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

Casey Schaufler casey at schaufler-ca.com
Fri Sep 3 18:49:47 UTC 2021

On 9/3/2021 8:26 AM, Vivek Goyal wrote:
> On Thu, Sep 02, 2021 at 03:34:17PM -0700, Casey Schaufler wrote:
>> 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:
>>>>>>>> https://lore.kernel.org/linux-fsdevel/20210708175738.360757-1-vgoyal@redhat.com/
>>>>>>>> v1:
>>>>>>>> https://lore.kernel.org/linux-fsdevel/20210625191229.1752531-1-vgoyal@redhat.co
>>>>>>>> +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 at redhat.com> 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.
>>>>>>   https://lore.kernel.org/selinux/CAEjxPJ4411vL3+Ab-J0yrRTmXoEf8pVR3x3CSRgPjfzwiUcDtw@mail.gmail.com/T/#mddea4cec7a68c3ee5e8826d650020361030209d6
>>>>>>   "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?
> Yes, I am not sure why we are so opposed to the idea of using 
> user.* xattrs for storing the guest security.selinux xattrs by virtiofsd.
> And I am trying to understand that. And this discussion should help.
> With virtiofsd, we want to keep all shared directory trees in a
> parent directory which has read/write/search permissions only for root
> so that no unpriviliged process can get to files in shared directory
> and do any of the operations.
> For example, /var/lib/containers/storage setup by podman has
> following permissions.
> ll -d /var/lib/containers/storage/
> drwx------. 10 root root 4096 Jun 18  2020 /var/lib/containers/storage/
> Now I should be able to create /var/lib/containers/storage/shared1
> directory and ask virtiofsd to export "share1" to guest. Unprivileged
> process on host can not open any of the files in shared1 dir, hence
> should not be able to modify any data/metadata associated with the
> file.
> If this assumption is correct, then I should be able to use "user.*"
> xattrs without having to worry about unprivileged processes modifying
> security labels of guest/nested-guest.

The only attributes you can really count on to protect an object
are the attributes on the object itself. Path based protections are
not reliable.

>> 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.
> I am relying on unpriviliged processes not having permissions to
> read/search in shared directory. And I guess I have to. Even if
> I use "trusted.*" xattrs, what about file data. Unpriviliged
> processes can modify file data and that's going to be equally
> problematic, isn't it? So why are we so focussed only on
> security label part of it.

We are concentrating on your proposed clever mapping trick.
We are doing so because it won't work, and when it blows up
into a full security bruhaha someone is going to try putting
the blame on the xattr mechanism.

> You have mentioned that file data is not necessarily
> that big a problem because UID 1000 on host and UID 1000 in guest
> should be treated same. But I am not sure we can do that. In kata
> container model, guest images are untrusted. So they can simply cook
> up UID 1000 or UID 0 or any other UID. Now there can be use
> cases where we are ready to trust guest and treat UID 1000
> on host and guest in same way. But primary model I am focussing
> on is that guest shared directory remains isolated from other
> users on host.

I don't have the time just now to examine the rest of virtiofs.
I am quite afraid that you are making dangerous assumptions on
a number of things. It sure doesn't sound like you've thought
through all of the implications of sharing between the host and

>>> 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.
> Ok, so does we have to have capability for every LSM to block write
> to user xattr. I mean in above example, virtiofsd is relying on DAC so that
> unprivileged processes can't modify user xattr labels. If we were to
> use "trusted.*" xattr then we are relying on CAP_SYS_ADMIN.
> IOW, it will be nice if one or more LSMs can provide mechanism fine
> grained enough to block write to user xattr by unwanted application
> and that provides extra level of security. But should it be mandatory?

No. The behavior of user xattrs is just fine the way it is.
It works as designed. 

>>>  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.
> I agree that its easy to make mistakes. But making mistakes is already
> disastrous. So lets say and admin makes mistake and unprivileged
> processes can open/read/write files in shared directory on host.
> Assume I am using "trusted" xattrs to store guest's security labels.
> Now file's data can be modified on host in unwated way and be very
> problematic.
> Guest can drop a setuid root binary in shared directory and unprivileged
> process on host can run it and take over the system.
> Isn't that even bigger problem. So to me making sure shared directory
> is not reachable by unprivileged processes is absolute must requirement
> for virtiofsd (when running as root). If we break that assumption,
> we already have much bigger problems.

I can't help but think you probably do already have much bigger problems.

>>> 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.
> You are assuming untrusted entities can have access to the shared dir. But
> assumption in this model is, shared directory is not reachable by
> unprivileged entities. If we break that requirement, there are much
> bigger issues to deal with then just security attributes.

That's a bad assumption. You have all sorts of issues.

>> 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.
> Ok, I understand your desire that security xattrs as seen by guest kernel
> should be protected by something stronger than simply user xattr.
>> 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?
> I am relying on shared directory not accessible to unprivileged processes.
> If that assumption is broken, I guess,  all bets are off. Until and unless
> one designs SELinux (or other LSM) policy in such a way so that
> only virtiofsd can read/write to these shared directories.
> I think Dan Walsh has got SELinux policy written for atleast kata
> containers case.
> So I guess we will rely on MAC to block unwanted access if users
> make a mistake? Is that the idea? I guess that's why you want
> a stronger mechansim to store guest security xattrs on host. If
> users make a mistake then we have a fallback path?
>>>  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.
> Hmm..., so if a user id 1000 inside guest can't modify security
> xattrs then same user on host should be allowed to bypass that
> by modifying user xattr. I agree with that.
> Just that I am relying on shared directory not being accessible
> to uid 1000 on host and you don't want to rely on that because
> users can easily make mistake. And that's why want to stronger
> form of mechanism to store security xattrs.
>>>  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.
> I am relying on changing direcotry permissions to allow read/write/search
> permission to root only.

Then you have a real mess of problems coming your way. Sorry.

> Thanks
> Vivek

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