Re: nfsd vurlerability submit

Patrick Goetz
2021-01-22T13:23:49Z

Thanks for engaging; this has been informative.

On 1/21/21 7:30 PM, bfields@fieldses.org wrote:
> On Thu, Jan 21, 2021 at 05:19:32PM -0600, Patrick Goetz wrote:
>> On 1/21/21 4:04 PM, bfields@fieldses.org wrote:
>>> As I said, NFS allows you to look up objects by filehandle (so,
>>> basically by inode number), not just by path
>>
>> Except surely this doesn't buy you much if you don't have root
>> access to the system? Is this all only an issue when the
>> filesystems are exported with no_root_squash?
>>
>> I feel like I must be missing something, but it seems to me that if
>> I'm not root, I'm not going to be able to access inodes I don't have
>> permissions to access even when directly connected to the exporting
>> server.
>
> If an attacker has access to the network (so they can send their own
> hand-crafted NFS requests), then filehandle guessing allows them to
> bypass the normal process of looking up a file. So if you were
> depending on lookup permissions along that path, or on hiding that path
> somehow, you're out of luck.
>
> But it doesn't let them bypass the permissions on the file itself once
> they get there. If the permissions on the file don't allow read, the
> server still won't let them read it.
>

That's probably good enough. Security through obscurity isn't a good
idea, so file/directory level permissions should be atomically correct
and not rely on directory hierarchies, restricted direct access by
users, or anything like this.

I didn't not know about the filehandle guessing thing and will keep that
in mind for the next NFS server I deploy.

>>>> It's not practical to making everything you export its own partition;
>>>> although I suppose one could do this with ZFS datasets.
>>>
>>> I'd be happy to hear about any use cases where that's not practical.
>>
>> Sure. The xray example is taken from one of my research groups which
>> collects thousands of very large electron microscopy images, along
>> with some xray data. I will certainly design this differently in the
>> next iteration (most likely using ZFS), but our current server has a
>> 519T attached storage device which presents itself as a single
>> device: /dev/sdg. Different groups need access to different classes
>> of data, which I export separately and with are presented on the
>> workstations as /xray, /EM, etc..
>>
>> Yes, I could partition the storage device, but then I run into the
>> usual issues where one partition runs out of space while others are
>> barely utilized. This is one good reason to switch to ZFS datasets.
>> The other is that -- with 450T+ of ever changing data, currently
>> rsync backups are almost impossible. I'm hoping zfs send/receive is
>> going to save me here.
>>
>>> As Christophe pointed out, xfs/ext4 project ids are another option.
>>
>> I must have missed this one, but it just leaves me more confused.
>> Project ID's are filesystem metadata, yet this affords better
>> boundary enforcement than a bind mount?
>
> Right. The project ID is stored in the inode, so it's easy to look up
> from the filehandle. (Whereas figuring out what paths might lead to
> that inode is a little tricker.)
>
>> Also, the only use case for Project ID's I was able to find are
>> project quotas, so am not even sure how this would be implemented, and
>> used by NFS.
>
> Project ID's were implemented for quotas, but they also have the
> characteristics to work well as NFS export boundaries.
>
> That said, I think Christoph was suggesting this is something we *could*
> support, not something that we now do. Though looking at it quickly, I
> think it shouldn't take much code at all. I'll put it on my list....
>
> Other options for doing this kind of thing might be btrfs subvolumes or
> lvm thin provisioning. I haven't personally used either, but they
> should both work now.
>
> --b.
>