From: Ben Hutchings <ben@decadent.org.uk>
To: "Theodore Ts'o" <tytso@mit.edu>
Cc: Linux Kernel Developers List <linux-kernel@vger.kernel.org>,
ewust@umich.edu, zakir@umich.edu, nadiah@cs.ucsd.edu,
jhalderm@umich.edu,
Linus Torvalds <torvalds@linux-foundation.org>,
stable@vger.kernel.org
Subject: Re: [PATCH 02/12] random: make 'add_interrupt_randomness()' do something sane
Date: Sun, 08 Jul 2012 03:01:09 +0100 [thread overview]
Message-ID: <1341712869.25597.203.camel@deadeye.wl.decadent.org.uk> (raw)
In-Reply-To: <1341614704-24965-3-git-send-email-tytso@mit.edu>
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On Fri, 2012-07-06 at 18:44 -0400, Theodore Ts'o wrote:
> 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.
Seems to be once per 64 interrupts. add_interrupt_randomness() calls
fast_mix() with nbytes = 20, so it increments fast_pool->count by 20.
add_interrupt_randomness() returns early if (fast_pool->count & 255),
and 64 * 20 == 0x500.
[...]
> +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;
> +}
[...]
This seems like a comparatively expensive operation to add to every
interrupt. :-/
Ben.
--
Ben Hutchings
Life would be so much easier if we could look at the source code.
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next prev parent reply other threads:[~2012-07-08 2:01 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-07-06 22:44 [PATCH 00/12] /dev/random fixups Theodore Ts'o
2012-07-06 22:44 ` [PATCH 01/12] random: fix up sparse warnings Theodore Ts'o
2012-07-06 22:44 ` [PATCH 02/12] random: make 'add_interrupt_randomness()' do something sane Theodore Ts'o
2012-07-08 2:01 ` Ben Hutchings [this message]
2012-07-06 22:44 ` [PATCH 03/12] random: use lockless techniques in the interrupt path Theodore Ts'o
2012-07-06 22:44 ` [PATCH 04/12] random: create add_device_randomness() interface Theodore Ts'o
2012-07-06 22:44 ` [PATCH 05/12] usb: feed USB device information to the /dev/random driver Theodore Ts'o
2012-07-06 23:02 ` Jonathan Nieder
2012-07-06 23:18 ` Greg KH
2012-07-06 23:26 ` Theodore Ts'o
2012-07-07 1:08 ` Jonathan Nieder
2012-07-06 22:44 ` [PATCH 06/12] net: feed /dev/random with the MAC address when registering a device Theodore Ts'o
2012-07-06 22:44 ` [PATCH 07/12] random: use the arch-specific rng in xfer_secondary_pool Theodore Ts'o
2012-07-07 17:11 ` [PATCH] random: only use gathered bytes from arch_get_random_long Kees Cook
2012-07-07 18:23 ` Theodore Ts'o
2012-07-07 23:20 ` Kees Cook
2012-07-08 1:06 ` [PATCH 07/12] random: use the arch-specific rng in xfer_secondary_pool Ben Hutchings
2012-07-08 1:41 ` Theodore Ts'o
2012-07-08 2:06 ` Ben Hutchings
2012-07-06 22:45 ` [PATCH 08/12] random: add new get_random_bytes_arch() function Theodore Ts'o
2012-07-06 22:45 ` [PATCH 09/12] random: add tracepoints for easier debugging and verification Theodore Ts'o
2012-07-06 22:45 ` [PATCH 10/12] MAINTAINERS: Theodore Ts'o is taking over the random driver Theodore Ts'o
2012-07-06 22:45 ` [PATCH 11/12] rtc: wm831x: Feed the write counter into device_add_randomness() Theodore Ts'o
2012-07-06 22:45 ` [PATCH 12/12] mfd: wm831x: Feed the device UUID " Theodore Ts'o
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