of: Fix modalias string generation

stable inclusion
from stable-v4.19.283
commit d72e2dc104e65827798e116f9e4853b85488d3e8
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I7BZ5U
CVE: NA

--------------------------------

[ Upstream commit b19a4266 ]

The helper generating an OF based modalias (of_device_get_modalias())
works fine, but due to the use of snprintf() internally it needs a
buffer one byte longer than what should be needed just for the entire
string (excluding the '\0'). Most users of this helper are sysfs hooks
providing the modalias string to users. They all provide a PAGE_SIZE
buffer which is way above the number of bytes required to fit the
modalias string and hence do not suffer from this issue.

There is another user though, of_device_request_module(), which is only
called by drivers/usb/common/ulpi.c. This request module function is
faulty, but maybe because in most cases there is an alternative, ULPI
driver users have not noticed it.

In this function, of_device_get_modalias() is called twice. The first
time without buffer just to get the number of bytes required by the
modalias string (excluding the null byte), and a second time, after
buffer allocation, to fill the buffer. The allocation asks for an
additional byte, in order to store the trailing '\0'. However, the
buffer *length* provided to of_device_get_modalias() excludes this extra
byte. The internal use of snprintf() with a length that is exactly the
number of bytes to be written has the effect of using the last available
byte to store a '\0', which then smashes the last character of the
modalias string.

Provide the actual size of the buffer to of_device_get_modalias() to fix
this issue.

Note: the "str[size - 1] = '\0';" line is not really needed as snprintf
will anyway end the string with a null byte, but there is a possibility
that this function might be called on a struct device_node without
compatible, in this case snprintf() would not be executed. So we keep it
just to avoid possible unbounded strings.

Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Peter Chen <peter.chen@kernel.org>
Fixes: 9c829c09 ("of: device: Support loading a module with OF based modalias")
Signed-off-by: NMiquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NSrinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20230404172148.82422-2-srinivas.kandagatla@linaro.orgSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NSasha Levin <sashal@kernel.org>
Signed-off-by: NYongqiang Liu <liuyongqiang13@huawei.com>

drivers/of/device.c

@@ -258,12 +258,15 @@ int of_device_request_module(struct device *dev)
 	if (size < 0)
 		return size;
 
-	str = kmalloc(size + 1, GFP_KERNEL);
+	/* Reserve an additional byte for the trailing '\0' */
+	size++;
+
+	str = kmalloc(size, GFP_KERNEL);
 	if (!str)
 		return -ENOMEM;
 
 	of_device_get_modalias(dev, str, size);
-	str[size] = '\0';
+	str[size - 1] = '\0';
 	ret = request_module(str);
 	kfree(str);