linux-kernel - Re: [PATCH v4 3/5] security: Allow all LSMs to provide xattrs for inode_init

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Message-ID: <66df8ebe-1ed2-c839-8a5f-cc0a26bad6c3@schaufler-ca.com>
Date:   Thu, 17 Nov 2022 09:40:20 -0800
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Mimi Zohar <zohar@...ux.ibm.com>,
        Roberto Sassu <roberto.sassu@...weicloud.com>,
        dmitry.kasatkin@...il.com, paul@...l-moore.com, jmorris@...ei.org,
        serge@...lyn.com, stephen.smalley.work@...il.com,
        eparis@...isplace.org
Cc:     linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org, selinux@...r.kernel.org,
        reiserfs-devel@...r.kernel.org, linux-kernel@...r.kernel.org,
        keescook@...omium.org, nicolas.bouchinet@...p-os.org,
        Roberto Sassu <roberto.sassu@...wei.com>,
        casey@...aufler-ca.com
Subject: Re: [PATCH v4 3/5] security: Allow all LSMs to provide xattrs for
 inode_init_security hook

On 11/17/2022 9:24 AM, Mimi Zohar wrote:
> On Thu, 2022-11-17 at 09:18 -0800, Casey Schaufler wrote:
>> On 11/17/2022 8:05 AM, Mimi Zohar wrote:
>>> hOn Thu, 2022-11-10 at 10:46 +0100, Roberto Sassu wrote:
>>>> From: Roberto Sassu <roberto.sassu@...wei.com>
>>>>
>>>> Currently, security_inode_init_security() supports only one LSM providing
>>>> an xattr and EVM calculating the HMAC on that xattr, plus other inode
>>>> metadata.
>>>>
>>>> Allow all LSMs to provide one or multiple xattrs, by extending the security
>>>> blob reservation mechanism. Introduce the new lbs_xattr field of the
>>>> lsm_blob_sizes structure, so that each LSM can specify how many xattrs it
>>>> needs, and the LSM infrastructure knows how many xattr slots it should
>>>> allocate.
>>> Perhaps supporting per LSM multiple xattrs is a nice idea, but EVM
>>> doesn't currently support it.  The LSM xattrs are hard coded in
>>> evm_config_default_xattrnames[],  based on whether the LSM is
>>> configured.  Additional security xattrs may be included in the
>>> security.evm calculation, by extending the list via
>>> security/integrity/evm/evm_xattrs.
>> Smack uses multiple xattrs. All file system objects have a SMACK64
>> attribute, which is used for access control. A program file may have
>> a SMACK64EXEC attribute, which is the label the program will run with.
>> A library may have a SMACK64MMAP attribute to restrict loading. A
>> directory may have a SMACK64TRANSMUTE attribute, which modifies the
>> new object creation behavior.
>>
>> The point being that it may be more than a "nice idea" to support
>> multiple xattrs. It's not a hypothetical situation.
> And each of these addiitonal Smack xattrs are already defined in 
> evm_config_default_xattrnames[].

Then I'm confused by the statement that "EVM doesn't currently support it".