linux-kernel - Re: [PATCH v6 0/3] implement OA2_CRED

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Date: Tue, 7 May 2024 21:58:20 +1000
From: Aleksa Sarai <cyphar@...har.com>
To: stsp <stsp2@...dex.ru>
Cc: 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>, Christian Brauner <brauner@...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 v6 0/3] implement OA2_CRED_INHERIT flag for openat2()

On 2024-05-07, stsp <stsp2@...dex.ru> wrote:
> 07.05.2024 10:50, Aleksa Sarai пишет:
> > If you are a privileged process which plans to change users,
> 
> Not privileged at all. But I think what you say is still possible with
> userns?

It is possible to configure MOUNT_ATTR_IDMAP in a user namespace but
there are some restrictions that I suspect will make this complicated.
If you try to do something with a regular filesystem you'll probably run
into issues because you won't have CAP_SYS_ADMIN in the super block's
userns. But you could probably do it with tmpfs.

> > A new attack I just thought of while writing this mail is that because
> > there is no RESOLVE_NO_XDEV requirement, it should be possible for the
> > process to get an arbitrary write primitive by creating a new
> > userns+mountns and then bind-mounting / underneath the directory.
> Doesn't this need a write perm to a
> directory? In his case this is not a threat,
> because you are not supposed to have a
> write perm to that dir. OA2_CRED_INHERIT
> is the only way to write.

No, bind-mounts don't require write permission. As long as you can
resolve the target path you can bind-mount on top of it, so if there's a
subdirectory you can bind-mount / underneath (and if there is only a
file you can bind-mount any file you want to access/overwrite instead).

There are restrictions on mounting through /proc/self/fd/... but they
don't apply here (all files opened by a process doing setns/unshare have
their vfsmounts updated to be from the new mount namespace, meaning you
can do mounts through them with /proc/self/fd/... without issue.)

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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