Re: overlayfs access checks on underlying layers

[Stephen Smalley]

On 12/4/18 9:45 AM, Miklos Szeredi wrote:
> On Tue, Dec 4, 2018 at 3:28 PM Stephen Smalley <sds@xxxxxxxxxxxxx> wrote:
> > On 12/4/18 8:32 AM, Miklos Szeredi wrote:
>
> > > My proposed sequence would be
> > >
> > > a) check task's creds against overlay inode, fail -> return fail, otherwise:
> > > b) check mounter's creds against lower inode, success -> return
> > > success, otherwise:
> > > c) copy up inode, fail -> return fail, otherwise
> > > d) check mounter's creds against upper inode, return result.
> > >
> > > So, unlike write access to regular files, write access to special
> > > files don't necessarily result in copy-up.
> > >
> > > You say this is an escalation of privilege, but I don't get it how.
> > > As DWalsh points out downthread, if mounter cannot create device
> > > files, then the copy-up will simply fail.  If mounter can create
> > > device files, then this is not an escalation of privilege for the
> > > mounter.
> >
> > Yes, in that case there isn't an escalation of privilege for the mounter
> > (I acknowledged that above).  I'm still not sure copy-up of special
> > files is a good idea though:
> >
> > - In the case of device files, there is the potential for mischief by
> > the client task in misusing the mounter's privileges to gain access to
> > otherwise unusable device node (nodev lower vs upper?),
>
> The mount flag "nodev" on lower or upper has no effect on the overlay,
> and never had.
>
> > - In the case of sockets or fifos, what useful result do you get from a
> > copy-up? Accessing the copy isn't going to yield the same result as
> > accessing the original.
>
> This is a misconception.  Overlayfs is a filesystem (that uses other
> filesystems as backing store), but it's more a filesystem and less a
> vfs hack (and trying to completely get out of the vfs hack business),
> and copy up has no effect on I/O being performed on a socket or FIFO:
> it's the same object before and after the copy up, and it's a
> different object from either the lower or upper nodes.   Same as NFS:
> fifo on server is logically different object than fifo having the same
> name on any number of clients.
>
> > For executables, this copy-up behavior might trigger a lot of undesired
> > copies of non-executable files from the lower dir into the upper, even
> > if we ultimately end up denying the execute.
>
> That would only happen if
>
>    - task is allowed exec on overlay
>    - mounter is denied exec on lower
>    - copy up is allowed
>    - mounter is denied exec on upper
>
> In fact in the model where upper object inherits context from overlay
> this is pretty much impossible, AFAICT.

I think the above is the situation in Android (mounter is denied exec to 
lower and upper to prevent unauthorized code execution, but is allowed 
to copy-up in order to support writes by the client).  However, since 
they need override_creds=off or similar anyway, I guess it doesn't matter.

Ok, I concede the point.  Not sure what that means though for v4.20.