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

From: Casey Schaufler
Date: Thu Sep 02 2021 - 18:34:36 EST

On 9/2/2021 1:06 PM, Vivek Goyal wrote:
> On Thu, Sep 02, 2021 at 11:55:11AM -0700, Casey Schaufler wrote:
>> On 9/2/2021 10:42 AM, Vivek Goyal wrote:
>>> On Thu, Sep 02, 2021 at 01:05:01PM -0400, Vivek Goyal wrote:
>>>> On Thu, Sep 02, 2021 at 08:43:50AM -0700, Casey Schaufler wrote:
>>>>> On 9/2/2021 8:22 AM, Vivek Goyal wrote:
>>>>>> Hi,
>>>>>> This is V3 of the patch. Previous versions were posted here.
>>>>>> v2:
>>>>>> v1:
>>>>>> +m/
>>>>>> Changes since v2
>>>>>> ----------------
>>>>>> - Do not call inode_permission() for special files as file mode bits
>>>>>> on these files represent permissions to read/write from/to device
>>>>>> and not necessarily permission to read/write xattrs. In this case
>>>>>> now user.* extended xattrs can be read/written on special files
>>>>>> as long as caller is owner of file or has CAP_FOWNER.
>>>>>> - Fixed "man xattr". Will post a patch in same thread little later. (J.
>>>>>> Bruce Fields)
>>>>>> - Fixed xfstest 062. Changed it to run only on older kernels where
>>>>>> user extended xattrs are not allowed on symlinks/special files. Added
>>>>>> a new replacement test 648 which does exactly what 062. Just that
>>>>>> it is supposed to run on newer kernels where user extended xattrs
>>>>>> are allowed on symlinks and special files. Will post patch in
>>>>>> same thread (Ted Ts'o).
>>>>>> Testing
>>>>>> -------
>>>>>> - Ran xfstest "./check -g auto" with and without patches and did not
>>>>>> notice any new failures.
>>>>>> - Tested setting "user.*" xattr with ext4/xfs/btrfs/overlay/nfs
>>>>>> filesystems and it works.
>>>>>> Description
>>>>>> ===========
>>>>>> Right now we don't allow setting user.* xattrs on symlinks and special
>>>>>> files at all. Initially I thought that real reason behind this
>>>>>> restriction is quota limitations but from last conversation it seemed
>>>>>> that real reason is that permission bits on symlink and special files
>>>>>> are special and different from regular files and directories, hence
>>>>>> this restriction is in place. (I tested with xfs user quota enabled and
>>>>>> quota restrictions kicked in on symlink).
>>>>>> This version of patch allows reading/writing user.* xattr on symlink and
>>>>>> special files if caller is owner or priviliged (has CAP_FOWNER) w.r.t inode.
>>>>> This part of your project makes perfect sense. There's no good
>>>>> security reason that you shouldn't set user.* xattrs on symlinks
>>>>> and/or special files.
>>>>> However, your virtiofs use case is unreasonable.
>>>> Ok. So we can merge this patch irrespective of the fact whether virtiofs
>>>> should make use of this mechanism or not, right?
>> I don't see a security objection. I did see that Andreas Gruenbacher
>> <agruenba@xxxxxxxxxx> has objections to the behavior.
>>>>>> Who wants to set user.* xattr on symlink/special files
>>>>>> -----------------------------------------------------
>>>>>> I have primarily two users at this point of time.
>>>>>> - virtiofs daemon.
>>>>>> - fuse-overlay. Giuseppe, seems to set user.* xattr attrs on unpriviliged
>>>>>> fuse-overlay as well and he ran into similar issue. So fuse-overlay
>>>>>> should benefit from this change as well.
>>>>>> Why virtiofsd wants to set user.* xattr on symlink/special files
>>>>>> ----------------------------------------------------------------
>>>>>> In virtiofs, actual file server is virtiosd daemon running on host.
>>>>>> There we have a mode where xattrs can be remapped to something else.
>>>>>> For example security.selinux can be remapped to
>>>>>> user.virtiofsd.securit.selinux on the host.
>>>>> As I have stated before, this introduces a breach in security.
>>>>> It allows an unprivileged process on the host to manipulate the
>>>>> security state of the guest. This is horribly wrong. It is not
>>>>> sufficient to claim that the breach requires misconfiguration
>>>>> to exploit. Don't do this.
>>>> So couple of things.
>>>> - Right now whole virtiofs model is relying on the fact that host
>>>> unpriviliged users don't have access to shared directory. Otherwise
>>>> guest process can simply drop a setuid root binary in shared directory
>>>> and unpriviliged process can execute it and take over host system.
>>>> So if virtiofs makes use of this mechanism, we are well with-in
>>>> the existing constraints. If users don't follow the constraints,
>>>> bad things can happen.
>>>> - I think Smalley provided a solution for your concern in other thread
>>>> we discussed this issue.
>>>> "So for example if the host policy says that only virtiofsd can set
>>>> attributes on those files, then the guest MAC labels along with all
>>>> the other attributes are protected against tampering by any other
>>>> process on the host."
>> You can't count on SELinux policy to address the issue on a
>> system running Smack.
>> Or any other user of system.* xattrs,
>> be they in the kernel or user space. You can't even count on
>> SELinux policy to be correct. virtiofs has to present a "safe"
>> situation regardless of how security.* xattrs are used and
>> regardless of which, if any, LSMs are configured. You can't
>> do that with user.* attributes.
> Lets take a step back. Your primary concern with using user.* xattrs
> by virtiofsd is that it can be modified by unprivileged users on host.
> And our solution to that problem is hide shared directory from
> unprivileged users.

You really don't see how fragile that is, do you? How a single
errant call to rename(), chmod() or chown() on the host can expose
the entire guest to exploitation. That's not even getting into
the bag of mount() tricks.

> In addition to that, LSMs on host can block setting "user.*" xattrs by
> virtiofsd domain only for additional protection.

Try thinking outside the SELinux box briefly, if you possibly can.
An LSM that implements just Bell & LaPadula isn't going to have a
"virtiofs domain". Neither is a Smack "3 domain" system. Smack doesn't
distinguish writing user xattrs from writing other file attributes
in policy. Your argument requires a fine grained policy a'la SELinux.
And an application specific SELinux policy at that.

> 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.

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

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. Otherwise you can't trust the guest with host data.

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 on the host, the unprivileged user on the host
can defeat the privilege requirements on the guest. That's why.

> 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.

> 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.

> (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