Re: [PATCH v5 0/3] implement OA2_CRED_INHERIT flag for openat2()

On 2024-04-28, Andy Lutomirski [off-list ref] wrote:

quoted

On Apr 26, 2024, at 6:39 AM, Stas Sergeev [off-list ref] wrote:
This patch-set implements the OA2_CRED_INHERIT flag for openat2() syscall.
It is needed to perform an open operation with the creds that were in
effect when the dir_fd was opened, if the dir was opened with O_CRED_ALLOW
flag. This allows the process to pre-open some dirs and switch eUID
(and other UIDs/GIDs) to the less-privileged user, while still retaining
the possibility to open/create files within the pre-opened directory set.

I’ve been contemplating this, and I want to propose a different solution.

First, the problem Stas is solving is quite narrow and doesn’t
actually need kernel support: if I want to write a user program that
sandboxes itself, I have at least three solutions already.  I can make
a userns and a mountns; I can use landlock; and I can have a separate
process that brokers filesystem access using SCM_RIGHTS.

But what if I want to run a container, where the container can access
a specific host directory, and the contained application is not aware
of the exact technology being used?  I recently started using
containers in anger in a production setting, and “anger” was
definitely the right word: binding part of a filesystem in is
*miserable*.  Getting the DAC rules right is nasty.  LSMs are worse.
Podman’s “bind,relabel” feature is IMO utterly disgusting.  I think I
actually gave up on making one of my use cases work on a Fedora
system.

Here’s what I wanted to do, logically, in production: pick a host
directory, pick a host *principal* (UID, GID, label, etc), and have
the *entire container* access the directory as that principal. This is
what happens automatically if I run the whole container as a userns
with only a single UID mapped, but I don’t really want to do that for
a whole variety and of reasons.

So maybe reimagining Stas’ feature a bit can actually solve this
problem.  Instead of a special dirfd, what if there was a special
subtree (in the sense of open_tree) that captures a set of creds and
does all opens inside the subtree using those creds?

This isn’t a fully formed proposal, but I *think* it should be
generally fairly safe for even an unprivileged user to clone a subtree
with a specific flag set to do this. Maybe a capability would be
needed (CAP_CAPTURE_CREDS?), but it would be nice to allow delegating
this to a daemon if a privilege is needed, and getting the API right
might be a bit tricky.

Tying this to an actual mount rather than a file handle sounds like a
more plausible proposal than OA2_CRED_INHERIT, but it just seems that
this is going to re-create all of the work that went into id-mapped
mounts but with the extra-special step of making the generic VFS
permissions no longer work normally (unless the idea is that everything
would pretend to be owned by current_fsuid()?).

IMHO it also isn't enough to just make open work, you need to make all
operations work (which leads to a non-trivial amount of
filesystem-specific handling), which is just idmapped mounts. A lot of
work was put into making sure that is safe, and collapsing owners seems
like it will cause a lot of headaches.

I also find it somewhat amusing that this proposal is to basically give
up on multi-user permissions for this one directory tree because it's
too annoying to deal with. In that case, isn't chmod 777 a simpler
solution? (I'm being a bit flippant, of course there is a difference,
but the net result is that all users in the container would have the
same permissions with all of the fun issues that implies.)

In short, AFAICS idmapped mounts pretty much solve this problem (minus
the ability to collapse users, which I suspect is not a good idea in
general)?

Then two different things could be done:

1. The subtree could be used unmounted or via /proc magic links. This
would be for programs that are aware of this interface.

2. The subtree could be mounted, and accessed through the mount would
use the captured creds.

(Hmm. What would a new open_tree() pointing at this special subtree do?)


With all this done, if userspace wired it up, a container user could
do something like:

—bind-capture-creds source=dest

And the contained program would access source *as the user who started
the container*, and this would just work without relabeling or
fiddling with owner uids or gids or ACLs, and it would continue to
work even if the container has multiple dynamically allocated subuids
mapped (e.g. one for “root” and one for the actual application).

Bonus points for the ability to revoke the creds in an already opened
subtree. Or even for the creds to automatically revoke themselves when
the opener exits (or maybe when a specific cred-pinning fd goes away).

(This should work for single files as well as for directories.)

New LSM hooks or extensions of existing hooks might be needed to make
LSMs comfortable with this.

What do you all think?

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>