Re: [PATCH 04/14] evm: re-release

[Stephen Smalley]

On Wed, 2010-04-21 at 17:49 -0400, Mimi Zohar wrote:
> EVM protects a file's security extended attributes against integrity
> attacks. It maintains an HMAC-sha1 value across the extended attributes,
> storing the value as the extended attribute 'security.evm'. EVM has gone
> through a number of iterations, initially as an LSM module, subsequently
> as a LIM integrity provider, and now, when co-located with a security_
> hook, embedded directly in the security_ hook, similar to IMA.
> 
> This is the first part of a local file integrity verification system.
> While this part does authenticate the selected extended attributes, and
> cryptographically bind them to the inode, coming extensions will bind
> other directory and inode metadata for more complete protection.  The
> set of protected security extended attributes is configured at compile.
> 
> EVM depends on the Kernel Key Retention System to provide it with the
> kernel master key for the HMAC operation.  The kernel master key is
> securely loaded onto the root's keyring, typically by 'loadkernkey',
> which either uses the TPM sealed secret key, if available, or a
> password requested from the console.  To signal EVM, that the key has
> been loaded onto the keyring, 'echo 1 > <securityfs>/evm'. This is
> normally done in the initrd, which has already been measured as part
> of the trusted boot. (Refer to http://linux-ima.sourceforge.net/#EVM.)

I don't remember this dependency on the kernel key system in prior
incarnations of EVM.  Can you explain the rationale for using it, and
the implications?

-- 
Stephen Smalley
National Security Agency

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