[PATCH] fix double fetch in wmi_ioctl.

[JingYi Hou]

There is a double fetch problem in function wmi_ioctl.
After second fetch overwrite the length to avoid this.
buf->length is not used now,but it may in the future.
It is good to get it fixed up.

Signed-off-by: JingYi Hou <houjingyi...@gmail.com>
---
 drivers/platform/x86/wmi.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/drivers/platform/x86/wmi.c b/drivers/platform/x86/wmi.c
index 7b26b6ccf1a0..628b9730b4d7 100644
--- a/drivers/platform/x86/wmi.c
+++ b/drivers/platform/x86/wmi.c
@@ -823,6 +823,7 @@ static long wmi_ioctl(struct file *filp, unsigned int cmd, 
unsigned long arg)
        struct wmi_ioctl_buffer *buf = NULL;
        struct wmi_driver *wdriver = NULL;
        int ret;
+       __u64 length;
 
        if (_IOC_TYPE(cmd) != WMI_IOC)
                return -ENOTTY;
@@ -833,24 +834,24 @@ static long wmi_ioctl(struct file *filp, unsigned int 
cmd, unsigned long arg)
 
        mutex_lock(&wblock->char_mutex);
        buf = wblock->handler_data;
-       if (get_user(buf->length, &input->length)) {
+       if (get_user(length, &input->length)) {
                dev_dbg(&wblock->dev.dev, "Read length from user failed\n");
                ret = -EFAULT;
                goto out_ioctl;
        }
        /* if it's too small, abort */
-       if (buf->length < wblock->req_buf_size) {
+       if (length < wblock->req_buf_size) {
                dev_err(&wblock->dev.dev,
                        "Buffer %lld too small, need at least %lld\n",
-                       buf->length, wblock->req_buf_size);
+                       length, wblock->req_buf_size);
                ret = -EINVAL;
                goto out_ioctl;
        }
        /* if it's too big, warn, driver will only use what is needed */
-       if (buf->length > wblock->req_buf_size)
+       if (length > wblock->req_buf_size)
                dev_warn(&wblock->dev.dev,
                        "Buffer %lld is bigger than required %lld\n",
-                       buf->length, wblock->req_buf_size);
+                       length, wblock->req_buf_size);
 
        /* copy the structure from userspace */
        if (copy_from_user(buf, input, wblock->req_buf_size)) {
@@ -860,6 +861,9 @@ static long wmi_ioctl(struct file *filp, unsigned int cmd, 
unsigned long arg)
                goto out_ioctl;
        }
 
+       /* Override length in case it is changed between two userspace fetches 
*/
+       buf->length = length;
+
        /* let the driver do any filtering and do the call */
        wdriver = container_of(wblock->dev.dev.driver,
                               struct wmi_driver, driver);
-- 
2.20.1