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    SubjectRe: Seeding /dev/random not working
    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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