lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  PHC 
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date:   Fri, 30 Oct 2020 11:03:32 -0500
From:   "Serge E. Hallyn" <>
To:     Seth Forshee <>
Cc:     "Eric W. Biederman" <>,
        Aleksa Sarai <>,
        Christian Brauner <>,
        Alexander Viro <>,
        Christoph Hellwig <>,,
        John Johansen <>,
        James Morris <>,
        Mimi Zohar <>,
        Dmitry Kasatkin <>,
        Stephen Smalley <>,
        Casey Schaufler <>,
        Arnd Bergmann <>,
        Andreas Dilger <>,
        OGAWA Hirofumi <>,
        Geoffrey Thomas <>,
        Mrunal Patel <>,
        Josh Triplett <>,
        Andy Lutomirski <>,
        Amir Goldstein <>,
        Miklos Szeredi <>,
        Theodore Tso <>, Alban Crequy <>,
        Tycho Andersen <>,
        David Howells <>,
        James Bottomley <>,
        Jann Horn <>,
        St├ęphane Graber <>,
        Lennart Poettering <>,, Phil Estes <>,
        Serge Hallyn <>,
        Kees Cook <>,
        Todd Kjos <>, Jonathan Corbet <>,,,,,,,,
Subject: Re: [PATCH 00/34] fs: idmapped mounts

On Fri, Oct 30, 2020 at 10:07:48AM -0500, Seth Forshee wrote:
> On Thu, Oct 29, 2020 at 11:37:23AM -0500, Eric W. Biederman wrote:
> > First and foremost: A uid shift on write to a filesystem is a security
> > bug waiting to happen.  This is especially in the context of facilities
> > like iouring, that play very agressive games with how process context
> > makes it to  system calls.
> > 
> > The only reason containers were not immediately exploitable when iouring
> > was introduced is because the mechanisms are built so that even if
> > something escapes containment the security properties still apply.
> > Changes to the uid when writing to the filesystem does not have that
> > property.  The tiniest slip in containment will be a security issue.
> > 
> > This is not even the least bit theoretical.  I have seem reports of how
> > shitfs+overlayfs created a situation where anyone could read
> > /etc/shadow.
> This bug was the result of a complex interaction with several
> contributing factors. It's fair to say that one component was overlayfs
> writing through an id-shifted mount, but the primary cause was related
> to how copy-up was done coupled with allowing unprivileged overlayfs
> mounts in a user ns. Checks that the mounter had access to the lower fs
> file were not done before copying data up, and so the file was copied up
> temporarily to the id shifted upperdir. Even though it was immediately
> removed, other factors made it possible for the user to get the file
> contents from the upperdir.
> Regardless, I do think you raise a good point. We need to be wary of any
> place the kernel could open files through a shifted mount, especially
> when the open could be influenced by userspace.
> Perhaps kernel file opens through shifted mounts should to be opt-in.
> I.e. unless a flag is passed, or a different open interface used, the
> open will fail if the dentry being opened is subject to id shifting.
> This way any kernel writes which would be subject to id shifting will
> only happen through code which as been written to take it into account.

For my use cases, it would be fine to require opt-in at original fs
mount time by init_user_ns admin.  I.e.
    mount -o allow_idmap /dev/mapper/whoozit /whatzit
I'm quite certain I would always be sharing a separate LV or loopback or


Powered by blists - more mailing lists