stm: Potential read overflow in stm_char_policy_set_ioctl()

The "size" variable comes from the user so we need to verify that it's
large enough to hold an stp_policy_id struct.

Fixes: 7bd1d409 ("stm class: Introduce an abstraction for System Trace Module devices")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>

drivers/hwtracing/stm/core.c

@@ -566,7 +566,7 @@ static int stm_char_policy_set_ioctl(struct stm_file *stmf, void __user *arg)
 	if (copy_from_user(&size, arg, sizeof(size)))
 		return -EFAULT;
 
-	if (size >= PATH_MAX + sizeof(*id))
+	if (size < sizeof(*id) || size >= PATH_MAX + sizeof(*id))
 		return -EINVAL;
 
 	/*