2 files changed, 22 insertions, 2 deletions

diff --git a/include/linux/rfkill.h b/include/linux/rfkill.h
index 2ce29831feb6..f3d5812693d6 100644
--- a/include/linux/rfkill.h
+++ b/include/linux/rfkill.h
@@ -82,6 +82,20 @@ struct rfkill_event {
         __u8  soft, hard;
 } __packed;
 
+/*
+ * We are planning to be backward and forward compatible with changes
+ * to the event struct, by adding new, optional, members at the end.
+ * When reading an event (whether the kernel from userspace or vice
+ * versa) we need to accept anything that's at least as large as the
+ * version 1 event size, but might be able to accept other sizes in
+ * the future.
+ *
+ * One exception is the kernel -- we already have two event sizes in
+ * that we've made the 'hard' member optional since our only option
+ * is to ignore it anyway.
+ */
+#define RFKILL_EVENT_SIZE_V1    8
+
 /* ioctl for turning off rfkill-input (if present) */
 #define RFKILL_IOC_MAGIC        'R'
 #define RFKILL_IOC_NOINPUT      1
diff --git a/net/rfkill/core.c b/net/rfkill/core.c
index 79693fe2001e..47497c97c8d9 100644
--- a/net/rfkill/core.c
+++ b/net/rfkill/core.c
@@ -1076,10 +1076,16 @@ static ssize_t rfkill_fop_write(struct file *file, const char __user *buf,
         struct rfkill_event ev;
 
         /* we don't need the 'hard' variable but accept it */
-        if (count < sizeof(ev) - 1)
+        if (count < RFKILL_EVENT_SIZE_V1 - 1)
                 return -EINVAL;
 
-        if (copy_from_user(&ev, buf, sizeof(ev) - 1))
+        /*
+         * Copy as much data as we can accept into our 'ev' buffer,
+         * but tell userspace how much we've copied so it can determine
+         * our API version even in a write() call, if it cares.
+         */
+        count = min(count, sizeof(ev));
+        if (copy_from_user(&ev, buf, count))
                 return -EFAULT;
 
         if (ev.op != RFKILL_OP_CHANGE && ev.op != RFKILL_OP_CHANGE_ALL)