linux-kernel - Re: [PATCH v4 10/17] firmware_loader: Use security_post_load

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Date:   Thu, 06 Aug 2020 18:07:35 -0400
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Kees Cook <keescook@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Scott Branden <scott.branden@...adcom.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Takashi Iwai <tiwai@...e.de>, Jessica Yu <jeyu@...nel.org>,
        SeongJae Park <sjpark@...zon.de>,
        KP Singh <kpsingh@...omium.org>, linux-efi@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-integrity@...r.kernel.org, selinux@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 10/17] firmware_loader: Use security_post_load_data()

On Wed, 2020-07-29 at 10:58 -0700, Kees Cook wrote:
> Now that security_post_load_data() is wired up, use it instead
> of the NULL file argument style of security_post_read_file(),
> and update the security_kernel_load_data() call to indicate that a
> security_kernel_post_load_data() call is expected.
> 
> Wire up the IMA check to match earlier logic. Perhaps a generalized
> change to ima_post_load_data() might look something like this:
> 
>     return process_buffer_measurement(buf, size,
>                                       kernel_load_data_id_str(load_id),
>                                       read_idmap[load_id] ?: FILE_CHECK,
>                                       0, NULL);
> 
> Signed-off-by: Kees Cook <keescook@...omium.org>

Other than one change and one question below, it looks good.

Reviewed-by:  Mimi Zohar <zohar@...ux.ibm.com>

<snip>

> diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> index 85000dc8595c..1a7bc4c7437d 100644
> --- a/security/integrity/ima/ima_main.c
> +++ b/security/integrity/ima/ima_main.c

> @@ -706,7 +697,7 @@ int ima_load_data(enum kernel_load_data_id id, bool contents)
>  		}
>  		break;
>  	case LOADING_FIRMWARE:
> -		if (ima_enforce && (ima_appraise & IMA_APPRAISE_FIRMWARE)) {
> +		if (ima_enforce && (ima_appraise & IMA_APPRAISE_FIRMWARE) && !contents) {
>  			pr_err("Prevent firmware sysfs fallback loading.\n");

Appended signatures are limited to kernel modules and, more recently,
to the kexec kernel image, not firmware.  Without a file descriptor,
file signatures stored as an xattrs are not applicable either.  We
might as well fail earlier, rather than later.  Adding "!contents" is
unnecessary.

>  			return -EACCES;	/* INTEGRITY_UNKNOWN */
>  		}
> @@ -739,6 +730,15 @@ int ima_load_data(enum kernel_load_data_id id, bool contents)
>   */
>  int ima_post_load_data(char *buf, loff_t size, enum kernel_load_data_id load_id)
>  {
> +	if (load_id == LOADING_FIRMWARE) {
> +		if ((ima_appraise & IMA_APPRAISE_FIRMWARE) &&
> +		    (ima_appraise & IMA_APPRAISE_ENFORCE)) {
> +			pr_err("Prevent firmware loading_store.\n");
> +			return -EACCES; /* INTEGRITY_UNKNOWN */
> +		}
> +		return 0;
> +	}

Even with failing LOADING_FIRMWARE early in ima_load_data(), is this
still needed for fw_sysfs_loading()?

thanks,

Mimi

> +
>  	return 0;
>  }
>