Re: net: uninitialized loopback addr leaks to userspace

Vegard Nossum
2009-06-08T09:17:13Z

2009/6/8 Stephen Hemminger <shemminger@vyatta.com>:
> On Sat, 30 May 2009 22:23:24 +0200
> Vegard Nossum <vegard.nossum@gmail.com> wrote:
>
>> Hi,
>>
>> 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). This can be demonstrated by creating a new network
>> namespace (CLONE_NEWNET), which creates a new loopback device, then
>> call ioctl() with SIOCGIFHWADDR on "lo". If this is done in a loop,
>> with some background load, or by running multiple instances, random
>> data will start to show up in the returned address.
>>
>> [  406.750329] WARNING: kmemcheck: Caught 16-bit read from
>> uninitialized memory (ffff880007220974)
>> [  406.753555] 18a2d7060088ffff18a2d7060088ffff00000000010000000100000003000000
>> [  406.758862]  i i i i i i i i i i i i i i i i i u u u u u u u u u u u u u u u
>> [  406.766224]                                          ^
>> [  406.768792] Modules linked in:
>> [  406.770416] Pid: 757, comm: ifconfig Not tainted
>> 2.6.30-rc7-next-20090529 #404
>> [  406.772876] RIP: 0010:[<ffffffff80664789>]  [<ffffffff80664789>]
>> dev_ioctl+0x5d9/0x600
>> [  406.804677]  [<ffffffff8064ff75>] sock_ioctl+0x95/0x2a0
>> [  406.807242]  [<ffffffff802c35eb>] vfs_ioctl+0x1b/0x70
>> [  406.809348]  [<ffffffff802c36fa>] do_vfs_ioctl+0x8a/0x570
>> [  406.811419]  [<ffffffff802c3c79>] sys_ioctl+0x99/0xa0
>> [  406.813400]  [<ffffffff802f3941>] dev_ifsioc+0x81/0x2f0
>> [  406.815424]  [<ffffffff802f454d>] compat_sys_ioctl+0xed/0x3c0
>> [  406.817596]  [<ffffffff8022d476>] cstar_dispatch+0x7/0x26
>> [  406.819978]  [<ffffffffffffffff>] 0xffffffffffffffff
>>
>> This is the code that triggers the warning, in net/core/dev.c, around line 4150:
>>
>>     memcpy(ifr->ifr_hwaddr.sa_data, dev->dev_addr,
>>         min(sizeof ifr->ifr_hwaddr.sa_data, (size_t) dev->addr_len));
>>
>> So it's dev->dev_addr that is the pointer to the uninitialized data.
>>
>> I didn't know how to fix it.
>>
>
>
> The whole dev structure is zeroed in alloc_netdev(), kmemcheck
> is giving bogus warning.

Hi -- and sorry for being unclear. If I hadn't been sure that this was
a real error, I would have said so (or not reported it at all).

I investigated it now, and as can be seen in the report above, I am
using a -next kernel. It seems that the error was introduced in:

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)

So the error does not, as you say, exist in mainline Linux, but it's
not a bogus warning either :-)

Adding to Cc.

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