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

From	Cyrill Gorcunov <gorcunov@gmail.com>
Newsgroups	linux.kernel
Subject	Re: [RFC 1/2] fs,eventpoll: Add ability to install target file by its number
Date	2017-02-17 18:20 +0100
Message-ID	<tbUml-79M-9@gated-at.bofh.it> (permalink)
References	<tbMf7-1Su-1@gated-at.bofh.it> <tbU30-6Np-9@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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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!

Thread

[RFC 1/2] fs,eventpoll: Add ability to install target file by its number Cyrill Gorcunov <gorcunov@openvz.org> - 2017-02-17 09:40 +0100
  Re: [RFC 1/2] fs,eventpoll: Add ability to install target file by its number Andy Lutomirski <luto@amacapital.net> - 2017-02-17 18:00 +0100
    Re: [RFC 1/2] fs,eventpoll: Add ability to install target file by  its number Cyrill Gorcunov <gorcunov@gmail.com> - 2017-02-17 18:20 +0100
      Re: [RFC 1/2] fs,eventpoll: Add ability to install target file by its number Andy Lutomirski <luto@amacapital.net> - 2017-02-17 18:30 +0100
        Re: [RFC 1/2] fs,eventpoll: Add ability to install target file by  its number Cyrill Gorcunov <gorcunov@gmail.com> - 2017-02-17 19:10 +0100
          Re: [RFC 1/2] fs,eventpoll: Add ability to install target file by its  number Jason Baron <jbaron@akamai.com> - 2017-02-17 19:40 +0100
            Re: [RFC 1/2] fs,eventpoll: Add ability to install target file by  its number Cyrill Gorcunov <gorcunov@gmail.com> - 2017-02-17 20:50 +0100

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