Re: [PATCH 0/5] evm: Support signatures on stacked filesystem

Stefan Berger
2024-01-31T14:55:31Z

On 1/31/24 08:18, Amir Goldstein wrote:
> On Tue, Jan 30, 2024 at 11:46 PM Stefan Berger <stefanb@linux.ibm.com> wrote:
>>
>> EVM has recently been completely disabled on unsupported (e.g.,
>> overlayfs). This series now enables copy-up of "portable and immutable"
>> signatures on those filesystems and enables the enforcement of
>> "portable and immutable" as well as the "original" signatures on
>> previously unsupported filesystem when EVM is enabled with EVM_INIT_X509.
>> HMAC verification and generation remains disabled on those filesystems.
>>
>
> I am missing a high level description of what is in those "portable
> and immutable"
> signatures and how those signatures remain valid across copy up.
>

From 2/5:
"Portable and immutable EVM signatures can be copied up by stacked file-
system since the metadata their signature covers does not include file-
system-specific data such as a file's inode number, generation, and UUID."

Instead, the signatures cover file metadata such as file mode bits, uid,
and gid as well as xattrs, which can all be preserved unchanged across a
copy-up.

Reference:
https://elixir.bootlin.com/linux/v6.7.2/source/security/integrity/evm/evm_crypto.c#L169

> Thanks,
> Amir.
>