soc: qcom: socinfo: Avoid out of bounds read of serial number

On MSM8916 devices, the serial number exposed in sysfs is constant and does
not change across individual devices. It's always:

  db410c:/sys/devices/soc0$ cat serial_number
  2644893864

The firmware used on MSM8916 exposes SOCINFO_VERSION(0, 8), which does not
have support for the serial_num field in the socinfo struct. There is an
existing check to avoid exposing the serial number in that case, but it's
not correct: When checking the item_size returned by SMEM, we need to make
sure the *end* of the serial_num is within bounds, instead of comparing
with the *start* offset. The serial_number currently exposed on MSM8916
devices is just an out of bounds read of whatever comes after the socinfo
struct in SMEM.

Fix this by changing offsetof() to offsetofend(), so that the size of the
field is also taken into account.

Cc: stable@vger.kernel.org
Fixes: efb448d ("soc: qcom: Add socinfo driver")
Signed-off-by: Stephan Gerhold <stephan.gerhold@linaro.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20241230-qcom-socinfo-serialno-oob-v1-1-9b7a890da3da@linaro.org
Signed-off-by: Bjorn Andersson <andersson@kernel.org>

drivers/soc/qcom/socinfo.c

@@ -796,7 +796,7 @@ static int qcom_socinfo_probe(struct platform_device *pdev)
 	if (!qs->attr.soc_id || !qs->attr.revision)
 		return -ENOMEM;
 
-	if (offsetof(struct socinfo, serial_num) <= item_size) {
+	if (offsetofend(struct socinfo, serial_num) <= item_size) {
 		qs->attr.serial_number = devm_kasprintf(&pdev->dev, GFP_KERNEL,
 							"%u",
 							le32_to_cpu(info->serial_num));