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From:   Milan Zamazal <mzamazal@redhat.com>
To:     linux-iio@vger.kernel.org
Cc:     Milan Zamazal <mzamazal@redhat.com>, linux-kernel@vger.kernel.org,
        Jonathan Cameron <jic23@kernel.org>,
        Lars-Peter Clausen <lars@metafoo.de>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        =?UTF-8?q?Nuno=20S=C3=A1?= <noname.nuno@gmail.com>
Subject: [PATCH v3] iio: core: Prevent invalid memory access when there is no parent
Date:   Wed, 19 Jul 2023 10:32:08 +0200
Message-Id: <20230719083208.88149-1-mzamazal@redhat.com>
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Commit 813665564b3d ("iio: core: Convert to use firmware node handle
instead of OF node") switched the kind of nodes to use for label
retrieval in device registration.  Probably an unwanted change in that
commit was that if the device has no parent then NULL pointer is
accessed.  This is what happens in the stock IIO dummy driver when a
new entry is created in configfs:

  # mkdir /sys/kernel/config/iio/devices/dummy/foo
  BUG: kernel NULL pointer dereference, address: ...
  ...
  Call Trace:
  __iio_device_register
  iio_dummy_probe

Since there seems to be no reason to make a parent device of an IIO
dummy device mandatory, let’s prevent the invalid memory access in
__iio_device_register when the parent device is NULL.  With this
change, the IIO dummy driver works fine with configfs.

Fixes: 813665564b3d ("iio: core: Convert to use firmware node handle instead of OF node")
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Milan Zamazal <mzamazal@redhat.com>
---
Changes in v3:
 - The call trace in the commit message reduced to the minimum lines
   needed to explain the problem.

Changes in v2:
 - Added a source comment about the dummy IIO device.
 - Adjusted the backtrace cited in the commit message a bit.
 - Replaced `... != NULL' condition with `...'.
 - Dropped the unnecessary `fwnode != NULL' check (the involved calls
   do the right thing when the argument is NULL).
---
 drivers/iio/industrialio-core.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
index c117f50d0cf3..adcba832e6fa 100644
--- a/drivers/iio/industrialio-core.c
+++ b/drivers/iio/industrialio-core.c
@@ -1888,7 +1888,7 @@ static const struct iio_buffer_setup_ops noop_ring_setup_ops;
 int __iio_device_register(struct iio_dev *indio_dev, struct module *this_mod)
 {
 	struct iio_dev_opaque *iio_dev_opaque = to_iio_dev_opaque(indio_dev);
-	struct fwnode_handle *fwnode;
+	struct fwnode_handle *fwnode = NULL;
 	int ret;
 
 	if (!indio_dev->info)
@@ -1899,7 +1899,8 @@ int __iio_device_register(struct iio_dev *indio_dev, struct module *this_mod)
 	/* If the calling driver did not initialize firmware node, do it here */
 	if (dev_fwnode(&indio_dev->dev))
 		fwnode = dev_fwnode(&indio_dev->dev);
-	else
+	/* The default dummy IIO device has no parent */
+	else if (indio_dev->dev.parent)
 		fwnode = dev_fwnode(indio_dev->dev.parent);
 	device_set_node(&indio_dev->dev, fwnode);
 
-- 
2.40.1