linux-kernel - Re: [RFC 1/2] fs,eventpoll: Add ability to install target file by its number

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Date:   Fri, 17 Feb 2017 20:11:36 +0300
From:   Cyrill Gorcunov <gorcunov@...il.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...uxfoundation.org>,
        Andrew Vagin <avagin@...tuozzo.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Kirill Kolyshkin <kir@...nvz.org>,
        Andrey Vagin <avagin@...nvz.org>
Subject: Re: [RFC 1/2] fs,eventpoll: Add ability to install target file by
 its number

On Fri, Feb 17, 2017 at 08:52:59AM -0800, Andy Lutomirski wrote:
> >
> > To resolve this tie lets add EPOLL_CTL_DUP operation which simply takes
> > target file descriptor number and installs it into a caller's file table,
> > thus we can use kcmp() syscall and figure out which exactly file to be
> > added into eventpoll on restore procedure.
> 
> This is a scary thing to let an unprivileged process do.
> 
> I'm wondering if there might be a nicer way to address this using a
> better interface in /proc.

Well, I tend to agree. Need to add security checking if the target
file is accessable by a caller. As to better interface to procfs
nothing comes to mind immediately. Another potential problem is that
since it is never guaranteed that target file number listed in fdinfo
matching existing /proc/pid/fd/N, so that I think we will have to
use this dup functionality for every target file, which of course
not that fast. Probably need to think more if I manage to invent
some better and faster interface to find where exactly target file
belong in the whole process tree of a container.

Thanks for pointing about this security problem, Andy!