Re: [PATCH v4 next 2/3] modules:capabilities: automatic module loading restriction

[Kees Cook]

On Mon, May 22, 2017 at 4:57 AM, Djalal Harouni <tixxdz@xxxxxxxxx> wrote:
> [...]
> diff --git a/kernel/module.c b/kernel/module.c
> index 4a3665f..ce7a146 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -282,6 +282,8 @@ module_param(sig_enforce, bool_enable_only, 0644);
>
>  /* Block module loading/unloading? */
>  int modules_disabled = 0;
> +int modules_autoload_mode = MODULES_AUTOLOAD_ALLOWED;
> +const int modules_autoload_max = MODULES_AUTOLOAD_DISABLED;
>  core_param(nomodule, modules_disabled, bint, 0);
>
>  /* Waiting for a module to finish initializing? */
> @@ -4296,6 +4298,46 @@ struct module *__module_text_address(unsigned long addr)
>  }
>  EXPORT_SYMBOL_GPL(__module_text_address);
>
> +/**
> + * may_autoload_module - Determine whether a module auto-load operation
> + * is permitted
> + * @kmod_name: The module name
> + * @allow_cap: if positive, may allow to auto-load the module if this capability
> + * is set
> + *
> + * Determine whether a module auto-load operation is allowed or not. The check
> + * uses the sysctl "modules_autoload_mode" value.
> + *
> + * This allows to have more control on automatic module loading, and align it
> + * with explicit load/unload module operations. The kernel contains several
> + * modules, some of them are not updated often and may contain bugs and
> + * vulnerabilities.
> + *
> + * The "allow_cap" is passed by callers to explicitly note that the module has
> + * the appropriate alias and that the "allow_cap" capability is set. This is
> + * for backward compatibility, the aim is to have a clear picture where:
> + *
> + * 1) Implicit module loading is allowed
> + * 2) Implicit module loading as with the explicit one requires CAP_SYS_MODULE.
> + * 3) Implicit module loading as with the explicit one can be disabled.
> + *
> + * Returns 0 if the module request is allowed or -EPERM if not.
> + */
> +int may_autoload_module(char *kmod_name, int allow_cap)
> +{
> +       if (modules_autoload_mode == MODULES_AUTOLOAD_ALLOWED)
> +               return 0;
> +       else if (modules_autoload_mode == MODULES_AUTOLOAD_PRIVILEGED) {
> +               /* Check CAP_SYS_MODULE then allow_cap if valid */
> +               if (capable(CAP_SYS_MODULE) ||
> +                   (allow_cap > 0 && capable(allow_cap)))

With the allow_cap check already happening in my suggestion for
__request_module(), it's not needed here. (In fact, it's not even
really needed to plumb this into the hook, I don't think?

Regardless, I remain a fan. :)

-Kees

-- 
Kees Cook
Pixel Security