[PATCH v3] gpio: cdev: fix kernel stack leak to user-space in error path

[Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>]

If we fail to acquire the GPIO chip guard in gpio_desc_to_lineinfo(), we
return immediately before zeroing the info struct we'll end up passing
to the user-space later in lineinfo_get_v1(). This may leak the kernel
stack contents. Make gpio_desc_to_lineinfo() return int so that the
-ENODEV returned on failure to acquire the guard can be propagated to
the callers.

While not strictly necessary: move the memset() before trying to acquire
the SRCU read lock too for good measure.

Fixes: d83cee3d2bb1 ("gpio: protect the pointer to gpio_chip in gpio_device with SRCU")
Cc: stable@vger.kernel.org
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260912123529.7951-1-tzungbi%40kernel.org?part=3
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
---
Changes in v3:
- Don't emit an error log message from the line info notifier if we fail
  to acquire the chip guard
- Link to v2: https://patch.msgid.link/20260921-gpio-cdev-stack-leak-fixes-v2-1-3307c4320352@oss.qualcomm.com

Changes in v2:
- Make gpio_desc_to_lineinfo() return an integer return value so that
  the error code returned in case of a failure to acquire the guard can
  be propagated down the stack
- Link to v1: https://patch.msgid.link/20260917-gpio-cdev-stack-leak-fixes-v1-1-2bb52879e49c@oss.qualcomm.com
---
 drivers/gpio/gpiolib-cdev.c | 32 +++++++++++++++++++++++++-------
 1 file changed, 25 insertions(+), 7 deletions(-)

diff --git a/drivers/gpio/gpiolib-cdev.c b/drivers/gpio/gpiolib-cdev.c
index b5da6e8566dcd777ef4e7784989b722416c07fd3..270edfdc5da203d072eee362aead0a55d9247fa4 100644
--- a/drivers/gpio/gpiolib-cdev.c
+++ b/drivers/gpio/gpiolib-cdev.c
@@ -2174,18 +2174,19 @@ static void gpio_v2_line_info_changed_to_v1(
 
 #endif /* CONFIG_GPIO_CDEV_V1 */
 
-static void gpio_desc_to_lineinfo(struct gpio_desc *desc,
-				  struct gpio_v2_line_info *info, bool atomic)
+static int gpio_desc_to_lineinfo(struct gpio_desc *desc,
+				 struct gpio_v2_line_info *info, bool atomic)
 {
 	u32 debounce_period_us;
 	unsigned long dflags;
 	const char *label;
 
+	memset(info, 0, sizeof(*info));
+
 	CLASS(gpio_chip_guard, guard)(desc);
 	if (!guard.gc)
-		return;
+		return -ENODEV;
 
-	memset(info, 0, sizeof(*info));
 	info->offset = gpiod_hwgpio(desc);
 
 	if (desc->name)
@@ -2260,6 +2261,8 @@ static void gpio_desc_to_lineinfo(struct gpio_desc *desc,
 							debounce_period_us;
 		info->num_attrs++;
 	}
+
+	return 0;
 }
 
 struct gpio_chardev_data {
@@ -2311,6 +2314,7 @@ static int lineinfo_get_v1(struct gpio_chardev_data *cdev, void __user *ip,
 	struct gpio_desc *desc;
 	struct gpioline_info lineinfo;
 	struct gpio_v2_line_info lineinfo_v2;
+	int ret;
 
 	if (copy_from_user(&lineinfo, ip, sizeof(lineinfo)))
 		return -EFAULT;
@@ -2328,7 +2332,10 @@ static int lineinfo_get_v1(struct gpio_chardev_data *cdev, void __user *ip,
 			return -EBUSY;
 	}
 
-	gpio_desc_to_lineinfo(desc, &lineinfo_v2, false);
+	ret = gpio_desc_to_lineinfo(desc, &lineinfo_v2, false);
+	if (ret)
+		return ret;
+
 	gpio_v2_line_info_to_v1(&lineinfo_v2, &lineinfo);
 
 	if (copy_to_user(ip, &lineinfo, sizeof(lineinfo))) {
@@ -2346,6 +2353,7 @@ static int lineinfo_get(struct gpio_chardev_data *cdev, void __user *ip,
 {
 	struct gpio_desc *desc;
 	struct gpio_v2_line_info lineinfo;
+	int ret;
 
 	if (copy_from_user(&lineinfo, ip, sizeof(lineinfo)))
 		return -EFAULT;
@@ -2365,7 +2373,10 @@ static int lineinfo_get(struct gpio_chardev_data *cdev, void __user *ip,
 		if (test_and_set_bit(lineinfo.offset, cdev->watched_lines))
 			return -EBUSY;
 	}
-	gpio_desc_to_lineinfo(desc, &lineinfo, false);
+
+	ret = gpio_desc_to_lineinfo(desc, &lineinfo, false);
+	if (ret)
+		return ret;
 
 	if (copy_to_user(ip, &lineinfo, sizeof(lineinfo))) {
 		if (watch)
@@ -2491,6 +2502,7 @@ static int lineinfo_changed_notify(struct notifier_block *nb,
 	struct lineinfo_changed_ctx *ctx;
 	struct gpio_desc *desc = data;
 	struct file *fp;
+	int ret;
 
 	if (!test_bit(gpiod_hwgpio(desc), cdev->watched_lines))
 		return NOTIFY_DONE;
@@ -2521,7 +2533,13 @@ static int lineinfo_changed_notify(struct notifier_block *nb,
 
 	ctx->chg.event_type = action;
 	ctx->chg.timestamp_ns = ktime_get_ns();
-	gpio_desc_to_lineinfo(desc, &ctx->chg.info, true);
+
+	ret = gpio_desc_to_lineinfo(desc, &ctx->chg.info, true);
+	if (ret) {
+		fput(fp);
+		return NOTIFY_DONE;
+	}
+
 	/* Keep the GPIO device alive until we emit the event. */
 	ctx->gdev = gpio_device_get(desc->gdev);
 	ctx->cdev = cdev;

---
base-commit: 5c4d4169604b335c38bbc79bc1fc03042981fc6f
change-id: 20260917-gpio-cdev-stack-leak-fixes-84ad8918a6e0

Best regards,
-- 
Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>