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

[Mimi Zohar]

Hi Roberto,

On Thu, 2020-06-18 at 18:01 +0200, Roberto Sassu wrote:
> Granting metadata write is safe if the HMAC key is not loaded, as it won't
> let an attacker obtain a valid HMAC from corrupted xattrs. evm_write_key()
> however does not allow it if any key is loaded, including a public key,
> which should not be a problem.
> 

Why is the existing hebavior a problem?  What is the problem being
solved?

> This patch allows setting EVM_ALLOW_METADATA_WRITES if the EVM_INIT_HMAC
> flag is not set.
> 
> Cc: stable@xxxxxxxxxxxxxxx # 4.16.x
> Fixes: ae1ba1676b88e ("EVM: Allow userland to permit modification of EVM-protected metadata")
> Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> ---
>  security/integrity/evm/evm_secfs.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/security/integrity/evm/evm_secfs.c b/security/integrity/evm/evm_secfs.c
> index cfc3075769bb..92fe26ace797 100644
> --- a/security/integrity/evm/evm_secfs.c
> +++ b/security/integrity/evm/evm_secfs.c
> @@ -84,7 +84,7 @@ static ssize_t evm_write_key(struct file *file, const char __user *buf,
>  	 * keys are loaded.
>  	 */
>  	if ((i & EVM_ALLOW_METADATA_WRITES) &&
> -	    ((evm_initialized & EVM_KEY_MASK) != 0) &&
> +	    ((evm_initialized & EVM_INIT_HMAC) != 0) &&
>  	    !(evm_initialized & EVM_ALLOW_METADATA_WRITES))
>  		return -EPERM;

>  

Documentation/ABI/testing/evm needs to be updated as well.

thanks,

Mimi