linux-hardening - [PATCH v4 111/115] pwm: Ensure a struct pwm has the same lifetime as its pwm

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Date: Wed,  6 Dec 2023 12:45:05 +0100
From: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To: Thierry Reding <thierry.reding@...il.com>
Cc: kernel@...gutronix.de,
	Kees Cook <keescook@...omium.org>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	linux-pwm@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: [PATCH v4 111/115] pwm: Ensure a struct pwm has the same lifetime as its pwm_chip

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().

Reviewed-by: Gustavo A. R. Silva <gustavoars@...nel.org> # for struct_size() and __counted_by()
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
---
 drivers/pwm/core.c  | 27 ++++++++++-----------------
 include/linux/pwm.h |  2 +-
 2 files changed, 11 insertions(+), 18 deletions(-)

diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
index ce6f4b397468..b3ba08aa0b14 100644
--- a/drivers/pwm/core.c
+++ b/drivers/pwm/core.c
@@ -200,15 +200,16 @@ static bool pwm_ops_check(const struct pwm_chip *chip)
 
 static void *pwmchip_priv(struct pwm_chip *chip)
 {
-	return (void *)chip + sizeof(*chip);
+	return (void *)chip + struct_size(chip, pwms, chip->npwm);
 }
 
 struct pwm_chip *devm_pwmchip_alloc(struct device *parent, unsigned int npwm, size_t sizeof_priv)
 {
 	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)
@@ -220,6 +221,13 @@ struct pwm_chip *devm_pwmchip_alloc(struct device *parent, unsigned int npwm, si
 
 	pwmchip_set_drvdata(chip, pwmchip_priv(chip));
 
+	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);
@@ -236,7 +244,6 @@ EXPORT_SYMBOL_GPL(devm_pwmchip_alloc);
  */
 int __pwmchip_add(struct pwm_chip *chip, struct module *owner)
 {
-	unsigned int i;
 	int ret;
 
 	if (!chip || !chip->dev || !chip->ops || !chip->npwm)
@@ -256,28 +263,16 @@ 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);
 
 	if (IS_ENABLED(CONFIG_OF))
@@ -307,8 +302,6 @@ 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 6a5b245e8951..eb91306ba28e 100644
--- a/include/linux/pwm.h
+++ b/include/linux/pwm.h
@@ -301,8 +301,8 @@ struct pwm_chip {
 
 	/* only used internally by the PWM framework */
 	bool uses_pwmchip_alloc;
-	struct pwm_device *pwms;
 	void *driver_data;
+	struct pwm_device pwms[] __counted_by(npwm);
 };
 
 static inline struct device *pwmchip_parent(struct pwm_chip *chip)
-- 
2.42.0