Re: [PATCH v4 3/5] security: Allow all LSMs to provide xattrs for inode_init_security hook

[Casey Schaufler]

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@xxxxxxxxxx>
>>
>> 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.