NTB: ntb_tool: uninitialized heap data in tool_fn_write()

The call to:

ret = simple_write_to_buffer(buf, size, offp, ubuf, size);

will return success if it is able to write even one byte to "buf".
The value of "*offp" controls which byte. This could result in
reading uninitialized data when we do the sscanf() on the next line.

This code is not really desigined to handle partial writes where
*offp is non-zero and the "buf" is preserved and re-used between writes.
Just ban partial writes and replace the simple_write_to_buffer() with
copy_from_user().

Fixes: 578b881ba9c4 ("NTB: Add tool test client")
Signed-off-by: Dan Carpenter <[e-mail]>
Signed-off-by: Jon Mason <[e-mail]>

drivers/ntb/test/ntb_tool.c

···
 	u64 bits;
 	int n;
 
+	if (*offp)
+		return 0;
+
 	buf = kmalloc(size + 1, GFP_KERNEL);
 	if (!buf)
 		return -ENOMEM;
 
-	ret = simple_write_to_buffer(buf, size, offp, ubuf, size);
-	if (ret < 0) {
+	if (copy_from_user(buf, ubuf, size)) {
 		kfree(buf);
-		return ret;
+		return -EFAULT;
 	}
 
 	buf[size] = 0;