Re: [PATCH v5 06/12] evm: Ignore INTEGRITY_NOLABEL/INTEGRITY_NOXATTRS if conditions are safe

[Mimi Zohar]

Hi Roberto,

On Wed, 2021-04-07 at 12:52 +0200, Roberto Sassu wrote:
> When a file is being created, LSMs can set the initial label with the
> inode_init_security hook. If no HMAC key is loaded, the new file will have
> LSM xattrs but not the HMAC. It is also possible that the file remains
> without protected xattrs after creation if no active LSM provided it.
> 
> Unfortunately, EVM will deny any further metadata operation on new files,
> as evm_protect_xattr() will always return the INTEGRITY_NOLABEL error, or
> INTEGRITY_NOXATTRS if no protected xattrs exist. This would limit the
> usability of EVM when only a public key is loaded, as commands such as cp
> or tar with the option to preserve xattrs won't work.
> 
> This patch ignores these errors when they won't be an issue, if no HMAC key
> is loaded and cannot be loaded in the future (which can be enforced by
> setting the EVM_SETUP_COMPLETE initialization flag).
> 
> Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> ---
>  security/integrity/evm/evm_main.c | 23 ++++++++++++++++++++++-
>  1 file changed, 22 insertions(+), 1 deletion(-)
> 
> diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
> index 998818283fda..6556e8c22da9 100644
> --- a/security/integrity/evm/evm_main.c
> +++ b/security/integrity/evm/evm_main.c
> @@ -90,6 +90,24 @@ static bool evm_key_loaded(void)
>  	return (bool)(evm_initialized & EVM_KEY_MASK);
>  }
>  
> +/*
> + * Ignoring INTEGRITY_NOLABEL/INTEGRITY_NOXATTRS is safe if no HMAC key
> + * is loaded and the EVM_SETUP_COMPLETE initialization flag is set.
> + */
> +static bool evm_ignore_error_safe(enum integrity_status evm_status)
> +{
> +	if (evm_initialized & EVM_INIT_HMAC)
> +		return false;
> +
> +	if (!(evm_initialized & EVM_SETUP_COMPLETE))
> +		return false;
> +
> +	if (evm_status != INTEGRITY_NOLABEL && evm_status != INTEGRITY_NOXATTRS)
> +		return false;
> +
> +	return true;
> +}
> +
>  static int evm_find_protected_xattrs(struct dentry *dentry)
>  {
>  	struct inode *inode = d_backing_inode(dentry);
> @@ -354,6 +372,8 @@ static int evm_protect_xattr(struct dentry *dentry, const char *xattr_name,
>  				    -EPERM, 0);
>  	}
>  out:
> +	if (evm_ignore_error_safe(evm_status))
> +		return 0;

I agree with the concept, but the function name doesn't provide enough
context.  Perhaps defining a function more along the lines of
"evm_hmac_disabled()" would be more appropriate and at the same time
self documenting.

>  	if (evm_status != INTEGRITY_PASS)
>  		integrity_audit_msg(AUDIT_INTEGRITY_METADATA, d_backing_inode(dentry),
>  				    dentry->d_name.name, "appraise_metadata",
> @@ -515,7 +535,8 @@ int evm_inode_setattr(struct dentry *dentry, struct iattr *attr)
>  		return 0;
>  	evm_status = evm_verify_current_integrity(dentry);
>  	if ((evm_status == INTEGRITY_PASS) ||
> -	    (evm_status == INTEGRITY_NOXATTRS))
> +	    (evm_status == INTEGRITY_NOXATTRS) ||
> +	    (evm_ignore_error_safe(evm_status)))

It would also remove the INTEGRITY_NOXATTRS test duplication here.

thanks,

Mimi

>  		return 0;
>  	integrity_audit_msg(AUDIT_INTEGRITY_METADATA, d_backing_inode(dentry),
>  			    dentry->d_name.name, "appraise_metadata",