linux-hardening - Re: [PATCH v3 105/108] pwm: Ensure a struct pwm has the same lifetime as its pwm

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Date: Tue, 21 Nov 2023 09:53:56 -0600
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
 Thierry Reding <thierry.reding@...il.com>, Kees Cook <keescook@...omium.org>
Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>, linux-pwm@...r.kernel.org,
 kernel@...gutronix.de, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v3 105/108] pwm: Ensure a struct pwm has the same lifetime
 as its pwm_chip



On 11/21/23 07:50, Uwe Kleine-König wrote:
> It's required to not free the memory underlying a requested PWM
> while a consumer still has a reference to it. While currently a pwm_chip
> doesn't life long enough in all cases, linking the struct pwm to the
> pwm_chip results in the right lifetime as soon as the pwmchip is living
> long enough. This happens with the following commits.
> 
> Note this is a breaking change for all pwm drivers that don't use
> pwmchip_alloc().
> 
> Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>

Reviewed-by: Gustavo A. R. Silva <gustavoars@...nel.org> # for struct_size() and __counted_by()

Thanks for including __counted_by(). :)
--
Gustavo

> ---
>   drivers/pwm/core.c  | 26 ++++++++++----------------
>   include/linux/pwm.h |  2 +-
>   2 files changed, 11 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
> index 15942210aa08..029aa1c69591 100644
> --- a/drivers/pwm/core.c
> +++ b/drivers/pwm/core.c
> @@ -198,7 +198,7 @@ static bool pwm_ops_check(const struct pwm_chip *chip)
>   
>   void *pwmchip_priv(struct pwm_chip *chip)
>   {
> -	return (void *)chip + sizeof(*chip);
> +	return (void *)chip + struct_size(chip, pwms, chip->npwm);
>   }
>   EXPORT_SYMBOL_GPL(pwmchip_priv);
>   
> @@ -206,8 +206,9 @@ struct pwm_chip *devm_pwmchip_alloc(struct device *parent, unsigned int npwm, si
>   {
>   	struct pwm_chip *chip;
>   	size_t alloc_size;
> +	unsigned int i;
>   
> -	alloc_size = size_add(sizeof(*chip), sizeof_priv);
> +	alloc_size = size_add(struct_size(chip, pwms, npwm), sizeof_priv);
>   
>   	chip = devm_kzalloc(parent, alloc_size, GFP_KERNEL);
>   	if (!chip)
> @@ -217,6 +218,13 @@ struct pwm_chip *devm_pwmchip_alloc(struct device *parent, unsigned int npwm, si
>   	chip->npwm = npwm;
>   	chip->uses_pwmchip_alloc = true;
>   
> +	for (i = 0; i < chip->npwm; i++) {
> +		struct pwm_device *pwm = &chip->pwms[i];
> +
> +		pwm->chip = chip;
> +		pwm->hwpwm = i;
> +	}
> +
>   	return chip;
>   }
>   EXPORT_SYMBOL_GPL(devm_pwmchip_alloc);
> @@ -234,7 +242,6 @@ EXPORT_SYMBOL_GPL(devm_pwmchip_alloc);
>   int __pwmchip_add(struct pwm_chip *chip, struct module *owner)
>   {
>   	int ret;
> -	unsigned i;
>   
>   	if (!chip || !chip->dev || !chip->ops || !chip->npwm)
>   		return -EINVAL;
> @@ -253,26 +260,15 @@ int __pwmchip_add(struct pwm_chip *chip, struct module *owner)
>   
>   	chip->owner = owner;
>   
> -	chip->pwms = kcalloc(chip->npwm, sizeof(*chip->pwms), GFP_KERNEL);
> -	if (!chip->pwms)
> -		return -ENOMEM;
> -
>   	mutex_lock(&pwm_lock);
>   
>   	ret = idr_alloc(&pwmchip_idr, chip, 0, 0, GFP_KERNEL);
>   	if (ret < 0) {
>   		mutex_unlock(&pwm_lock);
> -		kfree(chip->pwms);
>   		return ret;
>   	}
>   
>   	chip->id = ret;
> -	for (i = 0; i < chip->npwm; i++) {
> -		struct pwm_device *pwm = &chip->pwms[i];
> -
> -		pwm->chip = chip;
> -		pwm->hwpwm = i;
> -	}
>   
>   	mutex_unlock(&pwm_lock);
>   
> @@ -303,8 +299,6 @@ void pwmchip_remove(struct pwm_chip *chip)
>   	idr_remove(&pwmchip_idr, chip->id);
>   
>   	mutex_unlock(&pwm_lock);
> -
> -	kfree(chip->pwms);
>   }
>   EXPORT_SYMBOL_GPL(pwmchip_remove);
>   
> diff --git a/include/linux/pwm.h b/include/linux/pwm.h
> index b8e70ee01d31..a7294ef1495d 100644
> --- a/include/linux/pwm.h
> +++ b/include/linux/pwm.h
> @@ -302,7 +302,7 @@ struct pwm_chip {
>   
>   	/* only used internally by the PWM framework */
>   	bool uses_pwmchip_alloc;
> -	struct pwm_device *pwms;
> +	struct pwm_device pwms[] __counted_by(npwm);
>   };
>   
>   static inline struct device *pwmchip_parent(struct pwm_chip *chip)