linux-kernel - Re: [PATCH 5/6] ima: Allow appraisal of digitally signed files only

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Date:	Tue, 5 Mar 2013 14:13:50 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	zohar@...ux.vnet.ibm.com, linux-security-module@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, dmitry.kasatkin@...el.com
Subject: Re: [PATCH 5/6] ima: Allow appraisal of digitally signed files only

On Thu, Feb 14, 2013 at 02:55:44PM -0500, Vivek Goyal wrote:
> Currently ima appraises all the files as specified by the rule. So
> if one wants to create a system where only few executables are
> signed, that system will not work with IMA.
> 
> With secureboot, one needs to disable kexec so that unsigned kernels
> can't be booted. To avoid this problem, it was proposed that sign
> /sbin/kexec binary and if signatures are verified successfully, give
> an special capability to the /sbin/kexec process. And in secureboot
> mode processes with that special capability can invoke sys_kexec()
> system call.
> 
> So there is a need for IMA to allow appraising only signed binaries.
> Unsigned binaries will pass the appraisal too, but will not get the
> special capability. (Capability patches for that are yet to be written).
> 
> This patch adds new option, appraise_type=imasig_optional to allow
> appraisal to pass even if no signatures are present on the file. If
> signatures are present, then it has to be valid digital signature,
> otherwise appraisal will fail.
> 
> Signed-off-by: Vivek Goyal <vgoyal@...hat.com>
> ---
>  Documentation/ABI/testing/ima_policy |    2 +-
>  security/integrity/ima/ima_main.c    |   14 ++++++++++++--
>  security/integrity/ima/ima_policy.c  |    2 ++
>  security/integrity/integrity.h       |    1 +
>  4 files changed, 16 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/ABI/testing/ima_policy b/Documentation/ABI/testing/ima_policy
> index de16de3..cc69872 100644
> --- a/Documentation/ABI/testing/ima_policy
> +++ b/Documentation/ABI/testing/ima_policy
> @@ -30,7 +30,7 @@ Description:
>  			uid:= decimal value
>  			fowner:=decimal value
>  		lsm:  	are LSM specific
> -		option:	appraise_type:= [imasig]
> +		option:	appraise_type:= [imasig] | [optional]
>  
>  		default policy:
>  			# PROC_SUPER_MAGIC
> diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> index 3e751a9..da9e348 100644
> --- a/security/integrity/ima/ima_main.c
> +++ b/security/integrity/ima/ima_main.c
> @@ -207,8 +207,18 @@ out_digsig:
>  		rc = -EACCES;
>  out:
>  	mutex_unlock(&inode->i_mutex);
> -	if ((rc && must_appraise) && (ima_appraise & IMA_APPRAISE_ENFORCE))
> -		return -EACCES;
> +	if ((rc && must_appraise) && (ima_appraise & IMA_APPRAISE_ENFORCE)) {
> +		/*
> +		 * If IMA_APPRAISAL_OPT is set, then access is allowed
> +		 * even if hash or digital signatures are not present.
> +		 */
> +		if ((iint->flags & IMA_APPRAISAL_OPT) &&
> +		     (rc == INTEGRITY_XATTR_NOTSUPP ||
> +		      rc == INTEGRITY_IMA_NOLABEL))
> +			return 0;
> +		else
> +			return -EACCES;

I think there is problem here. "appraise_type=optional" can be specified
per rule/hook. So two different hooks can specify two different rules.

appraise func=MMAP_CHECK appraise_type=optional
appraise func=BPRM_CHECK

I think if a file is first mmaped(), then appraisal will take place and
IMA_APPRAISAL_OPT will be set in iint->flags.

Later when BPRM_CHECK hook gets executed, and it will return success
based on IMA_APPRAISAL_OPT even if there was no label. And that's not
what exec() expects.

So storing IMA_APPRAISAL_OPT in iint->flags seems wrong (espectially as
part of IMA_ACTION_FLAGS bits). I think only bits which are valid
across all rules/hooks should be stored here.

Any property which is hook/rule specific should either not be stored
or should be stroed in hook specific property area.

We don't have enough space to store more hook specific properties, so 
I will explore the option of passing around this flag when hook is being
executed and then discard it.

Thanks
Vivek
 
> +	}
>  	return 0;
>  }
>  
> diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
> index 4adcd0f..fd92dc3d4 100644
> --- a/security/integrity/ima/ima_policy.c
> +++ b/security/integrity/ima/ima_policy.c
> @@ -598,6 +598,8 @@ static int ima_parse_rule(char *rule, struct ima_rule_entry *entry)
>  			ima_log_string(ab, "appraise_type", args[0].from);
>  			if ((strcmp(args[0].from, "imasig")) == 0)
>  				entry->flags |= IMA_DIGSIG_REQUIRED;
> +			else if ((strcmp(args[0].from, "optional")) == 0)
> +				entry->flags |= IMA_APPRAISAL_OPT;
>  			else
>  				result = -EINVAL;
>  			break;
> diff --git a/security/integrity/integrity.h b/security/integrity/integrity.h
> index 0ae08fc..4d330a7 100644
> --- a/security/integrity/integrity.h
> +++ b/security/integrity/integrity.h
> @@ -29,6 +29,7 @@
>  #define IMA_ACTION_FLAGS	0xff000000
>  #define IMA_DIGSIG		0x01000000
>  #define IMA_DIGSIG_REQUIRED	0x02000000
> +#define IMA_APPRAISAL_OPT	0x04000000
>  
>  #define IMA_DO_MASK		(IMA_MEASURE | IMA_APPRAISE | IMA_AUDIT | \
>  				 IMA_APPRAISE_SUBMASK)
> -- 
> 1.7.7.6
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