linux-kernel - Re: [PATCH v3 01/11] evm: Execute evm_inode_init_security() only when an HMAC key is loaded

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Date:   Wed, 02 Dec 2020 12:03:34 -0500
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Roberto Sassu <roberto.sassu@...wei.com>, mjg59@...gle.com
Cc:     linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        silviu.vlasceanu@...wei.com, stable@...r.kernel.org
Subject: Re: [PATCH v3 01/11] evm: Execute evm_inode_init_security() only
 when an HMAC key is loaded

On Wed, 2020-11-11 at 10:22 +0100, Roberto Sassu wrote:
> evm_inode_init_security() requires an HMAC key to calculate the HMAC on
> initial xattrs provided by LSMs. However, it checks generically whether a
> key has been loaded, including also public keys, which is not correct as
> public keys are not suitable to calculate the HMAC.
> 
> Originally, support for signature verification was introduced to verify a
> possibly immutable initial ram disk, when no new files are created, and to
> switch to HMAC for the root filesystem. By that time, an HMAC key should
> have been loaded and usable to calculate HMACs for new files.
> 
> More recently support for requiring an HMAC key was removed from the
> kernel, so that signature verification can be used alone. Since this is a
> legitimate use case, evm_inode_init_security() should not return an error
> when no HMAC key has been loaded.
> 
> This patch fixes this problem by replacing the evm_key_loaded() check with
> a check of the EVM_INIT_HMAC flag in evm_initialized.
> 
> Cc: stable@...r.kernel.org # 4.5.x
> Fixes: 26ddabfe96b ("evm: enable EVM when X509 certificate is loaded")
> Signed-off-by: Roberto Sassu <roberto.sassu@...wei.com>
> Reviewed-by: Mimi Zohar <zohar@...ux.ibm.com>
> ---
>  security/integrity/evm/evm_main.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
> index 76d19146d74b..001e001eae01 100644
> --- a/security/integrity/evm/evm_main.c
> +++ b/security/integrity/evm/evm_main.c
> @@ -530,7 +530,8 @@ int evm_inode_init_security(struct inode *inode,
>  	struct evm_xattr *xattr_data;
>  	int rc;
>  
> -	if (!evm_key_loaded() || !evm_protected_xattr(lsm_xattr->name))
> +	if (!(evm_initialized & EVM_INIT_HMAC) ||
> +	    !evm_protected_xattr(lsm_xattr->name))
>  		return 0;
>  
>  	xattr_data = kzalloc(sizeof(*xattr_data), GFP_NOFS);

Let's update the function description to make it explicit.  Something
like: "evm_inode_init_security - initializes security.evm HMAC value"

Mimi