linux-kernel - Re: [PATCH net-next v2 2/6] net: Block MSG_SENDPAGE_* from being passed to sendmsg() by userspace

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Message-ID: <ZHd9vCcBNtjkqeqg@corigine.com>
Date:   Wed, 31 May 2023 19:02:52 +0200
From:   Simon Horman <simon.horman@...igine.com>
To:     David Howells <dhowells@...hat.com>
Cc:     netdev@...r.kernel.org, Chuck Lever <chuck.lever@...cle.com>,
        Boris Pismenny <borisp@...dia.com>,
        John Fastabend <john.fastabend@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        David Ahern <dsahern@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v2 2/6] net: Block MSG_SENDPAGE_* from being
 passed to sendmsg() by userspace

On Wed, May 31, 2023 at 01:45:24PM +0100, David Howells wrote:
> It is necessary to allow MSG_SENDPAGE_* to be passed into ->sendmsg() to
> allow sendmsg(MSG_SPLICE_PAGES) to replace ->sendpage().  Unblocking them
> in the network protocol, however, allows these flags to be passed in by
> userspace too[1].
> 
> Fix this by marking MSG_SENDPAGE_NOPOLICY, MSG_SENDPAGE_NOTLAST and
> MSG_SENDPAGE_DECRYPTED as internal flags, which causes sendmsg() to object
> if they are passed to sendmsg() by userspace.  Network protocol ->sendmsg()
> implementations can then allow them through.
> 
> Note that it should be possible to remove MSG_SENDPAGE_NOTLAST once
> sendpage is removed as a whole slew of pages will be passed in in one go by

Hi David,

on the off-chance that you need to respin for some other reason:

	s/in in/in/

> splice through sendmsg, with MSG_MORE being set if it has more data waiting
> in the pipe.

...