evm_inode_init_security and module stacking

[Mimi Zohar]

On Thu, 2019-01-17 at 16:47 -0800, Casey Schaufler wrote:
> security_inode_init_security() currently calls at most one
> of selinux_inode_init_security() and smack_inode_init_security().
> It then sends the result to evm_inode_init_security to create
> the security.evm attribute. This isn't going to work on a system
> that has both SELinux and Smack.

Calculating security.evm based on multiple xattrs sounded really
familiar.  Looking back at the git log, 9d8f13ba3f48 ("security: new
security_inode_init_security API adds function callback") addressed
filesystems wanting to be able to write all the xattrs at the same
time and prepared for multiple LSM xattr support.

> I see two options:
> 	- create security.evm with the information from all
> 	  security modules that provide inode_init_security hooks
> 	- create a separate attribute for each module,
> 	  security.evm-selinux and security.evm-smack in the
> 	  current case.
> 
> How would you like to have it work? I am agnostic, although the
> separate attributes would be easier for the infrastructure.

Having separate attributes for each LSM module would require re-
calculating the hmac for each one, any time any of the other file
metadata changed.  That doesn't sound like a good idea.

Mimi