udp: Fix __ip_append_data()'s handling of MSG_SPLICE_PAGES

udp: Fix __ip_append_data()'s handling of MSG_SPLICE_PAGES

__ip_append_data() can get into an infinite loop when asked to splice into
a partially-built UDP message that has more than the frag-limit data and up
to the MTU limit.  Something like:

        pipe(pfd);
        sfd = socket(AF_INET, SOCK_DGRAM, 0);
        connect(sfd, ...);
        send(sfd, buffer, 8161, 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
__ip_append_data() goes negative in two places, and, in the second place,
this gets subtracted from the length remaining, thereby increasing it.

This happens when pagedlen > 0 (which happens for MSG_ZEROCOPY and
MSG_SPLICE_PAGES), because the terms in:

        copy = datalen - transhdrlen - fraggap - pagedlen;

then mostly cancel when pagedlen is substituted for, leaving just -fraggap.
This causes:

        length -= copy + transhdrlen;

to increase the length to more than the amount of data in msg->msg_iter,
which causes skb_splice_from_iter() to be unable to fill the request and it
returns less than 'copied' - which means that length never gets to 0 and we
never exit the loop.

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.

[!] Note that this ought to also affect MSG_ZEROCOPY, but MSG_ZEROCOPY
avoids the problem by simply assuming that everything asked for got copied,
not just the amount that was in the iterator.  This is a potential bug for
the future.

Fixes: 7ac7c987850c ("udp: Convert udp_sendpage() to use MSG_SPLICE_PAGES")
Reported-by: syzbot+f527b971b4bdc8e79f9e@syzkaller.appspotmail.com
Link: https://lore.kernel.org/r/000000000000881d0606004541d1@google.com/
Signed-off-by: David Howells <dhowells@redhat.com>
cc: David Ahern <dsahern@kernel.org>
cc: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/1420063.1690904933@warthog.procyon.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>