linux-kernel - Re: [PATCH 2/6] ima: Return INTEGRITY_FAIL if digital signature can't be verified

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Message-ID: <1362404916.4392.25.camel@falcor1>
Date:	Mon, 04 Mar 2013 08:48:36 -0500
From:	Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	linux-security-module@...r.kernel.org,
	linux-kernel@...r.kernel.org, dmitry.kasatkin@...el.com
Subject: Re: [PATCH 2/6] ima: Return INTEGRITY_FAIL if digital signature
 can't be verified

On Thu, 2013-02-14 at 14:55 -0500, Vivek Goyal wrote:
> Digital signature verification happens using integrity_digsig_verify().
> Curently we set integrity to FAIL for all error codes except -EOPNOTSUPP.
> This sounds out of line.
> 
> - If appropriate kernel code is not compiled in to verify signature of
>   a file, then prractically it is a failed signature.
> 
> - For so many other possible errors we are setting the status to fail.
>   For example, -EINVAL, -ENOKEY, -ENOMEM, -EINVAL, -ENOTSUPP etc, it
>   beats me that why -EOPNOTSUPP is special.
> 
> This patch should make the semantics more consistent. That is, if digital
> signature is present in security.ima, then any error happened during
> signature processing leads to status INTEGRITY_FAIL.
> 
> AFAICS, it should not have any user visible effect on existing
> application. In some cases we will start returning INTEGRITY_FAIL
> instead of INTEGRITY_UNKNOWN. And process_measurement() will deny access
> to file both in case of INTEGRITY_UNKNOWN and INTEGRITY_FAIL.
> 
> Signed-off-by: Vivek Goyal <vgoyal@...hat.com>

A number of patches in this patchset more finely differentiate return
codes, which is good.  I agree with you totally that there is no good
reason for -EOPNOTSUPP to be handled differently.  Unfortunately, the
initramfs is CPIO, which doesn't support xattrs.  With the proposed
change and 'ima_appraise_tcb' flag enabled, we wouldn't be able to boot.
I really dislike hard coding policy in the kernel.

thanks,

Mimi

> ---
>  security/integrity/ima/ima_appraise.c |    4 +---
>  1 files changed, 1 insertions(+), 3 deletions(-)
> 
> diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
> index 3710f44..6f1eeb8 100644
> --- a/security/integrity/ima/ima_appraise.c
> +++ b/security/integrity/ima/ima_appraise.c
> @@ -178,9 +178,7 @@ int ima_appraise_measurement(int func, struct integrity_iint_cache *iint,
>  					     xattr_value->digest, rc - 1,
>  					     iint->ima_xattr.digest,
>  					     IMA_DIGEST_SIZE);
> -		if (rc == -EOPNOTSUPP) {
> -			status = INTEGRITY_UNKNOWN;
> -		} else if (rc) {
> +		if (rc) {
>  			cause = "invalid-signature";
>  			status = INTEGRITY_FAIL;
>  		} else {



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