Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Message-ID: <b9f1a31e9b2dfb7a7167574a39652932263488e8.camel@linux.ibm.com>
Subject: Re: [PATCH v3 06/11] evm: Ignore INTEGRITY_NOLABEL if no HMAC key
 is loaded
From:   Mimi Zohar <zohar@linux.ibm.com>
To:     Roberto Sassu <roberto.sassu@huawei.com>, mjg59@google.com
Cc:     linux-integrity@vger.kernel.org,
        linux-security-module@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        silviu.vlasceanu@huawei.com
Date:   Thu, 03 Dec 2020 15:42:44 -0500
In-Reply-To: <20201111092302.1589-7-roberto.sassu@huawei.com>
References: <20201111092302.1589-1-roberto.sassu@huawei.com>
         <20201111092302.1589-7-roberto.sassu@huawei.com>
Content-Type: text/plain; charset="ISO-8859-15"
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit
Precedence: bulk

Hi Roberto,

On Wed, 2020-11-11 at 10:22 +0100, Roberto Sassu wrote:
> When a file is being created, LSMs can set the initial label with the
> inode_init_security hook. If no HMAC key is loaded, the new file will have
> LSM xattrs but not the HMAC.
> 
> Unfortunately, EVM will deny any further metadata operation on new files,
> as evm_protect_xattr() will always return the INTEGRITY_NOLABEL error. This
> would limit the usability of EVM when only a public key is loaded, as
> commands such as cp or tar with the option to preserve xattrs won't work.
> 
> Ignoring this error won't be an issue if no HMAC key is loaded, as the
> inode is locked until the post hook, and EVM won't calculate the HMAC on
> metadata that wasn't previously verified. Thus this patch checks if an
> HMAC key is loaded and if not, ignores INTEGRITY_NOLABEL.

I'm not sure what problem this patch is trying to solve. 
evm_protect_xattr() is only called by evm_inode_setxattr() and
evm_inode_removexattr(), which first checks whether
EVM_ALLOW_METADATA_WRITES is enabled.

Mimi