Re: [PATCH] x86/ima: require signed kernel modules

[Jessica Yu <jeyu@xxxxxxxxxx>]

+++ Mimi Zohar [31/01/19 14:18 -0500]:
> Require signed kernel modules on systems with secure boot mode enabled.
>
> To coordinate between appended kernel module signatures and IMA
> signatures, only define an IMA MODULE_CHECK policy rule if
> CONFIG_MODULE_SIG is not enabled.
>
> This patch defines a function named set_module_sig_required() and renames
> is_module_sig_enforced() to is_module_sig_enforced_or_required().  The
> call to set_module_sig_required() is dependent on CONFIG_IMA_ARCH_POLICY
> being enabled.
>
> Signed-off-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>
> ---
> arch/x86/kernel/ima_arch.c        |  9 ++++++++-
> include/linux/module.h            |  7 ++++++-
> kernel/module.c                   | 15 +++++++++++----
> security/integrity/ima/ima_main.c |  2 +-
> 4 files changed, 26 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/kernel/ima_arch.c b/arch/x86/kernel/ima_arch.c
> index e47cd9390ab4..96a023238a83 100644
> --- a/arch/x86/kernel/ima_arch.c
> +++ b/arch/x86/kernel/ima_arch.c
> @@ -64,12 +64,19 @@ static const char * const sb_arch_rules[] = {
> 	"appraise func=KEXEC_KERNEL_CHECK appraise_type=imasig",
> #endif /* CONFIG_KEXEC_VERIFY_SIG */
> 	"measure func=KEXEC_KERNEL_CHECK",
> +#if !IS_ENABLED(CONFIG_MODULE_SIG)
> +	"appraise func=MODULE_CHECK appraise_type=imasig",
> +#endif
> +	"measure func=MODULE_CHECK",
> 	NULL
> };
>
> const char * const *arch_get_ima_policy(void)
> {
> -	if (IS_ENABLED(CONFIG_IMA_ARCH_POLICY) && arch_ima_get_secureboot())
> +	if (IS_ENABLED(CONFIG_IMA_ARCH_POLICY) && arch_ima_get_secureboot()) {
> +		if (IS_ENABLED(CONFIG_MODULE_SIG))
> +			set_module_sig_required();
> 		return sb_arch_rules;
> +	}
> 	return NULL;
> }
> diff --git a/include/linux/module.h b/include/linux/module.h
> index 8fa38d3e7538..af51c8ec755f 100644
> --- a/include/linux/module.h
> +++ b/include/linux/module.h
> @@ -659,7 +659,8 @@ static inline bool is_livepatch_module(struct module *mod)
> }
> #endif /* CONFIG_LIVEPATCH */
>
> -bool is_module_sig_enforced(void);
> +bool is_module_sig_enforced_or_required(void);
> +void set_module_sig_required(void);
>
> #else /* !CONFIG_MODULES... */
>
> @@ -780,6 +781,10 @@ static inline bool is_module_sig_enforced(void)
> 	return false;
> }
>
> +static inline void set_module_sig_required(void)
> +{
> +}
> +
> /* Dereference module function descriptor */
> static inline
> void *dereference_module_function_descriptor(struct module *mod, void *ptr)
> diff --git a/kernel/module.c b/kernel/module.c
> index 2ad1b5239910..70a9709d19eb 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -275,16 +275,23 @@ static void module_assert_mutex_or_preempt(void)
>
> static bool sig_enforce = IS_ENABLED(CONFIG_MODULE_SIG_FORCE);
> module_param(sig_enforce, bool_enable_only, 0644);
> +static bool sig_required;
>
> /*
>  * Export sig_enforce kernel cmdline parameter to allow other subsystems rely
>  * on that instead of directly to CONFIG_MODULE_SIG_FORCE config.
>  */
> -bool is_module_sig_enforced(void)
> +bool is_module_sig_enforced_or_required(void)
> {
> -	return sig_enforce;
> +	return sig_enforce || sig_required;
> }

Hi Mimi,

Just wondering, is there any particular reason why a distinction is
made between sig_enforce and sig_required? Doesn't sig_enforce imply
that signed modules are required? In other words, why introduce
another variable instead of just using sig_enforce? It may be
confusing in the case of a user looking at /sys/module/module/parameters/sig_enforce
and it having a value of 0 yet module signatures are being required by ima.

Thanks,

Jessica

> -EXPORT_SYMBOL(is_module_sig_enforced);
> +EXPORT_SYMBOL(is_module_sig_enforced_or_required);
> +
> +void set_module_sig_required(void)
> +{
> +	sig_required = true;
> +}
> +EXPORT_SYMBOL(set_module_sig_required);
>
> /* Block module loading/unloading? */
> int modules_disabled = 0;
> @@ -2789,7 +2796,7 @@ static int module_sig_check(struct load_info *info, int flags)
> 	}
>
> 	/* Not having a signature is only an error if we're strict. */
> -	if (err == -ENOKEY && !is_module_sig_enforced())
> +	if (err == -ENOKEY && !is_module_sig_enforced_or_required())
> 		err = 0;
>
> 	return err;
> diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> index 357edd140c09..bbaf87f688be 100644
> --- a/security/integrity/ima/ima_main.c
> +++ b/security/integrity/ima/ima_main.c
> @@ -563,7 +563,7 @@ int ima_load_data(enum kernel_load_data_id id)
> 		}
> 		break;
> 	case LOADING_MODULE:
> -		sig_enforce = is_module_sig_enforced();
> +		sig_enforce = is_module_sig_enforced_or_required();
>
> 		if (ima_enforce && (!sig_enforce
> 				    && (ima_appraise & IMA_APPRAISE_MODULES))) {
> --
> 2.7.5