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From: Borislav Petkov <bp@suse.de>
To: speck@linutronix.de
Subject: [MODERATED] Additional sampling fun
Date: Fri, 28 Feb 2020 17:21:40 +0100	[thread overview]
Message-ID: <20200228162140.GB24511@zn.tnic> (raw)
In-Reply-To: <20200220081420.GA3328448@kroah.com>

On Thu, Feb 20, 2020 at 09:14:20AM +0100, speck for Greg KH wrote:
> Then we need to stop using RDRAND internally for our "give me a random
> number api" which has spread to more and more parts of the kernel.
> Here's a patch that does so:
> 	https://lore.kernel.org/lkml/20200216161836.1976-1-Jason@zx2c4.com/
> which I'm going to advise get merged now and backported to the stable
> branches.

So one of our guys - Nicolai Stange - was looking at this
wrt backporting it to trees and there's another problem in
add_interrupt_randomness() which could potentially turn out
to be nasty.

We asked him to write it up for speck@ (he's not subscribed) so that we
can discuss it here first. Here is the deal in his own words:

"In the context of the get_random_long() patch posted at [1], I noticed
that there's also a RDSEED insn issued from the interrupt path, which
perhaps might have undesired effects performance-wise.

More specifically, add_interrupt_randomness() would issue one RDSEED
either once a second or every 64 interrupts, whichever comes first:

  void add_interrupt_randomness(int irq, int irq_flags)
    fast_mix(fast_pool); /* increments fast_pool->count */
    if ((fast_pool->count < 64) &&
         !time_after(now, fast_pool->last + HZ))
    fast_pool->last = now;
    if (arch_get_random_seed_long(&seed)) {}

So while this certainly won't matter much on average, I'm still
wondering whether or not this RDSEED could potentially cause IRQ
latency spikes relevant e.g. to -RT and/or under high IRQ load?

FWIW, the commit introducing the arch_get_random_seed_long() invocation
to add_interrupt_randomness() was commit 83664a6928a4 ("random: Use
arch_get_random_seed*() at init time and once a second").

I can only guess, but I think the motivation for mixing
arch_get_random_seed_long() from the interrupt path was probably to
sync that with the IRQ rate. That is, to make sure that the entropy
mixed from RDSEED doesn't dominate the interrupt entropy source.

[1] https://lkml.kernel.org/r/20200216161836.1976-1-Jason@zx2c4.com



SUSE Software Solutions Germany GmbH, GF: Felix Imendörffer, HRB 36809, AG Nürnberg

  parent reply	other threads:[~2020-02-28 16:21 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-19 22:45 [MODERATED] [PATCH 0/2] more sampling fun 0 mark gross
2020-02-20  1:53 ` [MODERATED] " mark gross
2020-02-20  8:14 ` Greg KH
2020-02-20 14:27   ` Greg KH
2020-02-20 15:40     ` mark gross
2020-02-20 16:18       ` Greg KH
2020-02-20 14:55   ` Andi Kleen
2020-02-20 15:05     ` Greg KH
2020-02-20 16:55       ` Andi Kleen
2020-02-20 21:51     ` Josh Poimboeuf
2020-02-20 22:15       ` Andi Kleen
2020-02-20 22:59         ` Kees Cook
2020-02-20 15:09   ` mark gross
2020-02-28 16:21   ` Borislav Petkov [this message]
2020-02-28 16:34     ` [MODERATED] Re: Additional sampling fun Greg KH
2020-02-28 17:38       ` mark gross
2020-02-28 17:44         ` Thomas Gleixner
2020-02-28 18:09           ` [MODERATED] " Luck, Tony
2020-02-28 18:40             ` Borislav Petkov
2020-02-28 21:53             ` Thomas Gleixner
2020-03-03  1:03               ` [MODERATED] " Luck, Tony

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