lkml.org 
[lkml]   [2023]   [Aug]   [2]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRE: [PATCH net-next] udp6: Fix __ip6_append_data()'s handling of MSG_SPLICE_PAGES
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.

\
 
 \ /
  Last update: 2023-08-02 16:27    [W:0.060 / U:1.204 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site