From: George Spelvin <lkml@SDF.ORG>
To: Willy Tarreau <w@1wt.eu>
Cc: netdev@vger.kernel.org, aksecurity@gmail.com,
torvalds@linux-foundation.org, edumazet@google.com,
Jason@zx2c4.com, luto@kernel.org, keescook@chromium.org,
tglx@linutronix.de, peterz@infradead.org, tytso@mit.edu,
lkml.mplumb@gmail.com, stephen@networkplumber.org, fw@strlen.de,
George Spelvin <lkml@SDF.ORG>
Subject: Re: [DRAFT PATCH] random32: make prandom_u32() output unpredictable
Date: Sun, 9 Aug 2020 17:06:39 +0000 [thread overview]
Message-ID: <20200809170639.GB25124@SDF.ORG> (raw)
In-Reply-To: <20200809093805.GA7928@1wt.eu>
On Sun, Aug 09, 2020 at 11:38:05AM +0200, Willy Tarreau wrote:
> So I gave it a quick test under Qemu and it didn't show any obvious
> performance difference compared to Tausworthe, which is a good thing,
> even though there's a significant amount of measurement noise in each
> case.
Thank you very much! I'm not quite sure how to benchmark this.
The whole idea is that it's *not* used in a tight cache-hot loop.
Hopefully someone already has a test setup so I don't have to invent
one.
> However it keeps the problem that the whole sequence is entirely
> determined at the moment of reseeding, so if one were to be able to
> access the state, e.g. using l1tf/spectre/meltdown/whatever, then
> this state could be used to predict the whole ongoing sequence for
> the next minute. What some view as a security feature, others will
> see as a backdoor :-/ That's why I really like the noise approach.
> Even just the below would significantly harm that capability because
> that state alone isn't sufficient anymore to pre-compute all future
> values:
>
> --- a/lib/random32.c
> +++ b/lib/random32.c
> @@ -375,6 +375,7 @@ static u32 siprand_u32(struct siprand_state *s)
> {
> unsigned long v0 = s->v[0], v1 = s->v[1], v2 = s->v[2], v3 = s->v[3];
>
> + v0 += get_cycles();
> SIPROUND(v0, v1, v2, v3);
> SIPROUND(v0, v1, v2, v3);
> s->v[0] = v0; s->v[1] = v1; s->v[2] = v2; s->v[3] = v3;
As long as:
1) The periodic catastrophic reseeding remains, and
2) You use fresh measurements, not the exact same bits
that add_*_randomness feeds into /dev/random
then it doesn't do any real harm, so if it makes you feel better...
But I really want to stress how weak a design drip-reseeding is.
If an attacker has enough local machine access to do a meltdown-style attack,
then they can calibrate the TSC used in get_cycles very accurately, so the
remaining timing uncertainty is very low. This makes a brute-force attack on
one or two reseedings quite easy. I.e. if you can see every other output,
It's straightforward to figure out the ones in between.
I wonder if, on general principles, it would be better to use a more
SipHash style mixing in of the sample:
m = get_cycles();
v3 ^= m;
SIPROUND(v0, v1, v2, v3);
SIPROUND(v0, v1, v2, v3);
v0 ^= m;
Not sure if it's worth the extra register (and associated spill/fill).
