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Open Source and information security mailing list archives
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Date: Fri, 30 Oct 2020 11:03:32 -0500 From: "Serge E. Hallyn" <serge@...lyn.com> To: Seth Forshee <seth.forshee@...onical.com> Cc: "Eric W. Biederman" <ebiederm@...ssion.com>, Aleksa Sarai <cyphar@...har.com>, Christian Brauner <christian.brauner@...ntu.com>, Alexander Viro <viro@...iv.linux.org.uk>, Christoph Hellwig <hch@...radead.org>, linux-fsdevel@...r.kernel.org, John Johansen <john.johansen@...onical.com>, James Morris <jmorris@...ei.org>, Mimi Zohar <zohar@...ux.ibm.com>, Dmitry Kasatkin <dmitry.kasatkin@...il.com>, Stephen Smalley <stephen.smalley.work@...il.com>, Casey Schaufler <casey@...aufler-ca.com>, Arnd Bergmann <arnd@...db.de>, Andreas Dilger <adilger.kernel@...ger.ca>, OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>, Geoffrey Thomas <geofft@...reload.com>, Mrunal Patel <mpatel@...hat.com>, Josh Triplett <josh@...htriplett.org>, Andy Lutomirski <luto@...nel.org>, Amir Goldstein <amir73il@...il.com>, Miklos Szeredi <miklos@...redi.hu>, Theodore Tso <tytso@....edu>, Alban Crequy <alban@...volk.io>, Tycho Andersen <tycho@...ho.ws>, David Howells <dhowells@...hat.com>, James Bottomley <James.Bottomley@...senpartnership.com>, Jann Horn <jannh@...gle.com>, Stéphane Graber <stgraber@...ntu.com>, Lennart Poettering <lennart@...ttering.net>, smbarber@...omium.org, Phil Estes <estesp@...il.com>, Serge Hallyn <serge@...lyn.com>, Kees Cook <keescook@...omium.org>, Todd Kjos <tkjos@...gle.com>, Jonathan Corbet <corbet@....net>, containers@...ts.linux-foundation.org, linux-security-module@...r.kernel.org, linux-api@...r.kernel.org, linux-ext4@...r.kernel.org, linux-unionfs@...r.kernel.org, linux-audit@...hat.com, linux-integrity@...r.kernel.org, selinux@...r.kernel.org 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 tmpfs. -serge
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