Re: overlayfs access checks on underlying layers

From: Stephen Smalley
Date: Wed Nov 28 2018 - 14:42:00 EST

On 11/28/18 12:03 PM, Vivek Goyal wrote:
On Wed, Nov 28, 2018 at 11:00:09AM +0100, Miklos Szeredi wrote:
On Tue, Nov 27, 2018 at 10:05 PM Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:

On Tue, Nov 27, 2018 at 08:58:06PM +0100, Miklos Szeredi wrote:
[resending with fixed email address for Paul Moore]

Moving discussion from github[1] to here.

To summarize: commit 007ea44892e6 ("ovl: relax permission checking on
underlying layers") was added in 4.20-rc1 to make overlayfs access
checks on underlying "real" filesystems more consistent. The
discussion leading up to this commit can be found at [2]. The commit
broke some selinux-testsuite cases, possibly indicating a security
hole opened by this commit.

The model this patch tries to follow is that if "cp --preserve=all"
was allowed to the mounter from underlying layer to the overlay layer,
then operation is allowed. That means even if mounter's creds doesn't
provide permission to for example execute underying file X, if
mounter's creds provide sufficient permission to perform "cp
--preserve=all X Y" and original creds allow execute on Y, then the
operation is allowed. This provides consistency in the face of
copy-ups. Consistency is only provided in sane setups, where mounter
has sufficient privileges to access both the lower and upper layers.

[cc daniel walsh]

I think current selinux testsuite tests are written keeping these
rules in mind.

1. Check overlay inode creds in the context of task and underlying
inode creds (lower/upper), in the context of mounter.

2. For a lower inode, if said file is being copied up, then only
check MAY_READ on lower. This is equivalent to mounter creating
a copy of file and providing caller access to it (context mount).

For the case of special devices, we do not copy up these. So should
we continue to do check on lower inode in the context of mounter
(instead of not doing any check on lower at all).

Hmm, I'm trying to understand the logic... If we follow the "cp
--preserve=all" thing, than mounter needs to have CREATE permission
for the special file, not READ or WRITE. Does that make sense? Would
that help with the context= mount use case?

Ok. If we follow "cp --preserve=all" methodology, then checking for
mounter CREATE permission on upper for special files makes sense. Or
change logic to copy up this special file during open. I am assuming
we don't copy up special file during open as it is not necessary
for things to work but copying up will work as well?

So rules will become.

- Two levels of checks.
- For lower level inode, check MAY_READ for regular files. (including
- For special files, only make sure mounter can CREATE object in upper.

- What about checks on files on upper/. As of now we seem to check
access in mounter's context if it is regular file. Skip the checks
completely for special files and for executables.

While non-context mount should still be ok, but this means lot of
privilige granting to unprivileged process using context mounts. So
unprivileged process which could not open a device/socket/fifo for
read/write on host fs, can open it for those operations for context

IOW, for context mount case, an unprivileged user will gain lot of
privileges. But that seems to be the point of context mount anyway
on regular disks. If a disk is mounted using context mount option,
then all real labels are ignored and all access checking happens
using context label. We are doing similar thing. With one step extra
and that is making sure if mounter itself can not do certain operation
on host, that will still be denied.

This probably means that context= mounts should be used very carefully.
It will grant lot of priviliges to the process (and allow operations
which process could not do on host without overlayfs mount).

Case of device file still baffels me though. We don't do any mounter's
checks on device files. So if a device file is on upper which mounter
can't open for read but mounter is still granting priviliges to client
to open that device file. That's unintutive to me and seems counter
to the principle of that mounter can't give more priviliges than what
it itself can't do on host.

Dan, stephen, paul moore, does this sound ok to you folks from selinux
point of view.

It seems wrong to check CREATE when no file is being created, and doing so could lead to over-privileging of the mounter context, requiring one to allow a mounter context to create device nodes just to allow a client task context to read/write via already existing device nodes through an overlay.

Checking READ but not EXECUTE upon an execute check could permit a mounter to execute unauthorized code, if it can context mount from a readable-but-not-executable context to an executable context.

Note btw that cp --preserve=all doesn't quite operate as expected if dealing with a context mount. You can't preserve the original security context if copying to a context mount unless the two contexts happen to already match. So I'm not sure how that model applies in the case of a context mount.

Does the breaking commit (007ea44892e6) fix a real bug affecting users? If not, I'd recommend just reverting it.