Re: [RFC][PATCH] overlayfs: Redirect xattr ops on security.evm to security.evm_overlayfs

Roberto Sassu
2023-12-12T13:13:22Z

On 12.12.23 11:44, Amir Goldstein wrote:
> On Tue, Dec 12, 2023 at 12:25 PM Roberto Sassu
> <roberto.sassu@huaweicloud.com> wrote:
>>
>> On 11.12.23 19:01, Christian Brauner wrote:
>>>> The second problem is that one security.evm is not enough. We need two,
>>>> to store the two different HMACs. And we need both at the same time,
>>>> since when overlayfs is mounted the lower/upper directories can be
>>>> still accessible.
>>>
>>> "Changes to the underlying filesystems while part of a mounted overlay
>>> filesystem are not allowed. If the underlying filesystem is changed, the
>>> behavior of the overlay is undefined, though it will not result in a
>>> crash or deadlock."
>>>
>>> https://docs.kernel.org/filesystems/overlayfs.html#changes-to-underlying-filesystems
>>>
>>> So I don't know why this would be a problem.
>>
>> + Eric Snowberg
>>
>> Ok, that would reduce the surface of attack. However, when looking at:
>>
>> ovl: Always reevaluate the file signature for IMA
>>
>> Commit db1d1e8b9867 ("IMA: use vfs_getattr_nosec to get the
>> i_version")
>> partially closed an IMA integrity issue when directly modifying a file
>> on the lower filesystem. If the overlay file is first opened by a
>> user
>> and later the lower backing file is modified by root, but the extended
>> attribute is NOT updated, the signature validation succeeds with
>> the old
>> original signature.
>>
>> Ok, so if the behavior of overlayfs is undefined if the lower backing
>> file is modified by root, do we need to reevaluate? Or instead would be
>> better to forbid the write from IMA (legitimate, I think, since the
>> behavior is documented)? I just saw that we have d_real_inode(), we can
>> use it to determine if the write should be denied.
>>
>
> There may be several possible legitimate actions in this case, but the
> overall concept IMO should be the same as I said about EVM -
> overlayfs does not need an IMA signature of its own, because it
> can use the IMA signature of the underlying file.
>
> Whether overlayfs reads a file from lower fs or upper fs, it does not
> matter, the only thing that matters is that the underlying file content
> is attested when needed.
>
> The only incident that requires special attention is copy-up.
> This is what the security hooks security_inode_copy_up() and
> security_inode_copy_up_xattr() are for.
>
> When a file starts in state "lower" and has security.ima,evm xattrs
> then before a user changes the file, it is copied up to upper fs
> and suppose that security.ima,evm xattrs are copied as is?
>
> When later the overlayfs file content is read from the upper copy
> the security.ima signature should be enough to attest that file content
> was not tampered with between going from "lower" to "upper".
>
> security.evm may need to be fixed on copy up, but that should be
> easy to do with the security_inode_copy_up_xattr() hook. No?

It is not yet clear to me. EVM will be seeing the creation of a new
file, and for new files setting xattrs is already allowed.

Maybe the security_inode_copy_up*() would be useful for IMA/EVM to
authorize writes by overlayfs, which would be otherwise denied to the
others (according to my solution).

Still, would like to hear Mimi's opinion.

Thanks

Roberto