linux-kernel - Re: [PATCH v5 0/3] implement OA2_CRED

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Message-ID: <CALCETrWuVQ-ggnak40AX16PUnM43zhogceFN-3c_YAKZGvs5Og@mail.gmail.com>
Date: Tue, 7 May 2024 13:38:42 -0700
From: Andy Lutomirski <luto@...capital.net>
To: Christian Brauner <brauner@...nel.org>
Cc: Aleksa Sarai <cyphar@...har.com>, Stas Sergeev <stsp2@...dex.ru>, 
	"Serge E. Hallyn" <serge@...lyn.com>, linux-kernel@...r.kernel.org, 
	Stefan Metzmacher <metze@...ba.org>, Eric Biederman <ebiederm@...ssion.com>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Andy Lutomirski <luto@...nel.org>, Jan Kara <jack@...e.cz>, 
	Jeff Layton <jlayton@...nel.org>, Chuck Lever <chuck.lever@...cle.com>, 
	Alexander Aring <alex.aring@...il.com>, David Laight <David.Laight@...lab.com>, 
	linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org, 
	Paolo Bonzini <pbonzini@...hat.com>, Christian Göttsche <cgzones@...glemail.com>
Subject: Re: [PATCH v5 0/3] implement OA2_CRED_INHERIT flag for openat2()

On Tue, May 7, 2024 at 12:42 AM Christian Brauner <brauner@...nel.org> wrote:
>
> > With my kernel hat on, maybe I agree.  But with my *user* hat on, I
> > think I pretty strongly disagree.  Look, idmapis lousy for
> > unprivileged use:
> >
> > $ install -m 0700 -d test_directory
> > $ echo 'hi there' >test_directory/file
> > $ podman run -it --rm
> > --mount=type=bind,src=test_directory,dst=/tmp,idmap [debian-slim]
>
> $ podman run -it --rm --mount=type=bind,src=test_directory,dst=/tmp,idmap [debian-slim]
>
> as an unprivileged user doesn't use idmapped mounts at all. So I'm not
> sure what this is showing. I suppose you're talking about idmaps in
> general.

Meh, fair enough.  But I don't think this would have worked any better
with privilege.

Can idmaps be programmed by an otherwise unprivileged owner of a
userns and a mountns inside?

> Many idmappings to one is in principle possible and I've noted that idea
> down as a possible extension at
> https://github.com/uapi-group/kernel-features quite a while (2 years?) ago.
>
> > I haven't looked at the idmap implementation nearly enough to have any
> > opinion as to whether squashing UID is practical or whether there's
>
> It's doable. The interesting bit to me was that if we want to allow
> writes we'd need a way to determine what the uid/gid would be to write
> down. Imho, that's not super difficult to solve though. The most obvious
> one is that userspace can just determine it when creating the idmapped
> mount.

Seems reasonable to me.  If this is set up by someone unprivileged, it
would need to be that uid/gid.