[PATCH v6 03/11] evm: Refuse EVM_ALLOW_METADATA_WRITES only if an HMAC key is loaded

[Mimi Zohar]

On Wed, 2021-05-05 at 13:29 +0200, Roberto Sassu wrote:
> EVM_ALLOW_METADATA_WRITES is an EVM initialization flag that can be set to
> temporarily disable metadata verification until all xattrs/attrs necessary
> to verify an EVM portable signature are copied to the file. This flag is
> cleared when EVM is initialized with an HMAC key, to avoid that the HMAC is
> calculated on unverified xattrs/attrs.
> 
> Currently EVM unnecessarily denies setting this flag if EVM is initialized
> with a public key, which is not a concern as it cannot be used to trust
> xattrs/attrs updates. This patch removes this limitation.
> 
> Cc: stable at vger.kernel.org # 4.16.x
> Fixes: ae1ba1676b88e ("EVM: Allow userland to permit modification of EVM-protected metadata")
> Signed-off-by: Roberto Sassu <roberto.sassu at huawei.com>

Once the comments below are addressed, 

Reviewed-by:  Mimi Zohar <zohar at linux.ibm.com>

> ---
>  Documentation/ABI/testing/evm      | 5 +++--
>  security/integrity/evm/evm_secfs.c | 5 ++---
>  2 files changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/ABI/testing/evm b/Documentation/ABI/testing/evm
> index 3c477ba48a31..eb6d70fd6fa2 100644
> --- a/Documentation/ABI/testing/evm
> +++ b/Documentation/ABI/testing/evm
> @@ -49,8 +49,9 @@ Description:
>  		modification of EVM-protected metadata and
>  		disable all further modification of policy
>  
> -		Note that once a key has been loaded, it will no longer be
> -		possible to enable metadata modification.
> +		Note that once an HMAC key has been loaded, it will no longer
> +		be possible to enable metadata modification and, if it is
> +		already enabled, it will be disabled.

It's worth mentioning that echo'ing a new value is additive.  Once EVM
metadata modification is enabled, the only way of disabling it is by
enabling an HMAC key.  It's also worth mentioning that metadata writes
are only permitted once further changes to the EVM policy are disabled.
Perhaps the best way of explaining this is by including a new example -
echo 6> <securityfs>/evm.

>  
>  		Until key loading has been signaled EVM can not create
>  		or validate the 'security.evm' xattr, but returns
> diff --git a/security/integrity/evm/evm_secfs.c b/security/integrity/evm/evm_secfs.c
> index bbc85637e18b..860c48b9a0c3 100644
> --- a/security/integrity/evm/evm_secfs.c
> +++ b/security/integrity/evm/evm_secfs.c
> @@ -81,11 +81,10 @@ static ssize_t evm_write_key(struct file *file, const char __user *buf,
>  		return -EINVAL;
>  
>  	/* Don't allow a request to freshly enable metadata writes if
> -	 * keys are loaded.
> +	 * an HMAC key is loaded.
>  	 */

Please drop the word "freshly".  While updating the comment, please
move the sentence starting with "Don't" to a new line.

>  	if ((i & EVM_ALLOW_METADATA_WRITES) &&
> -	    ((evm_initialized & EVM_KEY_MASK) != 0) &&
> -	    !(evm_initialized & EVM_ALLOW_METADATA_WRITES))
> +	    (evm_initialized & EVM_INIT_HMAC) != 0)
>  		return -EPERM;
>  
>  	if (i & EVM_INIT_HMAC) {