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In-Reply-To: <20240507-verpennen-defekt-b6f2c9a46916@brauner>
From: Andy Lutomirski <luto@amacapital.net>
Date: Tue, 7 May 2024 13:38:42 -0700
Message-ID: <CALCETrWuVQ-ggnak40AX16PUnM43zhogceFN-3c_YAKZGvs5Og@mail.gmail.com>
Subject: Re: [PATCH v5 0/3] implement OA2_CRED_INHERIT flag for openat2()
To: Christian Brauner <brauner@kernel.org>
Cc: Aleksa Sarai <cyphar@cyphar.com>, Stas Sergeev <stsp2@yandex.ru>, 
	"Serge E. Hallyn" <serge@hallyn.com>, linux-kernel@vger.kernel.org, 
	Stefan Metzmacher <metze@samba.org>, Eric Biederman <ebiederm@xmission.com>, 
	Alexander Viro <viro@zeniv.linux.org.uk>, Andy Lutomirski <luto@kernel.org>, Jan Kara <jack@suse.cz>, 
	Jeff Layton <jlayton@kernel.org>, Chuck Lever <chuck.lever@oracle.com>, 
	Alexander Aring <alex.aring@gmail.com>, David Laight <David.Laight@aculab.com>, 
	linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org, 
	Paolo Bonzini <pbonzini@redhat.com>, =?UTF-8?Q?Christian_G=C3=B6ttsche?= <cgzones@googlemail.com>
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On Tue, May 7, 2024 at 12:42=E2=80=AFAM Christian Brauner <brauner@kernel.o=
rg> 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=3Dtype=3Dbind,src=3Dtest_directory,dst=3D/tmp,idmap [debian-sli=
m]
>
> $ podman run -it --rm --mount=3Dtype=3Dbind,src=3Dtest_directory,dst=3D/t=
mp,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?) ag=
o.
>
> > 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.