linux-kernel - [PATCH v2] gpiolib: Fix possible use after free on label

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Message-Id: <20181101131250.41636-1-smuchun@gmail.com>
Date:   Thu,  1 Nov 2018 21:12:50 +0800
From:   Muchun Song <smuchun@...il.com>
To:     linus.walleij@...aro.org, bgolaszewski@...libre.com
Cc:     linux-gpio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH v2] gpiolib: Fix possible use after free on label

gpiod_request_commit() copies the pointer to the label passed as
an argument only to be used later. But there's a chance the caller
could immediately free the passed string(e.g., local variable).
This could trigger a use after free when we use gpio label(e.g.,
gpiochip_unlock_as_irq(), gpiochip_is_requested()).

To be on the safe side: duplicate the string with kstrdup_const()
so that if an unaware user passes an address to a stack-allocated
buffer, we won't get the arbitrary label.

Also fix gpiod_set_consumer_name().

Signed-off-by: Muchun Song <smuchun@...il.com>
---
 drivers/gpio/gpiolib.c        | 25 +++++++++++++++++++++----
 include/linux/gpio/consumer.h |  6 ++++--
 2 files changed, 25 insertions(+), 6 deletions(-)

diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
index 25187403e3ac..550918268549 100644
--- a/drivers/gpio/gpiolib.c
+++ b/drivers/gpio/gpiolib.c
@@ -2270,6 +2270,12 @@ static int gpiod_request_commit(struct gpio_desc *desc, const char *label)
 	unsigned long		flags;
 	unsigned		offset;
 
+	if (label) {
+		label = kstrdup_const(label, GFP_KERNEL);
+		if (!label)
+			return -ENOMEM;
+	}
+
 	spin_lock_irqsave(&gpio_lock, flags);
 
 	/* NOTE:  gpio_request() can be called in early boot,
@@ -2280,6 +2286,7 @@ static int gpiod_request_commit(struct gpio_desc *desc, const char *label)
 		desc_set_label(desc, label ? : "?");
 		status = 0;
 	} else {
+		kfree_const(label);
 		status = -EBUSY;
 		goto done;
 	}
@@ -2296,6 +2303,7 @@ static int gpiod_request_commit(struct gpio_desc *desc, const char *label)
 
 		if (status < 0) {
 			desc_set_label(desc, NULL);
+			kfree_const(label);
 			clear_bit(FLAG_REQUESTED, &desc->flags);
 			goto done;
 		}
@@ -2391,6 +2399,7 @@ static bool gpiod_free_commit(struct gpio_desc *desc)
 			chip->free(chip, gpio_chip_hwgpio(desc));
 			spin_lock_irqsave(&gpio_lock, flags);
 		}
+		kfree_const(desc->label);
 		desc_set_label(desc, NULL);
 		clear_bit(FLAG_ACTIVE_LOW, &desc->flags);
 		clear_bit(FLAG_REQUESTED, &desc->flags);
@@ -3212,11 +3221,19 @@ EXPORT_SYMBOL_GPL(gpiod_cansleep);
  * @desc: gpio to set the consumer name on
  * @name: the new consumer name
  */
-void gpiod_set_consumer_name(struct gpio_desc *desc, const char *name)
+int gpiod_set_consumer_name(struct gpio_desc *desc, const char *name)
 {
-	VALIDATE_DESC_VOID(desc);
-	/* Just overwrite whatever the previous name was */
-	desc->label = name;
+	VALIDATE_DESC(desc);
+	if (name) {
+		name = kstrdup_const(name, GFP_KERNEL);
+		if (!name)
+			return -ENOMEM;
+	}
+
+	kfree_const(desc->label);
+	desc_set_label(desc, name);
+
+	return 0;
 }
 EXPORT_SYMBOL_GPL(gpiod_set_consumer_name);
 
diff --git a/include/linux/gpio/consumer.h b/include/linux/gpio/consumer.h
index 21ddbe440030..acc4279ad5e3 100644
--- a/include/linux/gpio/consumer.h
+++ b/include/linux/gpio/consumer.h
@@ -142,7 +142,7 @@ int gpiod_is_active_low(const struct gpio_desc *desc);
 int gpiod_cansleep(const struct gpio_desc *desc);
 
 int gpiod_to_irq(const struct gpio_desc *desc);
-void gpiod_set_consumer_name(struct gpio_desc *desc, const char *name);
+int gpiod_set_consumer_name(struct gpio_desc *desc, const char *name);
 
 /* Convert between the old gpio_ and new gpiod_ interfaces */
 struct gpio_desc *gpio_to_desc(unsigned gpio);
@@ -465,10 +465,12 @@ static inline int gpiod_to_irq(const struct gpio_desc *desc)
 	return -EINVAL;
 }
 
-static inline void gpiod_set_consumer_name(struct gpio_desc *desc, const char *name)
+static inline int gpiod_set_consumer_name(struct gpio_desc *desc,
+					  const char *name)
 {
 	/* GPIO can never have been requested */
 	WARN_ON(1);
+	return -EINVAL;
 }
 
 static inline struct gpio_desc *gpio_to_desc(unsigned gpio)
-- 
2.17.1