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

From: Stefan Berger <stefanb@linux.ibm.com>
To: Amir Goldstein <amir73il@gmail.com>
Cc: linux-integrity@vger.kernel.org,
	linux-security-module@vger.kernel.org,
	linux-unionfs@vger.kernel.org, linux-kernel@vger.kernel.org,
	paul@paul-moore.com, jmorris@namei.org, serge@hallyn.com,
	zohar@linux.ibm.com, roberto.sassu@huawei.com, miklos@szeredi.hu
Subject: Re: [PATCH 0/5] evm: Support signatures on stacked filesystem
Date: Wed, 31 Jan 2024 09:52:44 -0500	[thread overview]
Message-ID: <015c3a14-0d23-43ba-9593-3d4ff588f1bb@linux.ibm.com> (raw)
In-Reply-To: <CAOQ4uxg9VQ9LksxX5eFFg7E1VO_iFZNmNBaC3pED+a2=ixOhdw@mail.gmail.com>

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.
> 