next prev parent reply other threads:[~2020-08-09 17:11 UTC|newest]
Thread overview: 68+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-08 15:26 Flaw in "random32: update the net random state on interrupt and activity" George Spelvin
2020-08-08 17:07 ` Andy Lutomirski
2020-08-08 18:08 ` Willy Tarreau
2020-08-08 18:13 ` Linus Torvalds
2020-08-08 19:03 ` George Spelvin
2020-08-08 19:49 ` Andy Lutomirski
2020-08-08 21:29 ` George Spelvin
2020-08-08 17:44 ` Willy Tarreau
2020-08-08 18:19 ` Linus Torvalds
2020-08-08 18:53 ` Willy Tarreau
2020-08-08 20:47 ` George Spelvin
2020-08-08 20:52 ` Linus Torvalds
2020-08-08 22:27 ` George Spelvin
2020-08-09 2:07 ` Linus Torvalds
2020-08-11 16:01 ` Eric Dumazet
2020-08-08 19:18 ` Florian Westphal
2020-08-08 20:59 ` George Spelvin
2020-08-08 21:18 ` Willy Tarreau
2020-08-08 20:08 ` George Spelvin
2020-08-08 20:47 ` Linus Torvalds
2020-08-09 6:57 ` [DRAFT PATCH] random32: make prandom_u32() output unpredictable George Spelvin
2020-08-09 9:38 ` Willy Tarreau
2020-08-09 17:06 ` George Spelvin [this message]
2020-08-09 17:33 ` Willy Tarreau
2020-08-09 18:30 ` George Spelvin
2020-08-09 19:16 ` Willy Tarreau
2020-08-10 11:47 ` Willy Tarreau
2020-08-10 12:01 ` David Laight
2020-08-10 14:48 ` Willy Tarreau
2020-08-10 12:03 ` Florian Westphal
2020-08-10 14:53 ` Willy Tarreau
2020-08-10 16:31 ` Linus Torvalds
2020-08-10 16:58 ` Willy Tarreau
2020-08-10 17:45 ` Linus Torvalds
2020-08-10 18:01 ` Willy Tarreau
2020-08-10 21:04 ` Willy Tarreau
2020-08-11 5:26 ` George Spelvin
2020-08-11 5:37 ` Willy Tarreau
2020-08-11 3:47 ` George Spelvin
2020-08-11 3:58 ` Willy Tarreau
[not found] ` <fdbc7d7d-cba2-ef94-9bde-b3ccae0cfaac@gmail.com>
2020-08-09 21:10 ` Marc Plumb
2020-08-09 21:48 ` Linus Torvalds
2020-08-09 13:50 ` Randy Dunlap
[not found] ` <CANEQ_++a4YcwQQ2XhuguTono9=RxbSRVsMw08zLWBWJ_wxG2AQ@mail.gmail.com>
2020-08-09 16:08 ` George Spelvin
[not found] <CA+icZUVnsmf1kXPYFYufStQ_MxnLuxL+EWfDS2wQy1VbAEMwkA@mail.gmail.com>
2020-08-09 21:10 ` Sedat Dilek
[not found] ` <20200809235412.GD25124@SDF.ORG>
[not found] ` <20200810034948.GB8262@1wt.eu>
[not found] ` <20200811053455.GH25124@SDF.ORG>
[not found] ` <20200811054328.GD9456@1wt.eu>
[not found] ` <20200811062814.GI25124@SDF.ORG>
[not found] ` <20200811074538.GA9523@1wt.eu>
2020-08-11 10:51 ` Sedat Dilek
2020-08-11 11:01 ` Sedat Dilek
2020-08-12 3:21 ` Willy Tarreau
2020-08-13 7:53 ` Sedat Dilek
2020-08-13 8:06 ` Willy Tarreau
2020-08-13 8:13 ` Sedat Dilek
2020-08-13 8:27 ` Sedat Dilek
2020-08-13 14:00 ` Eric Dumazet
2020-08-13 16:02 ` Sedat Dilek
2020-08-14 15:32 ` Sedat Dilek
2020-08-14 16:05 ` Willy Tarreau
2020-08-14 16:17 ` Sedat Dilek
2020-08-16 15:01 ` Willy Tarreau
2020-08-16 16:48 ` Sedat Dilek
2020-08-20 3:05 ` Sedat Dilek
2020-08-20 4:33 ` Willy Tarreau
2020-08-20 4:42 ` Sedat Dilek
2020-08-20 6:08 ` Willy Tarreau
2020-08-20 6:58 ` Willy Tarreau
2020-08-20 8:05 ` Willy Tarreau
2020-08-20 12:08 ` Sedat Dilek
[not found] ` <CANEQ_+L+22Hkdqf38Zr0bfq16fcL1Ax2X9fToXV_niHKXCB8aA@mail.gmail.com>
2020-08-27 1:09 ` Willy Tarreau
2020-08-27 7:08 ` Sedat Dilek
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