linux-kernel - Re: overlayfs: caller_credentials option bypass creator

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Message-ID: <aecdeaca-74cf-54b1-3ea9-4751972074f5@android.com>
Date:   Mon, 18 Jun 2018 14:59:50 -0700
From:   Mark Salyzyn <salyzyn@...roid.com>
To:     Vivek Goyal <vgoyal@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Miklos Szeredi <miklos@...redi.hu>,
        Jonathan Corbet <corbet@....net>,
        linux-unionfs@...r.kernel.org, linux-doc@...r.kernel.org,
        Daniel Walsh <dwalsh@...hat.com>,
        Stephen Smalley <sds@...ho.nsa.gov>
Subject: Re: overlayfs: caller_credentials option bypass creator_cred

On 06/18/2018 12:43 PM, Vivek Goyal wrote:
> Will it be acceptable to write security policies in such a way so that
> mounter has access as well.
Unfortunately No. Policy of minimizing attack surface for a contained 
root service (init in this case). Just because it can mount, does not 
mean it can modify critical content; an attacker could use this to open 
a hole.

> Current model does assume that mounter has privileges on underlying files.

Only ones it appears to need is the workdir AFAIK, had to add ability to 
create in the <wordir> xattr in order to enable r/w mounts later. 
Although not all corners were tested, I did not see any copy_up issues 
b/c the caller had the privs in the Android security model when mounted 
with this new flag.

-- Mark