linux-kernel - Re: [PATCH v4 2/2] lib/vsprintf: Initialize vsprintf's pointer hash once the random core is ready.

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Message-ID: <YzLchr32qSKgN4vC@zx2c4.com>
Date:   Tue, 27 Sep 2022 13:20:38 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, Theodore Ts'o <tytso@....edu>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        John Ogness <john.ogness@...utronix.de>,
        Mike Galbraith <efault@....de>,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v4 2/2] lib/vsprintf: Initialize vsprintf's pointer hash
 once the random core is ready.

You added my Acked-by already (which is fine), but I thought I should
still add some final notes to that ack, for posterity:

On Tue, Sep 27, 2022 at 12:49:12PM +0200, Sebastian Andrzej Siewior wrote:
> The printk code invokes vnsprintf in order to compute the complete
> string before adding it into its buffer. This happens in an IRQ-off
> region which leads to a warning on PREEMPT_RT in the random code if the
> format strings contains a %p for pointer printing. This happens because
> the random core acquires locks which become sleeping locks on PREEMPT_RT
> which must not be acquired with disabled interrupts and or preemption
> disabled.
> By default the pointers are hashed which requires a random value on the
> first invocation (either by printk or another user which comes first.
> 
> One could argue that there is no need for printk to disable interrupts
> during the vsprintf() invocation which would fix the just mentioned
> problem. However printk itself can be invoked in a context with
> disabled interrupts which would lead to the very same problem.
> 
> Move the initialization of ptr_key into a worker and schedule it from
> subsys_initcall(). This happens early but after the workqueue subsystem
> is ready. Use get_random_bytes() to retrieve the random value if the RNG
> core is ready, otherwise schedule a worker in two seconds and try again.
> 
> Reported-by: Mike Galbraith <efault@....de>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Acked-by: Jason A. Donenfeld <Jason@...c4.com>

I really do hate the idea of polling every 2 seconds. But as we
discussed, this seems like the least bad solution, at least for now. If
we discover another bug in the tree that needs a gross solution like
that, then at that point, I'll move ahead with adding a notifier_block
to random.c, to avoid polling. But so long as this is a one-off (as we
understand it to be at the moment), this seems okay.

Acked-by: Jason A. Donenfeld <Jason@...c4.com>

Jason