netdev - Re: [bpf-next PATCH 4/7] net: do_tcp_sendpages flag to avoid SKBTX_SHARED

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Date:   Fri, 12 Jan 2018 13:11:46 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>, borkmann@...earbox.net,
        ast@...nel.org
Cc:     netdev@...r.kernel.org, kafai@...com
Subject: Re: [bpf-next PATCH 4/7] net: do_tcp_sendpages flag to avoid
 SKBTX_SHARED_FRAG

On 01/12/2018 12:46 PM, Eric Dumazet wrote:
> On Fri, 2018-01-12 at 12:26 -0800, John Fastabend wrote:
>> On 01/12/2018 12:10 PM, Eric Dumazet wrote:
>>> On Fri, 2018-01-12 at 10:10 -0800, John Fastabend wrote:
>>>> When calling do_tcp_sendpages() from in kernel and we know the data
>>>> has no references from user side we can omit SKBTX_SHARED_FRAG flag.
>>>> This patch adds an internal flag, NO_SKBTX_SHARED_FRAG that can be used
>>>> to omit setting SKBTX_SHARED_FRAG.
>>>>
>>>> Signed-off-by: John Fastabend <john.fastabend@...il.com>
>>>> ---
>>>>  include/linux/socket.h |    1 +
>>>>  net/ipv4/tcp.c         |    4 +++-
>>>>  2 files changed, 4 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/include/linux/socket.h b/include/linux/socket.h
>>>> index 9286a5a..add9360 100644
>>>> --- a/include/linux/socket.h
>>>> +++ b/include/linux/socket.h
>>>> @@ -287,6 +287,7 @@ struct ucred {
>>>>  #define MSG_SENDPAGE_NOTLAST 0x20000 /* sendpage() internal : not the last page */
>>>>  #define MSG_BATCH	0x40000 /* sendmmsg(): more messages coming */
>>>>  #define MSG_EOF         MSG_FIN
>>>> +#define MSG_NO_SHARED_FRAGS 0x80000 /* sendpage() internal : page frags are not shared */
>>>>  
>>>>  #define MSG_ZEROCOPY	0x4000000	/* Use user data in kernel path */
>>>>  #define MSG_FASTOPEN	0x20000000	/* Send data in TCP SYN */
>>>> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
>>>> index 7ac583a..56c6f49 100644
>>>> --- a/net/ipv4/tcp.c
>>>> +++ b/net/ipv4/tcp.c
>>>> @@ -995,7 +995,9 @@ ssize_t do_tcp_sendpages(struct sock *sk, struct page *page, int offset,
>>>>  			get_page(page);
>>>>  			skb_fill_page_desc(skb, i, page, offset, copy);
>>>>  		}
>>>> -		skb_shinfo(skb)->tx_flags |= SKBTX_SHARED_FRAG;
>>>> +
>>>> +		if (!(flags & MSG_NO_SHARED_FRAGS))
>>>> +			skb_shinfo(skb)->tx_flags |= SKBTX_SHARED_FRAG;
>>>>  
>>>>  		skb->len += copy;
>>>>  		skb->data_len += copy;
>>>
>>> What would prevent user space from using this flag ?
>>>
>>
>> Nothing in the current patches. So user could set this, change the data,
>> and then presumably get incorrect checksums with bad timing. Seems like
>> this should be blocked so we don't allow users to try and send bad csums.
> 
> Are you sure user can set it ? How would this happen ?
> 

Ah OK I thought you might have a path that I missed. Just
rechecked and I don't see any paths where user flags get
to sendpage without being masked.

> It would be nice to check (sorry I was lazy/busy and did not check
> before asking)

No problem.

The splice path using pipe_to_sendpage() already masks the
flags before sendpage is called. The only other call sites I
see are in o2net and lowcomms both places flags are hard coded
in-kernel.

So we should be safe.


>> How about masking the flags coming from userland? Alternatively could add
>> a bool to do_tcp_sendpages().
>>