linux-kernel - Re: [REVIEW][PATCH 2/6] vfs: Allow userns root to call mknod on owned filesystems.

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Date:   Thu, 24 May 2018 11:55:45 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Seth Forshee <seth.forshee@...onical.com>
Cc:     Linux Containers <containers@...ts.linux-foundation.org>,
        linux-fsdevel@...r.kernel.org,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Christian Brauner <christian@...uner.io>,
        linux-kernel@...r.kernel.org
Subject: Re: [REVIEW][PATCH 2/6] vfs: Allow userns root to call mknod on owned filesystems.

Seth Forshee <seth.forshee@...onical.com> writes:

> On Wed, May 23, 2018 at 06:25:34PM -0500, Eric W. Biederman wrote:
>> These filesystems already always set SB_I_NODEV so mknod will not be
>> useful for gaining control of any devices no matter their permissions.
>> This will allow overlayfs and applications to fakeroot to use device
>> nodes to represent things on disk.
>> 
>> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
>
> For a normal filesystem this does seem safe enough.
>
> However, I'd also like to see us allow unprivileged mounting for
> overlayfs, and there we need to worry about whether this would allow a
> mknod in an underlying filesystem which should not be allowed. That
> mknod will be subject to this same check in the underlying filesystem
> using the credentials of the user that mounted the overaly fs, which
> should be sufficient to ensure that the mknod is permitted.

Sufficient to ensure the mknod is not permitted on the underlying
filesystem.  I believe you mean.

> Thus this looks okay to me.
>
> Acked-by: Seth Forshee <seth.forshee@...onical.com>

Eric