Re: overlayfs access checks on underlying layers

[Paul Moore]

On Tue, Dec 4, 2018 at 9:40 AM Stephen Smalley <sds@xxxxxxxxxxxxx> wrote:
> On 12/3/18 6:27 PM, Paul Moore wrote:
> > On Thu, Nov 29, 2018 at 5:22 PM Daniel Walsh <dwalsh@xxxxxxxxxx> wrote:
> >> On 11/29/18 2:47 PM, Miklos Szeredi wrote:
> >>> On Thu, Nov 29, 2018 at 5:14 PM Stephen Smalley <sds@xxxxxxxxxxxxx> wrote:
> >>>
> >>>> Possibly I misunderstood you, but I don't think we want to copy-up on
> >>>> permission denial, as that would still allow the mounter to read/write
> >>>> special files or execute regular files to which it would normally be
> >>>> denied access, because the copy would inherit the context specified by
> >>>> the mounter in the context mount case.  It still represents an
> >>>> escalation of privilege for the mounter.  In contrast, the copy-up on
> >>>> write behavior does not allow the mounter to do anything it could not do
> >>>> already (i.e. read from the lower, write to the upper).
> >>> Let's get this straight:  when file is copied up, it inherits label
> >>> from context=, not from label of lower file?
> >>
> >> Yes, in the case of context mount, it will get the context mount directory.
> >>
> >> In the case of not context mount, it should maintain the label of  the
> >> lower.
> >>
> >>> Next question: permission to change metadata is tied to permission to
> >>> open?  Is it possible that open is denied, but metadata can be
> >>> changed?
> >>
> >> Yes, SElinux handles open differently then setattr.  Although I am not
> >> sure if any tools handle this.
> >>
> >>> DAC model allows this: metadata change is tied to ownership, not mode
> >>> bits.   And different capability flag.
> >>>
> >>> If the same is true for MAC, then the pre-v4.20-rc1 is already
> >>> susceptible to the privilege escalation you describe, right?
> >>
> >> After talking to Vivek, I am not sure their is a privilege escallation.
> >
> > More on this below, but this thread doesn't have me convinced, and we
> > are at -rc5 right now.  We need to come to some decision on this soon
> > because we are running out of time before v4.20 is released with this
> > code.
> >
> >> For device nodes, the mounter has to have the ability to create the
> >> devicenode with the context mount, if he can do this, then he can do it
> >> with or without Overlay.  This might lead to users making mistakes on
> >> security, but the model is sound. And I think this stands even in the
> >> case of the lower is mounted NODEV and the upper is not.  If the mounter
> >> can create a device on the upper with a particular label, then he does
> >> not need the lower.
> >
> > The problem I have when looking at the current code is that permission
> > is given, regardless of what is requested, for any special_file() on
> > an overlayfs mount.
> >
> > It also looks like the mounter's creds are used when checking
> > permissions regardless of the file has been copied up or not; I would
> > expect that the mounter's permissions would only used when checking
> > permissions against the lower inode, no?
>
> No, that's never been the model as far as I know.  mounter's permissions
> are checked to the underlying inode, whether upper or lower.  client's
> permissions are only checked to the overlay inode.  upper and lower are
> logically backing store - upper for writes and lower for reads from
> unmodified files.  Now, in theory, upper should always be labeled the
> same as overlay, so client check against overlay should already imply
> client access to upper, unless someone has manually relabeled upper
> outside of the overlay.

Okay, thanks for the clarification on the model.  This seems a little
odd at first, but I'm trying to convince myself that it makes sense.

-- 
paul moore
www.paul-moore.com