Subject: Re: [PATCH v3 2/3] packet: fill the gap of TPACKET_ALIGNMENT with zeros

From: Daniel Borkmann <[email-masked]>
Date: Mon, 16 Dec 2013 13:55:43 +0100

On 12/16/2013 01:41 PM, Atzm Watanabe wrote:
> At Mon, 16 Dec 2013 11:22:35 +0100,
> Daniel Borkmann wrote:
>>
>> On 12/16/2013 11:16 AM, David Laight wrote:
>>>>> +		memset(h.h2->tp_padding, 0,
>>>>> +		       hdrlen - offsetof(struct tpacket2_hdr, tp_padding));
>>>
>>> What is wrong with 'sizeof h.h2->tp_padding' ?
>>> The compiler will probably inline the memset into a couple of word
>>> sized writes of zero - probably not measurable.
>>> Not zeroing them might be leaking kernel memory contents (depends
>>> where the memory came from - might just be stale packet data).
>>
>> The ring buffer memory we're operating on comes from mmap(2) btw.
>
> Thank you for comments.
> In struct tpacket2_hdr, it seems that a padding member was really
> zeroing to fix information leak on commit
> 13fcb7bd322164c67926ffe272846d4860196dc6 ("af_packet: prevent information leak").
> So next time I'll try to zero tp_padding using sizof(h.h2->tp_padding)
> David proposed.  If you have any thoughts on this please share it with me.

Yep, 13fcb7bd was for struct tpacket_auxdata structure in packet_recvmsg()
that sits on the stack and copied uninitialized data to user space. But,
okay, lets go with the memset().