RE: [PATCH net-next] udp6: Fix __ip6_append_data()'s handling of MSG_SPLICE_PAGES

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

David Howells wrote:
> __ip6_append_data() can has a similar problem to __ip_append_data()[1] when
> asked to splice into a partially-built UDP message that has more than the
> frag-limit data and up to the MTU limit, but in the ipv6 case, it errors
> out with EINVAL.  This can be triggered with something like:
> 
>         pipe(pfd);
>         sfd = socket(AF_INET6, SOCK_DGRAM, 0);
>         connect(sfd, ...);
>         send(sfd, buffer, 8137, MSG_CONFIRM|MSG_MORE);
>         write(pfd[1], buffer, 8);
>         splice(pfd[0], 0, sfd, 0, 0x4ffe0ul, 0);
> 
> where the amount of data given to send() is dependent on the MTU size (in
> this instance an interface with an MTU of 8192).
> 
> The problem is that the calculation of the amount to copy in
> __ip6_append_data() goes negative in two places, but a check has been put
> in to give an error in this case.
> 
> This happens because when pagedlen > 0 (which happens for MSG_ZEROCOPY and
> MSG_SPLICE_PAGES), the terms in:
> 
>         copy = datalen - transhdrlen - fraggap - pagedlen;
> 
> then mostly cancel when pagedlen is substituted for, leaving just -fraggap.
> 
> Fix this by:
> 
>  (1) Insert a note about the dodgy calculation of 'copy'.
> 
>  (2) If MSG_SPLICE_PAGES, clear copy if it is negative from the above
>      equation, so that 'offset' isn't regressed and 'length' isn't
>      increased, which will mean that length and thus copy should match the
>      amount left in the iterator.
> 
>  (3) When handling MSG_SPLICE_PAGES, give a warning and return -EIO if
>      we're asked to splice more than is in the iterator.  It might be
>      better to not give the warning or even just give a 'short' write.
> 
>  (4) If MSG_SPLICE_PAGES, override the copy<0 check.
> 
> [!] Note that this should also affect MSG_ZEROCOPY, but that will return
> -EINVAL for the range of send sizes that requires the skbuff to be split.
> 
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
> cc: "David S. Miller" <davem@davemloft.net>
> cc: Eric Dumazet <edumazet@google.com>
> cc: Jakub Kicinski <kuba@kernel.org>
> cc: Paolo Abeni <pabeni@redhat.com>
> cc: David Ahern <dsahern@kernel.org>
> cc: Jens Axboe <axboe@kernel.dk>
> cc: Matthew Wilcox <willy@infradead.org>
> cc: netdev@vger.kernel.org
> Link: https://lore.kernel.org/r/000000000000881d0606004541d1@google.com/ [1]

Reviewed-by: Willem de Bruijn <willemb@google.com>

I'm beginning to understand your point that the bug is older and copy
should never end up equal to -fraglen. pagedlen includes all of
datalen, which includes fraggap. This is wrong, as fraggap is always
copied to skb->linear. Haven't really thought it through, but would
this solve it as well?

                        else {
                                alloclen = fragheaderlen + transhdrlen;
-                               pagedlen = datalen - transhdrlen;
+                               pagedlen = datalen - transhdrlen - fraggap;

After that copy no longer subtracts fraglen twice.

                        copy = datalen - transhdrlen - fraggap - pagedlen;

But don't mean to delay these targeted fixes for MSG_SPLICE_PAGES any
further.