linux-hardening - [PATCH v6 162/164] pwm: Ensure a struct pwm has the same lifetime as its pwm

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Message-ID:  <3eb4ed957bf37faf67a978816c4bf619176d8545.1707900770.git.u.kleine-koenig@pengutronix.de>
Date: Wed, 14 Feb 2024 10:33:29 +0100
From: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To: Kees Cook <keescook@...omium.org>,
	linux-pwm@...r.kernel.org
Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>,
	kernel@...gutronix.de,
	linux-hardening@...r.kernel.org
Subject: [PATCH v6 162/164] 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 cae93ccc32ed..c8303da06b62 100644
--- a/drivers/pwm/core.c
+++ b/drivers/pwm/core.c
@@ -458,7 +458,7 @@ EXPORT_SYMBOL_GPL(of_pwm_single_xlate);
 
 static void *pwmchip_priv(struct pwm_chip *chip)
 {
-	return (void *)chip + ALIGN(sizeof(*chip), PWMCHIP_ALIGN);
+	return (void *)chip + ALIGN(struct_size(chip, pwms, chip->npwm), PWMCHIP_ALIGN);
 }
 
 /* This is the counterpart to pwmchip_alloc */
@@ -472,8 +472,9 @@ struct pwm_chip *pwmchip_alloc(struct device *parent, unsigned int npwm, size_t
 {
 	struct pwm_chip *chip;
 	size_t alloc_size;
+	unsigned int i;
 
-	alloc_size = size_add(ALIGN(sizeof(*chip), PWMCHIP_ALIGN), sizeof_priv);
+	alloc_size = size_add(ALIGN(struct_size(chip, pwms, npwm), PWMCHIP_ALIGN), sizeof_priv);
 
 	chip = kzalloc(alloc_size, GFP_KERNEL);
 	if (!chip)
@@ -485,6 +486,13 @@ struct pwm_chip *pwmchip_alloc(struct device *parent, unsigned int npwm, size_t
 
 	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(pwmchip_alloc);
@@ -556,7 +564,6 @@ static bool pwm_ops_check(const struct pwm_chip *chip)
  */
 int __pwmchip_add(struct pwm_chip *chip, struct module *owner)
 {
-	unsigned int i;
 	int ret;
 
 	if (!chip || !pwmchip_parent(chip) || !chip->ops || !chip->npwm)
@@ -576,28 +583,16 @@ 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(&pwm_chips, 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))
@@ -627,8 +622,6 @@ void pwmchip_remove(struct pwm_chip *chip)
 	idr_remove(&pwm_chips, 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 94a642a88817..fbd8f5db938a 100644
--- a/include/linux/pwm.h
+++ b/include/linux/pwm.h
@@ -290,7 +290,7 @@ struct pwm_chip {
 	/* only used internally by the PWM framework */
 	bool uses_pwmchip_alloc;
 	void *driver_data;
-	struct pwm_device *pwms;
+	struct pwm_device pwms[] __counted_by(npwm);
 };
 
 static inline struct device *pwmchip_parent(const struct pwm_chip *chip)
-- 
2.43.0