Re: net: uninitialized loopback addr leaks to userspace

Vegard Nossum
2009-06-08T10:00:58Z

2009/6/7 John Dykstra <john.dykstra1@gmail.com>:
> On Sat, 2009-05-30 at 22:23 +0200, Vegard Nossum wrote:
>> It seems that loopback's hardware address is never initialized by the
>> kernel. So if userspace attempts to read this address before it has
>> been set, the kernel will return some uninitialized data (only 6
>> bytes, though).
>
> Thank you for the report, Vegard.
>
> I've been unable to reproduce the problem you describe, using
> 2.6-30-rc8, this test program and a couple of kernel builds for system
> load:
[...]
> ------------------------------------------------------------------
>
> Looking at the kernel code, it appears that all bytes of struct
> net_device, including the L2 address, are initialized to zeros at
> interface creation time.
>
> Can you spot a difference between your test procedures and mine that
> would enable me to reproduce the problem?

Hi,

I just tried your test program on a linux-next kernel, it works beautifully :-)

(I made one change: The stack grows downwards on x86, so I think you
should put child_stack + 16386 as the stack to clone()?)

As I wrote in reply to Stephen Hemminger, this problem seems to be
caused by a particular patch in linux-next:

commit f001fde5eadd915f4858d22ed70d7040f48767cf
Author: Jiri Pirko <jpirko@redhat.com>
Date: Tue May 5 02:48:28 2009 +0000

net: introduce a list of device addresses dev_addr_list (v6)

Thanks for testing.

Vegard

--
"The animistic metaphor of the bug that maliciously sneaked in while
the programmer was not looking is intellectually dishonest as it
disguises that the error is the programmer's own creation."
-- E. W. Dijkstra, EWD1036