netdev - Re: [PATCH RFC v2 00/12] socket sendmsg MSG

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Message-Id: <20170228.222845.1863678335516061180.davem@davemloft.net>
Date:   Tue, 28 Feb 2017 22:28:45 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     luto@...capital.net
Cc:     willemdebruijn.kernel@...il.com, mtk.manpages@...il.com,
        netdev@...r.kernel.org, willemb@...gle.com,
        linux-api@...r.kernel.org
Subject: Re: [PATCH RFC v2 00/12] socket sendmsg MSG_ZEROCOPY

From: Andy Lutomirski <luto@...capital.net>
Date: Tue, 28 Feb 2017 13:06:49 -0800

> 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.

These device are already choking, because as I stated this can already
be done via sendfile().

Networking card wise this isn't an issue, chips bring the entire packet
into their FIFO, compute checksums on the fly mid-stream, and then write
the 16-bit checksum field before starting to write the packet onto the
wire.

I think this is completely a non-issue, and we thought about this right
from the start when sendfile() support was added nearly two decades ago.
If network cards from back then didn't crap out in this situation I
think the ones out there now are probably ok.