_enabled = 0;

/* flag for if timekeeping is suspended */
int __read_mostly timekeeping_suspended;
@@ -833,8 +834,15 @@ void __init timekeeping_init(void)

write_seqcount_end(&timekeeper_seq);
raw_spin_unlock_irqrestore(&timekeeper_lock, flags);
+ timekeeper_enabled = 1;
}

+bool timekeeping_initialized(void)
+{
+ return timekeeper_enabled;
+}
+EXPORT_SYMBOL(timekeeping_initialized);
+
/* time in seconds when suspend began */
static struct timespec timekeeping_suspend_time;



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