Re: [PATCH 6/6] ima: Support appended signatures for appraisal

[Mimi Zohar]

Hi Thiago,

On Tue, 2017-04-18 at 17:17 -0300, Thiago Jung Bauermann wrote:
> This patch introduces the appended_imasig keyword to the IMA policy syntax
> to specify that a given hook should expect the file to have the IMA
> signature appended to it.  Here is how it can be used in a rule:
> 
> appraise func=KEXEC_KERNEL_CHECK appraise_type=appended_imasig
> appraise func=KEXEC_KERNEL_CHECK appraise_type=appended_imasig|imasig

> In the second form, IMA will accept either an appended signature or a
> signature stored in the extended attribute. In that case, it will first
> check whether there is an appended signature, and if not it will read it
> from the extended attribute.

An appended signature should be another place to look for a signature,
when a signature is required, but it shouldn't make a difference where
the signature is located. Â"imasig" could have implied to look for the
signature in both places - xattr or appended. ÂSo the new option is
just a hint - a performance improvement.

This might seem picayune, but the difference between "expect" vs.
"hint" impacts the code. (Further explanation inline.)

> The format of the appended signature is the same used for signed kernel
> modules. This means that the file can be signed with the scripts/sign-file
> tool, with a command line such as this:
> 
> $ sign-file sha256 privkey_ima.pem x509_ima.der vmlinux
> 
> This code only works for files that are hashed from a memory buffer, not
> for files that are read from disk at the time of hash calculation. In other
> words, only hooks that use kernel_read_file can support appended
> signatures.
> 
> The change in CONFIG_INTEGRITY_SIGNATURE to select CONFIG_KEYS instead of
> depending on it is to avoid a dependency recursion in
> CONFIG_IMA_APPRAISE_APPENDED_SIG, because CONFIG_MODULE_SIG_FORMAT selects
> CONFIG_KEYS and Kconfig complains that CONFIG_INTEGRITY_SIGNATURE depends
> on it.
> 
> Signed-off-by: Thiago Jung Bauermann <bauerman@xxxxxxxxxxxxxxxxxx>
> ---

snip

> diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> index 2aebb7984437..994ee420b2ec 100644
> --- a/security/integrity/ima/ima_main.c
> +++ b/security/integrity/ima/ima_main.c
> @@ -16,6 +16,9 @@
>   *	implements the IMA hooks: ima_bprm_check, ima_file_mmap,
>   *	and ima_file_check.
>   */
> +
> +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
> +
>  #include <linux/module.h>
>  #include <linux/file.h>
>  #include <linux/binfmts.h>
> @@ -228,9 +231,30 @@ static int process_measurement(struct file *file, char *buf, loff_t size,
> 
>  	template_desc = ima_template_desc_current();
>  	if ((action & IMA_APPRAISE_SUBMASK) ||
> -		    strcmp(template_desc->name, IMA_TEMPLATE_IMA_NAME) != 0)
> -		/* read 'security.ima' */
> -		xattr_len = ima_read_xattr(file_dentry(file), &xattr_value);
> +		    strcmp(template_desc->name, IMA_TEMPLATE_IMA_NAME) != 0) {
> +#ifdef CONFIG_IMA_APPRAISE_APPENDED_SIG

Using "ifdef" in C code is really discouraged. Normally, it is an
indication that the code needs to be re-factored. ÂAssuming we really
need a new CONFIG option, which I'm not sure that we do, I would move
the appended signature code to its own file, define stub functions in
ima.h, and update the Makefile.

> +		unsigned long digsig_req;
> +
> +		if (iint->flags & IMA_APPENDED_DIGSIG_REQUIRED) {
> +			if (!buf || !size)
> +				pr_err("%s doesn't support appended_imasig\n",
> +				       func_tokens[func]);

The policy parsing should prevent defining appended_imasig on
inappropriate hooks. ÂSince the iint->flags might be shared between
hooks, we might still need to test buf, but it could be simplified to:

if (!buf && iint->flags & IMA_APPENDED_DIGSIG_REQUIRED) {

> +			else
> +				ima_read_appended_sig(buf, &size, &xattr_value,
> +						      &xattr_len);
> +		}
> +
> +		/*
> +		 * Don't try to read the xattr if we require an appended
> +		 * signature but failed to get one.
> +		 */

If the appended_sig is just a hint as to where the signature is
located, then we should read the xattr, even if IMA_DIGSIG_REQUIRED is
not specified. Âima_appraise_measurement() should be updated to
require a signature if either IMA_DIGSIG_REQUIRED or
IMA_APPENDED_SIGNATURE_REQUIRED are specified. Â

Part of the confusion might be due to the naming
-"IMA_APPENEDED_SIGNATURE_REQUIRED".


> +		digsig_req = iint->flags & IMA_DIGSIG_REQUIRED_MASK;
> +		if (!xattr_len && digsig_req != IMA_APPENDED_DIGSIG_REQUIRED)
> +#endif /* CONFIG_IMA_APPRAISE_APPENDED_SIG */

Is limiting the "if" to the ifdef really necessary?Â

> +			/* read 'security.ima' */
> +			xattr_len = ima_read_xattr(file_dentry(file),
> +						   &xattr_value);
> +	}
> 

Suppose an appended signature and an xattr both exist (eg. kernel
modules), but for some reason the appended signature validation fails.
ÂThe code should somehow retry the signature validation with the
xattr.

>  	hash_algo = ima_get_hash_algo(xattr_value, xattr_len);
> 

Unfortunately, if the hash algorithm in the appended signature and the
xattr are not the same, then we would need to re-calculate the file
hash.

Mimi