linux-kernel - Re: [PATCH v6] random: defer fast pool mixing to worker

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Date:   Mon, 14 Feb 2022 15:47:34 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        "Theodore Ts'o" <tytso@....edu>,
        Sultan Alsawaf <sultan@...neltoast.com>,
        Jonathan Neuschäfer <j.neuschaefer@....net>,
        Dominik Brodowski <linux@...inikbrodowski.net>
Subject: Re: [PATCH v6] random: defer fast pool mixing to worker

On Mon, Feb 14, 2022 at 12:16 PM Sebastian Andrzej Siewior
<bigeasy@...utronix.de> wrote:
>
> On 2022-02-14 11:17:20 [+0100], Jason A. Donenfeld wrote:
> > On 2/14/22, Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:
> > > to
> > > | - Does anything anywhere call get_random_xx() before the worker has a
> > > |   chance to run?
> > >
> > > Once you queue a work item I don't think that the scheduler needs to put
> > > it on the CPU right away. It may have already have other tasks waiting
> > > including some with a RT priority.
> > > Also, the lock is irqsave() so they can be users in an interrupt
> > > handler. I remember the original reason why I made it irqsave is because
> > > something did kmalloc() and SLUB somehow asked for random bits.
> >
> > Right. So there are two sides of the questions: 1) how bad is this
> > actual race, and are there any drivers that do regularly get bit by
> > this? 2) There's a largeish window between workqueue_init_early()
> > setting up the system highprio workqueue, and workqueue_init()
> > enabling queued workers to actually run. Interrupts also get enabled
> > in the interim. Does anything get bit by that window?
>
> This is only important during boot-up, right?

Right. This is a pre-init window only. But a bunch of things are done
pre-init -- siphash secret keys, aslr seeds, and so forth.