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Date:   Tue, 28 Feb 2017 13:06:49 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Michael Kerrisk <mtk.manpages@...il.com>,
        netdev <netdev@...r.kernel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH RFC v2 00/12] socket sendmsg MSG_ZEROCOPY

On Tue, Feb 28, 2017 at 12:43 PM, Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
> On Tue, Feb 28, 2017 at 2:46 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> On Mon, Feb 27, 2017 at 10:57 AM, Michael Kerrisk
>> <mtk.manpages@...il.com> wrote:
>>> [CC += linux-api@...r.kernel.org]
>>>
>>> Hi Willem
>>>
>>
>>>> On a send call with MSG_ZEROCOPY, the kernel pins the user pages and
>>>> creates skbuff fragments directly from these pages. On tx completion,
>>>> it notifies the socket owner that it is safe to modify memory by
>>>> queuing a completion notification onto the socket error queue.
>>
>> What happens if the user writes to the pages while it's not safe?
>>
>> How about if you're writing to an interface or a route that has crypto
>> involved and a malicious user can make the data change in the middle
>> of a crypto operation, thus perhaps leaking the entire key?  (I
>> wouldn't be at all surprised if a lot of provably secure AEAD
>> constructions are entirely compromised if an attacker can get the
>> ciphertext and tag computed from a message that changed during the
>> computation.
>
> Operations that read or write payload, such as this crypto example,
> but also ebpf in tc or iptables, for instance, demand a deep copy using
> skb_copy_ubufs before the operation.
>
> This blacklist approach requires caution, but these paths should be
> few and countable. It is not possible to predict at the socket layer
> whether a packet will encounter any such operation, so white-listing
> a subset of end-to-end paths is not practical.

How about hardware that malfunctions if the packet changes out from
under it?  A whitelist seems quite a bit safer.

--Andy

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