linux-kernel - Re: [RFC PATCH v2 1/2] security: add fault injection capability

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Message-ID: <c768f42a-1370-5b38-4f89-357744fd9d5a@schaufler-ca.com>
Date:   Mon, 26 Oct 2020 09:20:38 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Aleksandr Nogikh <a.nogikh@...il.com>, jmorris@...ei.org,
        serge@...lyn.com, akinobu.mita@...il.com
Cc:     andreyknvl@...gle.com, dvyukov@...gle.com, elver@...gle.com,
        glider@...gle.com, keescook@...gle.com,
        linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        Aleksandr Nogikh <nogikh@...gle.com>,
        Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [RFC PATCH v2 1/2] security: add fault injection capability

On 10/26/2020 5:52 AM, Aleksandr Nogikh wrote:
> From: Aleksandr Nogikh <nogikh@...gle.com>
>
> Add a fault injection capability to call_int_hook macro. This will
> facilitate testing of fault tolerance of the code that invokes
> security hooks as well as the fault tolerance of the LSM
> implementations themselves.
>
> Add a KConfig option (CONFIG_FAIL_LSM_HOOKS) that controls whether the
> capability is enabled. In order to enable configuration from the user
> space, add the standard debugfs entries for fault injection (if
> CONFIG_FAULT_INJECTION_DEBUG_FS is enabled).
>
> Signed-off-by: Aleksandr Nogikh <nogikh@...gle.com>
> ---
> v2:
> - Renamed should_fail_lsm_hook() to should_fail_lsm_hook().
> ---
>  lib/Kconfig.debug   |  6 +++++
>  security/security.c | 53 ++++++++++++++++++++++++++++++++++++++++++---
>  2 files changed, 56 insertions(+), 3 deletions(-)
>
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 537cf3c2937d..80d289591e29 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -1803,6 +1803,12 @@ config FAIL_MAKE_REQUEST
>  	help
>  	  Provide fault-injection capability for disk IO.
>  
> +config FAIL_LSM_HOOKS
> +	bool "Fault-injection capability for LSM hooks"
> +	depends on FAULT_INJECTION
> +	help
> +	  Provide fault-injection capability for LSM hooks.
> +
>  config FAIL_IO_TIMEOUT
>  	bool "Fault-injection capability for faking disk interrupts"
>  	depends on FAULT_INJECTION && BLOCK
> diff --git a/security/security.c b/security/security.c
> index 69ff6e2e2cd4..1105ad0f6891 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -28,6 +28,7 @@
>  #include <linux/backing-dev.h>
>  #include <linux/string.h>
>  #include <linux/msg.h>
> +#include <linux/fault-inject.h>
>  #include <net/flow.h>
>  
>  #define MAX_LSM_EVM_XATTR	2
> @@ -669,6 +670,51 @@ static void __init lsm_early_task(struct task_struct *task)
>  		panic("%s: Early task alloc failed.\n", __func__);
>  }
>  
> +
> +#ifdef CONFIG_FAIL_LSM_HOOKS
> +
> +static struct {
> +	struct fault_attr attr;
> +	int retval;
> +} fail_lsm_hooks = {
> +	.attr = FAULT_ATTR_INITIALIZER,
> +	.retval = -EACCES
> +};
> +
> +static int __init setup_fail_lsm_hooks(char *str)
> +{
> +	return setup_fault_attr(&fail_lsm_hooks.attr, str);
> +}
> +__setup("fail_lsm_hooks=", setup_fail_lsm_hooks);
> +
> +static int lsm_hooks_inject_fail(void)
> +{
> +	return should_fail(&fail_lsm_hooks.attr, 1) ? fail_lsm_hooks.retval : 0;
> +}
> +
> +#ifdef CONFIG_FAULT_INJECTION_DEBUG_FS
> +
> +static int __init fail_lsm_hooks_debugfs(void)
> +{
> +	umode_t mode = S_IFREG | 0600;
> +	struct dentry *dir;
> +
> +	dir = fault_create_debugfs_attr("fail_lsm_hooks", NULL,
> +					&fail_lsm_hooks.attr);
> +	debugfs_create_u32("retval", mode, dir, &fail_lsm_hooks.retval);
> +	return 0;
> +}
> +
> +late_initcall(fail_lsm_hooks_debugfs);
> +
> +#endif /* CONFIG_FAULT_INJECTION_DEBUG_FS */
> +
> +#else
> +
> +static inline int lsm_hooks_inject_fail(void) { return 0; }
> +
> +#endif /* CONFIG_FAIL_LSM_HOOKS */
> +
>  /*
>   * The default value of the LSM hook is defined in linux/lsm_hook_defs.h and
>   * can be accessed with:
> @@ -707,16 +753,17 @@ static void __init lsm_early_task(struct task_struct *task)
>  	} while (0)
>  
>  #define call_int_hook(FUNC, IRC, ...) ({			\
> -	int RC = IRC;						\
> -	do {							\
> +	int RC = lsm_hooks_inject_fail();			\
> +	if (RC == 0) {								\

Injecting the failure here will prevent the loaded LSM hooks from
being called.

>  		struct security_hook_list *P;			\
> +		RC = IRC;								\
>  								\
>  		hlist_for_each_entry(P, &security_hook_heads.FUNC, list) { \
>  			RC = P->hook.FUNC(__VA_ARGS__);		\
>  			if (RC != 0)				\
>  				break;				\
>  		}						\
> -	} while (0);						\
> +	}							\

Injecting the failure here would allow the loaded LSM hooks to
be called. It shouldn't make a difference, but hooks with side-effects
are always possible. I don't have an issue either way.

>  	RC;							\
>  })
>