linux-kernel - Re: [PATCH 2/2 v3] lib/vsprintf: Initialize vsprintf's pointer hash once the random core is ready.

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Date:   Mon, 1 Aug 2022 14:36:48 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        John Ogness <john.ogness@...utronix.de>,
        Mike Galbraith <efault@....de>, Petr Mladek <pmladek@...e.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Theodore Ts'o <tytso@....edu>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/2 v3] lib/vsprintf: Initialize vsprintf's pointer hash
 once the random core is ready.

On Mon, Aug 01, 2022 at 11:34:26AM +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>
> ---
> v2…v3:
>    - schedule a worker every two seconds if the RNG core is not ready.
> 
>  lib/vsprintf.c |   46 +++++++++++++++++++++++++++-------------------
>  1 file changed, 27 insertions(+), 19 deletions(-)
> 
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -751,31 +751,39 @@ static int __init debug_boot_weak_hash_e
>  early_param("debug_boot_weak_hash", debug_boot_weak_hash_enable);
>  
>  static bool filled_random_ptr_key;
> +static siphash_key_t ptr_key __read_mostly;
> +static void fill_ptr_key_workfn(struct work_struct *work);
> +static DECLARE_DELAYED_WORK(fill_ptr_key_work, fill_ptr_key_workfn);
> +
> +static void fill_ptr_key_workfn(struct work_struct *work)
> +{
> +	if (!rng_is_initialized()) {
> +		queue_delayed_work(system_unbound_wq, &fill_ptr_key_work, HZ  * 2);
> +		return;
> +	}

This seems kind of crazy and over-complex. You only need this on RT,
right? Because of the raw lock issue? So just schedule it for the right
time on RT and not elsewhere.

I'll send a more basic patch.

Jason