[PATCH v3 4/4] fuse: define the filesystem as untrusted

[Chuck Lever]

> On Mar 14, 2018, at 1:50 PM, Mimi Zohar <zohar at linux.vnet.ibm.com> wrote:
> 
> On Wed, 2018-03-14 at 11:17 -0500, Eric W. Biederman wrote:
>> Mimi Zohar <zohar at linux.vnet.ibm.com> writes:
>> 
>>> On Wed, 2018-03-14 at 08:52 +0100, Stef Bon wrote:
>>>> I do not have any comments about the patches but a question.
>>>> I completely agree that the files can change without the VFS knowing
>>>> about it, but isn't that in general the case with filesystems with a
>>>> backend shared with others (network fs's?).
>>> 
>>> Right, the problem is not limited to fuse, but needs to be addressed
>>> before unprivileged fuse mounts are upstreamed.
>>> 
>>> Alban's response to this question:
>>> https://marc.info/?l=linux-kernel&m=151784020321045&w=2
>> 
>> Which goes to why it is a flag that get's set.
>> 
>> All of this just needs a follow-up patch to update every filesystem
>> that does not meet ima's requirements.
> 
> Currently files on remote filesystems are measured/appraised/audited
> once.  With the new flags, our options would be to either fail the
> signature verification or constantly re-measure/re-appraise files on
> remote file systems.  Neither option seems like the right solution.

Being new to this game, I may be making a bad assumption, but I thought
that the (NFSv4) change attribute can be used to detect remote mutations
to a file's content or metadata, so that the appraisal could be cached
as long as that attribute does not change. At least for NFSv4, clients
assume their data cache is valid while the change attribute remains the
same.

NFSv4 (and SMB) also has a mechanism where a server guarantees it will
report any other clients that want to update a file. This is an NFSv4
read delegation or an SMB oplock. NFSv4 clients use this mechanism to
cache file data quite aggressively, and it could also be used to
preserve or cache audit state on remote files.


> There's some very initial discussions on how to support file integrity
> on remote filesystems.  Chuck Lever has some thoughts on piggy-backing 
> on the fs-verity work being done.  From a very, very high level, IMA-
> appraisal would verify the file signature, but leave the integrity
> enforcement to the vfs/fs layer.  By integrating fs-verity or similar
> proposal with IMA, measurements would be included in the measurement
> list and keys used for file signature verification would use the same
> existing IMA-appraisal infrastructure.

>> Mimi I believe you said that the requirement is that all file changes
>> can be detected through the final __fput of a file that calls
>> ima_file_free.
> 
> Right, like for fuse, I don't believe this existing hook works for
> remote filesystems.
> 
> Mimi

--
Chuck Lever



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