[PATCH v3 0/1] Relax restrictions on user.* xattr

Casey Schaufler casey at schaufler-ca.com
Mon Sep 13 19:05:07 UTC 2021

On 9/6/2021 7:55 AM, Dr. David Alan Gilbert wrote:
> * Casey Schaufler (casey at schaufler-ca.com) wrote:
>> On 9/2/2021 1:06 PM, Vivek Goyal wrote:
>>>  If LSMs are not configured,
>>> then hiding the directory is the solution.
>> It's not a solution at all. It's wishful thinking that
>> some admin is going to do absolutely everything right, will
>> never make a mistake and will never, ever, read the mount(2)
>> man page.
> That is why we run our virtiofsd with a sandbox setup and seccomp; and
> frankly anything we can or could turn on we would.

That doesn't address my concern at all. Being able to create an
environment in which a feature can be used safely does not make
the feature safe.

> So why that's not a solution and only relying on CAP_SYS_ADMIN is the
> solution. I don't understand that part.

Sure you do. If you didn't, you wouldn't be so concerned about
requiring CAP_SYS_ADMIN. You're trying hard to avoid taking the
level of responsibility that running with privilege requires.
To do that, you're introducing a massive security hole, a backdoor
into the file system security attributes.

>> It comes back to your design, which is fundamentally flawed. You
>> can't store system security information in an attribute that can
>> be manipulated by untrusted entities. That's why we have system.*
>> xattrs. You want to have an attribute on the host that maps to a
>> security attribute on the guest. The host has to protect the attribute
>> on the guest with mechanisms of comparable strength as the guest's
>> mechanisms.
> Can you just explain this line to me a bit more: 
>> Otherwise you can't trust the guest with host data.
> Note we're not trying to trust the guest with the host data here;
> we're trying to allow the guest to store the data on the host, while
> trusting the host.

But you can't trust the host! You're allowing unprivileged processes
on the host to modify security state of the guest.

>> It's a real shame that CAP_SYS_ADMIN is so scary. The capability
>> mechanism as implemented today won't scale to the hundreds of individual
>> capabilities it would need to break CAP_SYS_ADMIN up. Maybe someday.
>> I'm not convinced that there isn't a way to accomplish what you're
>> trying to do without privilege, but this isn't it, and I don't know
>> what is. Sorry.
>>> Also if directory is not hidden, unprivileged users can change file
>>> data and other metadata.
>> I assumed that you've taken that into account. Are you saying that
>> isn't going to be done correctly either?
>>>  Why that's not a concern and why there is
>>> so much of focus only security xattr.
>> As with an NFS mount, the assumption is that UID 567 (or its magically
>> mapped equivalent) has the same access rights on both the server/host
>> and client/guest. I'm not worried about the mode bits because they are
>> presented consistently on both machines. If, on the other hand, an
>> attribute used to determine access is security.esprit on the guest and
>> user.security.esprit on the host, the unprivileged user on the host
>> can defeat the privilege requirements on the guest. That's why.
> We're OK with that;

I understand that. I  am  not  OK  with  that.

>  remember that the host can do wth it likes to the
> guest anyway

We're not talking about "the host", we're talking about an
unprivileged user on the host.

>  - it can just go in and poke at the guests RAM if it wants
> to do something evil to the guest.
> We wouldn't suggest using a scheme like this once you have
> encrypted/protected guest RAM for example (SEV/TDX etc)
>>>  If you were to block modification
>>> of file then you will have rely on LSMs.
>> No. We're talking about the semantics of the xattr namespaces.
>> LSMs can further constrain access to xattrs, but the basic rules
>> of access to the user.* and security.* attributes are different
>> in any case. This is by design.
> I'm happy if you can suggest somewhere else to store the guests xattr
> data other than in one of the hosts xattr's - the challenge is doing
> that in a non-racy way, and making sure that the xattr's never get
> associated with the wrong file as seen by a guest.

I'm sorry, but I've got a bunch of other stuff on my plate.
I've already suggested implementing xattr namespaces a'la user
namespaces, but I understand that is beyond the scope of your
current needs, and has its own set of dragons.

>>>  And if LSMs are not configured,
>>> then we will rely on shared directory not being visible.
>> LSMs are not the problem. LSMs use security.* xattrs, which is why
>> they come up in the discussion.
>>> Can you please help me understand why hiding shared directory from
>>> unprivileged users is not a solution
>> Maybe you can describe the mechanism you use to "hide" a shared directory
>> on the host. If the filesystem is mounted on the host it seems unlikely
>> that you can provide a convincing argument for sufficient protection.
> Why?

Because 99-44/100% of admins out there aren't as skilled at "hiding"
data as you are. Many (I almost said "most". I'm still not sure which.)
of them don't even know how to use mode bits correctly.

>  What can a guests fs mounted on the host, under one of the
> directories that's already typically used for container fs's do - it's
> already what fileservers, and existing container systems do.
> Dave
>>>  (With both LSMs configured or
>>> not configured on host). That's a requirement for virtiofs anyway. 
>>> And if we agree on that, then I don't see why using "user.*" xattrs
>>> for storing guest sercurity attributes is a problem.
>>> Thanks
>>> Vivek

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