Re: overlayfs: caller_credentials option bypass creator_cred

[Mark Salyzyn <salyzyn@android.com>]

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