Re: [PATCH v3] Allow to exclude specific file types in LoadPin

[Kees Cook <keescook@chromium.org>]

On Fri, May 31, 2019 at 11:25:53AM -0700, Ke Wu wrote:
> Linux kernel already provide MODULE_SIG and KEXEC_VERIFY_SIG to
> make sure loaded kernel module and kernel image are trusted. This
> patch adds a kernel command line option "loadpin.exclude" which
> allows to exclude specific file types from LoadPin. This is useful
> when people want to use different mechanisms to verify module and
> kernel image while still use LoadPin to protect the integrity of
> other files kernel loads.
> 
> Signed-off-by: Ke Wu <mikewu@google.com>
> ---
> Changelog since v2:
> - Make size of exclude_read_files and ignore_read_file_id to be
>   equal to the size of kernel_read_file_str.

Thanks! I've fixed this differently and it should be visible shortly.

-Kees

> 
> Changelog since v1:
> - Mark ignore_read_file_id with __ro_after_init.
> - Mark parse_exclude() with __init.
> - Use ARRAY_SIZE(ignore_read_file_id) instead of READING_MAX_ID.
> 
> 
>  Documentation/admin-guide/LSM/LoadPin.rst | 10 ++++++
>  security/loadpin/loadpin.c                | 42 +++++++++++++++++++++++
>  2 files changed, 52 insertions(+)
> 
> diff --git a/Documentation/admin-guide/LSM/LoadPin.rst b/Documentation/admin-guide/LSM/LoadPin.rst
> index 32070762d24c..716ad9b23c9a 100644
> --- a/Documentation/admin-guide/LSM/LoadPin.rst
> +++ b/Documentation/admin-guide/LSM/LoadPin.rst
> @@ -19,3 +19,13 @@ block device backing the filesystem is not read-only, a sysctl is
>  created to toggle pinning: ``/proc/sys/kernel/loadpin/enabled``. (Having
>  a mutable filesystem means pinning is mutable too, but having the
>  sysctl allows for easy testing on systems with a mutable filesystem.)
> +
> +It's also possible to exclude specific file types from LoadPin using kernel
> +command line option "``loadpin.exclude``". By default, all files are
> +included, but they can be excluded using kernel command line option such
> +as "``loadpin.exclude=kernel-module,kexec-image``". This allows to use
> +different mechanisms such as ``CONFIG_MODULE_SIG`` and
> +``CONFIG_KEXEC_VERIFY_SIG`` to verify kernel module and kernel image while
> +still use LoadPin to protect the integrity of other files kernel loads. The
> +full list of valid file types can be found in ``kernel_read_file_str``
> +defined in ``include/linux/fs.h``.
> diff --git a/security/loadpin/loadpin.c b/security/loadpin/loadpin.c
> index 055fb0a64169..baa8a5b08c53 100644
> --- a/security/loadpin/loadpin.c
> +++ b/security/loadpin/loadpin.c
> @@ -45,6 +45,12 @@ static void report_load(const char *origin, struct file *file, char *operation)
>  }
>  
>  static int enforce = IS_ENABLED(CONFIG_SECURITY_LOADPIN_ENFORCE);
> +/*
> + * The size should be READING_MAX_ID + 1 to be equal to the size of
> + * kernel_read_file_str.
> + */
> +static char *exclude_read_files[READING_MAX_ID + 1];
> +static int ignore_read_file_id[READING_MAX_ID + 1] __ro_after_init;
>  static struct super_block *pinned_root;
>  static DEFINE_SPINLOCK(pinned_root_spinlock);
>  
> @@ -129,6 +135,13 @@ static int loadpin_read_file(struct file *file, enum kernel_read_file_id id)
>  	struct super_block *load_root;
>  	const char *origin = kernel_read_file_id_str(id);
>  
> +	/* If the file id is excluded, ignore the pinning. */
> +	if ((unsigned int)id < ARRAY_SIZE(ignore_read_file_id) &&
> +	    ignore_read_file_id[id]) {
> +		report_load(origin, file, "pinning-excluded");
> +		return 0;
> +	}
> +
>  	/* This handles the older init_module API that has a NULL file. */
>  	if (!file) {
>  		if (!enforce) {
> @@ -187,10 +200,37 @@ static struct security_hook_list loadpin_hooks[] __lsm_ro_after_init = {
>  	LSM_HOOK_INIT(kernel_load_data, loadpin_load_data),
>  };
>  
> +static void __init parse_exclude(void)
> +{
> +	int i, j;
> +	char *cur;
> +
> +	for (i = 0; i < ARRAY_SIZE(exclude_read_files); i++) {
> +		cur = exclude_read_files[i];
> +		if (!cur)
> +			break;
> +		if (*cur == '\0')
> +			continue;
> +
> +		for (j = 0; j < ARRAY_SIZE(kernel_read_file_str); j++) {
> +			if (strcmp(cur, kernel_read_file_str[j]) == 0) {
> +				pr_info("excluding: %s\n",
> +					kernel_read_file_str[j]);
> +				ignore_read_file_id[j] = 1;
> +				/*
> +				 * Can not break, because one read_file_str
> +				 * may map to more than on read_file_id.
> +				 */
> +			}
> +		}
> +	}
> +}
> +
>  static int __init loadpin_init(void)
>  {
>  	pr_info("ready to pin (currently %senforcing)\n",
>  		enforce ? "" : "not ");
> +	parse_exclude();
>  	security_add_hooks(loadpin_hooks, ARRAY_SIZE(loadpin_hooks), "loadpin");
>  	return 0;
>  }
> @@ -203,3 +243,5 @@ DEFINE_LSM(loadpin) = {
>  /* Should not be mutable after boot, so not listed in sysfs (perm == 0). */
>  module_param(enforce, int, 0);
>  MODULE_PARM_DESC(enforce, "Enforce module/firmware pinning");
> +module_param_array_named(exclude, exclude_read_files, charp, NULL, 0);
> +MODULE_PARM_DESC(exclude, "Exclude pinning specific read file types");
> -- 
> 2.22.0.rc1.257.g3120a18244-goog
> 

-- 
Kees Cook