Re: Seeding /dev/random not working

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Matt Mackall a écrit :
On Tue, May 29, 2007 at 09:15:01AM -0400, Theodore Tso wrote:
Another thing which I noticed is that when Matt Mackall took over
maintainership of /dev/random, he apparently took out one of the
safeguards I had, which was that before, when entropy was extracted
from the pool the time stamp when it was extracted was mixed back into
the pool.  The theory was that an external attacker might not know
when a program might be calling /dev/random, so mixing in the time of
that entropy was extracted wouldn't hurt, and might help.  I'll submit
a patch to add that support back in, which will help you a little.

It's still there, and in the same place, it just looks different:

static void add_timer_randomness(struct timer_rand_state *state,
unsigned num)
{
...
        sample.jiffies = jiffies;
        sample.cycles = get_cycles();
        sample.num = num;
        add_entropy_words(&input_pool, (u32 *)&sample,
	sizeof(sample)/4);

Trouble is the write(2) interface calls add_entropy_words directly, as
does the pool initialization function.

We might as well mix jiffies and cycles in at init time in a manner
similar to the above. Something like this (untested):

Index: l/drivers/char/random.c
===================================================================
--- l.orig/drivers/char/random.c	2007-05-29 12:45:00.000000000 -0500
+++ l/drivers/char/random.c	2007-05-29 12:44:02.000000000 -0500
@@ -559,6 +559,26 @@ static struct timer_rand_state input_tim
 static struct timer_rand_state *irq_timer_state[NR_IRQS];
/*
+ * Mix a sample of the current time into the pool with no entropy
+ * accounting
+ */
+static long __add_timer_randomness(void)
+{
+	struct {
+		cycles_t cycles;
+		long jiffies;
+		unsigned num;
+	} sample;
+
+	sample.jiffies = jiffies;
+	sample.cycles = get_cycles();
+	sample.num = num;
+	add_entropy_words(&input_pool, (u32 *)&sample, sizeof(sample)/4);

Well, you need to pass 'num' argument I guess.

But... How is this supposed to work on 64 bits arches ?

Because of alignment, 'struct sample' will include a 4 bytes filler after 'unsigned num', and sizeof(sample) will include this (null) filler in entropy pool.

Shouldn't we use __attribute__((packed)) on 'struct sample' definition ?


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