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random: use registers from interrupted code for CPU's w/o a cycle counter

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For CPU's that don't have a cycle counter, or something equivalent
which can be used for random_get_entropy(), random_get_entropy() will
always return 0.  In that case, substitute with the saved interrupt
registers to add a bit more unpredictability.

Some folks have suggested hashing all of the registers
unconditionally, but this would increase the overhead of
add_interrupt_randomness() by at least an order of magnitude, and this
would very likely be unacceptable.

The changes in this commit have been benchmarked as mostly unaffecting
the overhead of add_interrupt_randomness() if the entropy counter is
present, and doubling the overhead if it is not present.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: Jörn Engel <joern@logfs.org>
This commit is contained in:
Theodore Ts'o 2014-06-15 16:59:24 -04:00
parent 0f734e6e76
commit ee3e00e9e7
1 changed files with 22 additions and 25 deletions
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47
drivers/char/random.c
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@ -551,9 +551,8 @@ static void mix_pool_bytes(struct entropy_store *r, const void *in,
struct fast_pool {
__u32 pool[4];
unsigned long last;
unsigned short reg_idx;
unsigned char count;
unsigned char notimer_count;
unsigned char rotate;
};
/*
@ -857,6 +856,17 @@ static void add_interrupt_bench(cycles_t start)
#define add_interrupt_bench(x)
#endif
static __u32 get_reg(struct fast_pool *f, struct pt_regs *regs)
{
__u32 *ptr = (__u32 *) regs;
if (regs == NULL)
return 0;
if (f->reg_idx >= sizeof(struct pt_regs) / sizeof(__u32))
f->reg_idx = 0;
return *(ptr + f->reg_idx++);
}
void add_interrupt_randomness(int irq, int irq_flags)
{
struct entropy_store *r;
@ -869,28 +879,23 @@ void add_interrupt_randomness(int irq, int irq_flags)
unsigned long seed;
int credit = 0;
if (cycles == 0)
cycles = get_reg(fast_pool, regs);
c_high = (sizeof(cycles) > 4) ? cycles >> 32 : 0;
j_high = (sizeof(now) > 4) ? now >> 32 : 0;
fast_pool->pool[0] ^= cycles ^ j_high ^ irq;
fast_pool->pool[1] ^= now ^ c_high;
ip = regs ? instruction_pointer(regs) : _RET_IP_;
fast_pool->pool[2] ^= ip;
fast_pool->pool[3] ^= ip >> 32;
fast_pool->pool[3] ^= (sizeof(ip) > 4) ? ip >> 32 :
get_reg(fast_pool, regs);
fast_mix(fast_pool);
if ((irq_flags & __IRQF_TIMER) == 0)
fast_pool->notimer_count++;
add_interrupt_bench(cycles);
if (cycles) {
if ((fast_pool->count < 64) &&
!time_after(now, fast_pool->last + HZ))
return;
} else {
/* CPU does not have a cycle counting register :-( */
if (fast_pool->count < 64)
return;
}
if ((fast_pool->count < 64) &&
!time_after(now, fast_pool->last + HZ))
return;
r = nonblocking_pool.initialized ? &input_pool : &nonblocking_pool;
if (!spin_trylock(&r->lock))
@ -910,18 +915,10 @@ void add_interrupt_randomness(int irq, int irq_flags)
}
spin_unlock(&r->lock);
/*
* If we have a valid cycle counter or if the majority of
* interrupts collected were non-timer interrupts, then give
* an entropy credit of 1 bit. Yes, this is being very
* conservative.
*/
if (cycles || (fast_pool->notimer_count >= 32))
credit++;
fast_pool->count = 0;
fast_pool->count = fast_pool->notimer_count = 0;
credit_entropy_bits(r, credit);
/* award one bit for the contents of the fast pool */
credit_entropy_bits(r, credit + 1);
}
#ifdef CONFIG_BLOCK