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

From: Eric Dumazet <[email-masked]>
Date: Fri, 12 Jan 2018 12:46:35 -0800

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 <[email-masked]>
> > > ---
> > >  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 ?

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

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