From: Willy Tarreau <w@1wt.eu>
To: George Spelvin <lkml@sdf.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Florian Westphal <fw@strlen.de>, Netdev <netdev@vger.kernel.org>,
Amit Klein <aksecurity@gmail.com>,
Eric Dumazet <edumazet@google.com>,
"Jason A. Donenfeld" <Jason@zx2c4.com>,
Andrew Lutomirski <luto@kernel.org>,
Kees Cook <keescook@chromium.org>,
Thomas Gleixner <tglx@linutronix.de>,
Peter Zijlstra <peterz@infradead.org>,
"Theodore Ts'o" <tytso@mit.edu>,
Marc Plumb <lkml.mplumb@gmail.com>,
Stephen Hemminger <stephen@networkplumber.org>
Subject: Re: [DRAFT PATCH] random32: make prandom_u32() output unpredictable
Date: Tue, 11 Aug 2020 07:37:38 +0200 [thread overview]
Message-ID: <20200811053738.GC9456@1wt.eu> (raw)
In-Reply-To: <20200811052649.GG25124@SDF.ORG>
On Tue, Aug 11, 2020 at 05:26:49AM +0000, George Spelvin wrote:
> On Mon, Aug 10, 2020 at 11:04:55PM +0200, Willy Tarreau wrote:
> > What could be improved is the way the input values are mixed (just
> > added hence commutative for now). I didn't want to call a siphash
> > round on the hot paths, but just shifting the previous noise value
> > before adding would work, such as the following for example:
> >
> > void prandom_u32_add_noise(a, b, c, d)
> > {
> > unsigned long *noise = get_cpu_ptr(&net_rand_noise);
> >
> > #if BITS_PER_LONG == 64
> > *noise = rol64(*noise, 7) + a + b + c + d;
> > #else
> > *noise = rol32(*noise, 7) + a + b + c + d;
> > #endif
> > put_cpu_ptr(&net_rand_noise);
> >
> > }
>
> If you think this is enough seed material, I'm fine with it.
>
> I don't hugely like the fact that you sum all the inputs, since
> entropy tends to be concentrated in the low-order words, and summing
> risks cancellation.
Yes I've figured this. But I thought it was still better than
a pure xor which would cancell the high bits from pointers.
> You can't afford even one SIPROUND as a non-cryptographic hash? E.g.
That's what I mentioned above, I'm still hesitating. I need to test.
>
> DEFINE_PER_CPU(unsigned long[4], net_rand_noise);
> EXPORT_SYMBOL(net_rand_noise);
>
> void prandom_u32_add_noise(a, b, c, d)
> {
> unsigned long *noise = get_cpu_ptr(&net_rand_noise);
>
> a ^= noise[0];
> b ^= noise[1];
> c ^= noise[2];
> d ^= noise[3];
> /*
> * This is not used cryptographically; it's just
> * a convenient 4-word hash function.
> */
> SIPROUND(a, b, c, d);
> noise[0] = a;
> noise[1] = b;
> noise[2] = c;
> put_cpu_ptr(&net_rand_noise);
> }
>
> (And then you mix in net_rand_noise[0].)
>
> Other options are HASH_MIX() from fs/namei.c, but that's
> more sequential.
>
> There's also a simple Xorshift generator.
I think a xorshift on each value will have roughly the same cost
as a single SIPROUND. But I've not yet completely eliminated these
options until I've tested. If we lose a few cycles per packet, that
might be OK.
Willy
next prev parent reply other threads:[~2020-08-11 5:38 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
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 [this message]
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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