Re: overlayfs access checks on underlying layers

From: Stephen Smalley
Date: Wed Dec 12 2018 - 09:49:44 EST

On 12/11/18 4:48 PM, Vivek Goyal wrote:
On Thu, Dec 06, 2018 at 03:26:26PM -0500, Stephen Smalley wrote:
On 12/5/18 8:43 AM, Vivek Goyal wrote:
On Tue, Dec 04, 2018 at 11:49:16AM -0500, Stephen Smalley wrote:
On 12/4/18 11:17 AM, Vivek Goyal wrote:
On Tue, Dec 04, 2018 at 11:05:46AM -0500, Stephen Smalley wrote:
On 12/4/18 10:42 AM, Vivek Goyal wrote:
On Tue, Dec 04, 2018 at 04:31:09PM +0100, Miklos Szeredi wrote:
On Tue, Dec 4, 2018 at 4:22 PM Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:

Having said that, this still create little anomaly when mknod to client
is not allowed on context label. So a device file, which is on lower
and client can not open it for read/write on host, it can now be opened
for read/write because mounter will allow access. So why it is different
that regular copy up. Well, in regular copy up, we created a copy of
the original object and allowed writing to that object (cp --preserve=all)
model. But in case of device file, writes will go to same original
object. (And not a separate copy).

That's true.

In that sense copy up of special file should result in upper having
the same label as of lower, right?

I guess that might be reasonable (if this behavior is a concern). So even
after copy up, client will not be able to read/write a device if it was
not allowed on lower.

Stephen, what do you think about retaining label of lower for device
files during copy up. What about socket/fifo.

We don't check client task access to the upper inode label, only to the
overlay, right? So the client is still free to access the device through
the overlay even if we preserve the lower inode label on the upper inode?
What do we gain?

That's only with latest code and Miklos said he will revert it for 4.20.

IOW, I am assuming that we will continue to check access to a file
on upper in the context of mounter. Otherwise, client will be able to access
files on upper/ which even mounter can't access.

I was assuming we're talking about the proposed solution, where we check
client access to the overlay (unchanged), mounter access to lower
(unchanged), copy-up if denied (new), mounter access to upper (new in the
sense that previously we didn't copy-up on denials).

In that situation, propagating the lower inode label to the upper inode only
impacts the mounter checks, and in that case makes copy-up pointless - if it
wasn't allowed to lower it won't be allowed to upper. If it is allowed,
then client task is free to access the device regardless as long as it has
permissions to the overlay inode. So I don't see what we gain by
propagating the lower inode label to the upper inode in the context mount
case, and it creates an inconsistency between special files and regular
ones.

If we agree on retaining lower label of lower device file on copy up, then
I am assuming we will change rule c) to copy up only non device files.
(because if you don't have access on lower, you will not have access
even after copy up).

There are other paths where copy up happnes. Like link or when file
metadata (ownership, permissions, timestmap) changes. In those cases,
if we retain the lower label over copy up, it probably will help.

IOW, just by creating a link to a device, one will not get access to
a device on upper which could not be accessed on lower.

Device files are special anyway. In regular files we are creating a
copy and user writes to copy. But that's not the case with device
files. So I guess these will have to be treated differently.

I don't understand what you are suggesting. In the case of a context mount,
the context specified by the mounter must be assigned to the upper inode for
any files that are copied up. Otherwise, changes to file data or metadata
made through the overlay will be visible under two different security
contexts simultaneously: the context of the overlay inode (i.e. the one
specified by the mounter) and the context of the upper inode (in your
suggestion, the context from the lower inode). This allows a violation of
MAC policy where one can leak data through an overlay to an unauthorized
context.

Hi Stephen,

Sorry, I don't understand this point of leaking data through overlay. Even
if we retain lower label on copy up (for device file), to open that file
process should have access on overlay context label and then mounter needs
to have access on upper inode (lower label). This is not different from
opening a file on lower. Just that metadata of this file on upper might
be different.

Can you elaborate a bit more on how this is leaking data through overlay
mount. If it is, then why accessing file on lower is not equivalent of
leaking of data.

In the container use case, retaining the lower label on copy-up for a context-mounted overlay permits a process in the container to leak the container data out to host files not labeled with the container label and thus potentially accessible to other containers or host processes. The container process appears to just be writing to files labeled with the container label via the overlay, but the written data and/or metadata is directly accessible through the lower label, which is likely readable to all/many containers and host processes.

In the multi-level security (MLS) use case, an analogy would a situation where you have an unclassified lower dir with some content to be shared read-only across all levels, and an overlay is context-mounted at each level with a corresponding upper dir and work dir private to that level. If a client process at secret performs a write to a file via the secret overlay, and if the written data is stored in a file in the upper dir that inherits the label from the lower file (unclassified), then the secret process can leak data to unclassified processes at will, violating the MLS policy.

The difference with the lower is that it is read-only and the mounter is explicitly choosing to export it under the new context for reading (but not for writing).

As a side note, the actual checking during a context mount isn't as granular as we might like here, since there is no overlay-specific logic and thus no individual checking of the lower, upper, and work directory labels.