linux-kernel - Re: [PATCH v1 2/2] hwrng: core: fix potential suspend/resume race condition

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Message-ID: <Yxg6OayyEFWIax/r@gondor.apana.org.au>
Date:   Wed, 7 Sep 2022 14:29:13 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Sven van Ashbrook <svenva@...omium.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Alex Levin <levinale@...gle.com>,
        Rajat Jain <rajatja@...gle.com>,
        Andrey Pronin <apronin@...gle.com>,
        Stephen Boyd <swboyd@...gle.com>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Eric Biggers <ebiggers@...gle.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Olivia Mackall <olivia@...enic.com>,
        linux-crypto@...r.kernel.org
Subject: Re: [PATCH v1 2/2] hwrng: core: fix potential suspend/resume race
 condition

On Wed, Aug 31, 2022 at 05:20:24PM +0000, Sven van Ashbrook wrote:
> The hwrng fill function runs as a normal kthread. This thread
> doesn't get frozen by the PM, i.e. it will keep running during,
> or in, system suspend. It may call the client driver's
> data_present()/data_read() functions during, or in, suspend;
> which may generate errors or warnings. For example, if the
> client driver uses an i2c bus, the following warning may be
> intermittently generated:
> 
>   i2c: Transfer while suspended
> 
> Fix by converting the delay polled kthread into an ordered work
> queue running a single, self-rearming delayed_work. Make the
> workqueue WQ_FREEZABLE, so the PM will drain any work items
> before going into suspend. This prevents client drivers from
> being accessed during, or in, suspend.
> 
> Tested on a Chromebook containing an cr50 tpm over i2c. The test
> consists of 31000 suspend/resume cycles. Occasional
> "i2c: Transfer while suspended" warnings are seen. After applying
> this patch, these warnings disappear.
> 
> This patch also does not appear to cause any regressions on the
> ChromeOS test queues.
> 
> Signed-off-by: Sven van Ashbrook <svenva@...omium.org>

The general concept seems to be fine.

> -		if (rc <= 0) {
> -			pr_warn("hwrng: no data available\n");
> -			msleep_interruptible(10000);
> -			continue;
> -		}
> +	if (!quality)
> +		return;
>  
> +	if (rc > 0) {
>  		/* If we cannot credit at least one bit of entropy,
>  		 * keep track of the remainder for the next iteration
>  		 */

You need to refresh your patch-set against the latest tree.  This
function has changed quite a bit.


> @@ -541,14 +536,12 @@ static void hwrng_manage_rngd(struct hwrng *rng)
>  	if (WARN_ON(!mutex_is_locked(&rng_mutex)))
>  		return;
>  
> -	if (rng->quality == 0 && hwrng_fill)
> -		kthread_stop(hwrng_fill);
> -	if (rng->quality > 0 && !hwrng_fill) {
> -		hwrng_fill = kthread_run(hwrng_fillfn, NULL, "hwrng");
> -		if (IS_ERR(hwrng_fill)) {
> -			pr_err("hwrng_fill thread creation failed\n");
> -			hwrng_fill = NULL;
> -		}
> +	if (rng->quality == 0 && is_hwrng_wq_running) {
> +		cancel_delayed_work(&hwrng_fill_dwork);
> +		is_hwrng_wq_running = false;
> +	} else if (rng->quality > 0 && !is_hwrng_wq_running) {
> +		mod_delayed_work(hwrng_wq, &hwrng_fill_dwork, 0);
> +		is_hwrng_wq_running = true;
>  	}
>  }
>  
> @@ -631,8 +624,7 @@ void hwrng_unregister(struct hwrng *rng)
>  	new_rng = get_current_rng_nolock();
>  	if (list_empty(&rng_list)) {
>  		mutex_unlock(&rng_mutex);
> -		if (hwrng_fill)
> -			kthread_stop(hwrng_fill);
> +		cancel_delayed_work_sync(&hwrng_fill_dwork);

What if hwrng_manage_rngd races against hwrng_unregister?

Thanks,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt