linux-kernel - Re: [RFC PATCH] fips: check whether a module registering an alg or template is signed

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Date:	Wed, 06 Feb 2013 09:02:46 +0100
From:	Stephan Mueller <stephan.mueller@...ec.com>
To:	Kyle McMartin <kyle@...hat.com>
CC:	linux-kernel@...r.kernel.org, David Howells <dhowells@...hat.com>,
	rusty@...tcorp.com.au, jstancek@...hat.com,
	herbert@...dor.hengli.com.au
Subject: Re: [RFC PATCH] fips: check whether a module registering an alg or
 template is signed

On 05.02.2013 23:58:30, +0100, Kyle McMartin <kyle@...hat.com> wrote:

Hi Kyle,

> fips mode needs to prevent unsigned modules from registering crypto
> algorithms, and currently panics if an unsigned module is attempted to
> be loaded. Instead, forbid (by returning -EINVAL) registering a crypto
> alg or template if fips mode is enabled and the module signature is not
> valid.

Just reading this paragraph, there is one missing puzzle piece: the
*entire* kernel crypto API must shut down, even if only one kernel
module with one cipher (or block chaining mode, ...) has a broken signature.

The overall requirement is: if one self test fails, the entire FIPS
140-2 crypto module must become unavailable. (please note and do not get
confused by the overload of the term "module" -- we have the KOs the
kernel loads, and we have something called a FIPS 140-2 module which is
the entire crypto "library" subject to a FIPS 140-2 validation)

This signature check is one self test required at runtime.

I added comments inline into the patch.

> 
> crypto_sig_check should return 1 (and allow the registration) if any
> of the following are true:
>  1/ fips is not enabled (but CONFIG_CRYPTO_FIPS is enabled.)
>  2/ the algorithm is built into the kernel (THIS_MODULE == NULL)
>  3/ the algorithm is in a module, and the module sig check passes
> and fail in any of the other cases.
> 
> Checking in crypto_check_alg and crypto_register_template seems to hit
> the callpoints as far as I can see.
> 
> Signed-off-by: Kyle McMartin <kyle@...hat.com>
> 
> ---
> 
> rusty,
> 
> How about something like this? It keeps the FIPS mess in the
> crypto/fips.c file (aside from something that goes away entirely in the
> !CONFIG_CRYPTO_FIPS case.)
> 
> regards, Kyle
> 
> --- a/crypto/algapi.c
> +++ b/crypto/algapi.c
> @@ -52,6 +52,9 @@ static int crypto_check_alg(struct crypto_alg *alg)
>  	if (alg->cra_priority < 0)
>  		return -EINVAL;
>  
> +	if (!crypto_sig_check(alg->cra_module))
> +		return -EINVAL;

Instead of an EINVAL, the kernel either must panic(), or a global flag
is introduced which is evaluated by every kernel crypto API call. If
that flag is, say, false, none of the kernel crypto API calls must succeed.
> +
>  	return crypto_set_driver_name(alg);
>  }
>  
> @@ -435,6 +438,11 @@ int crypto_register_template(struct crypto_template *tmpl)


I am wondering whether the modification of these two functions are
sufficient. As I wrote in a previous email, there are a number of
register functions the kernel crypto API exports and which are used.

>  			goto out;
>  	}
>  
> +	if (!crypto_sig_check(tmpl->module)) {
> +		err = -EINVAL;
> +		goto out;
> +	}
> +
>  	list_add(&tmpl->list, &crypto_template_list);
>  	crypto_notify(CRYPTO_MSG_TMPL_REGISTER, tmpl);
>  	err = 0;
> diff --git a/crypto/fips.c b/crypto/fips.c
> index 5539700..2ebbe0f 100644
> --- a/crypto/fips.c
> +++ b/crypto/fips.c
> @@ -15,6 +15,19 @@
>  int fips_enabled;
>  EXPORT_SYMBOL_GPL(fips_enabled);
>  
> +/* forbid loading modules in fips mode if the module is not signed */
> +int crypto_sig_check(struct module *m)
> +{
> +#if defined(CONFIG_MODULE_SIG)
> +	if (!fips_enabled || !m || (m && m->sig_ok))
> +		return 1;
> +	else
> +		return 0;

This code looks good.

> +#else
> +	return 1;
> +#endif
> +}
> +
>  /* Process kernel command-line parameter at boot time. fips=0 or fips=1 */
>  static int fips_enable(char *str)
>  {
> diff --git a/crypto/internal.h b/crypto/internal.h
> index 9ebedae..937bfaf 100644
> --- a/crypto/internal.h
> +++ b/crypto/internal.h
> @@ -139,5 +139,14 @@ static inline void crypto_notify(unsigned long val, void *v)
>  	blocking_notifier_call_chain(&crypto_chain, val, v);
>  }
>  
> +#if defined(CONFIG_CRYPTO_FIPS)
> +int crypto_sig_check(struct module *m);
> +#else
> +static inline int crypto_sig_check(struct module *m)
> +{
> +	return 1;
> +}
> +#endif
> +
>  #endif	/* _CRYPTO_INTERNAL_H */
>  
> 

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