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SubjectRe: [PATCH v4 2/3] security: Expose a mechanism to load lsm hooks dynamically at runtime
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On 3/6/2018 11:23 PM, Sargun Dhillon wrote:
> This patch adds dynamic security hooks. These hooks are designed to allow
> for safe runtime loading.
>
> These hooks are only run after all built-in, and major LSMs are run.
> The LSMs enabled by this feature must be minor LSMs, but they can poke
> at the security blobs, as the blobs should be initialized by the time
> their callback happens.
>
> There should be little runtime performance overhead for this feature,
> as it's protected behind static_keys, which are disabled by default,
> and are only enabled per-hook at runtime, when a module is loaded.
>
> Currently, the hook heads are separated for dynamic hooks, because
> it is not read-only like the hooks which are loaded at runtime.
>
> Some hooks are blacklisted, and attempting to load an LSM with any
> of them in use will fail.
>
> Signed-off-by: Sargun Dhillon <sargun@sargun.me>
> ---
> include/linux/lsm_hooks.h | 26 +++++-
> security/Kconfig | 9 +++
> security/inode.c | 13 ++-
> security/security.c | 198 ++++++++++++++++++++++++++++++++++++++++++++--
> 4 files changed, 234 insertions(+), 12 deletions(-)
>
> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> index d28c7f5b01c1..4e6351957dff 100644
> --- a/include/linux/lsm_hooks.h
> +++ b/include/linux/lsm_hooks.h
> @@ -28,6 +28,7 @@
> #include <linux/security.h>
> #include <linux/init.h>
> #include <linux/rculist.h>
> +#include <linux/module.h>
>
> /**
> * union security_list_options - Linux Security Module hook function list
> @@ -1968,6 +1969,9 @@ struct security_hook_list {
> enum lsm_hook head_idx;
> union security_list_options hook;
> char *lsm;
> +#ifdef CONFIG_SECURITY_DYNAMIC_HOOKS
> + struct module *owner;
> +#endif
> } __randomize_layout;
>
> /*
> @@ -1976,11 +1980,29 @@ struct security_hook_list {
> * care of the common case and reduces the amount of
> * text involved.
> */
> +#ifdef CONFIG_SECURITY_DYNAMIC_HOOKS
> +#define LSM_HOOK_INIT(HEAD, HOOK) \
> + { \
> + .head_idx = HOOK_IDX(HEAD), \
> + .hook = { .HEAD = HOOK }, \
> + .owner = THIS_MODULE, \
> + }
> +
> +#else
> #define LSM_HOOK_INIT(HEAD, HOOK) \
> { .head_idx = HOOK_IDX(HEAD), .hook = { .HEAD = HOOK } }
> +#endif
>
> -extern char *lsm_names;
> -
> +#ifdef CONFIG_SECURITY_DYNAMIC_HOOKS
> +extern int security_add_dynamic_hooks(struct security_hook_list *hooks,
> + int count, char *lsm);
> +#else
> +static inline int security_add_dynamic_hooks(struct security_hook_list *hooks,
> + int count, char *lsm)
> +{
> + return -EOPNOTSUPP;
> +}
> +#endif
> extern void security_add_hooks(struct security_hook_list *hooks, int count,
> char *lsm);
>
> diff --git a/security/Kconfig b/security/Kconfig
> index c4302067a3ad..481b93b0d4d9 100644
> --- a/security/Kconfig
> +++ b/security/Kconfig
> @@ -36,6 +36,15 @@ config SECURITY_WRITABLE_HOOKS
> bool
> default n
>
> +config SECURITY_DYNAMIC_HOOKS
> + bool "Runtime loadable (minor) LSMs via LKMs"
> + depends on SECURITY && SRCU
> + help
> + This enables LSMs which live in LKMs, and supports loading, and
> + unloading them safely at runtime. These LSMs must be minor LSMs.
> + They cannot circumvent the built-in LSMs.
> + If you are unsure how to answer this question, answer N.
> +
> config SECURITYFS
> bool "Enable the securityfs filesystem"
> help
> diff --git a/security/inode.c b/security/inode.c
> index 8dd9ca8848e4..89be07b044a5 100644
> --- a/security/inode.c
> +++ b/security/inode.c
> @@ -22,6 +22,10 @@
> #include <linux/security.h>
> #include <linux/lsm_hooks.h>
> #include <linux/magic.h>
> +#include <linux/mutex.h>
> +
> +extern char *lsm_names;
> +extern struct mutex lsm_lock;
>
> static struct vfsmount *mount;
> static int mount_count;
> @@ -312,8 +316,13 @@ static struct dentry *lsm_dentry;
> static ssize_t lsm_read(struct file *filp, char __user *buf, size_t count,
> loff_t *ppos)
> {
> - return simple_read_from_buffer(buf, count, ppos, lsm_names,
> - strlen(lsm_names));
> + ssize_t ret;
> +
> + mutex_lock(&lsm_lock);
> + ret = simple_read_from_buffer(buf, count, ppos, lsm_names,
> + strlen(lsm_names));
> + mutex_unlock(&lsm_lock);
> + return ret;
> }
>
> static const struct file_operations lsm_ops = {
> diff --git a/security/security.c b/security/security.c
> index b9fb297b824e..492d44dd0549 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -29,6 +29,7 @@
> #include <linux/backing-dev.h>
> #include <linux/string.h>
> #include <net/flow.h>
> +#include <linux/mutex.h>
>
> #define MAX_LSM_EVM_XATTR 2
>
> @@ -36,10 +37,18 @@
> #define SECURITY_NAME_MAX 10

I expect that this limit will be too small for loadable modules.

> static struct list_head security_hook_heads[__MAX_LSM_HOOK] __lsm_ro_after_init;
> -static ATOMIC_NOTIFIER_HEAD(lsm_notifier_chain);
> -
> #define HOOK_HEAD(NAME) (&security_hook_heads[HOOK_IDX(NAME)])
>
> +#ifdef CONFIG_SECURITY_DYNAMIC_HOOKS
> +static struct list_head dynamic_security_hook_heads[__MAX_LSM_HOOK];
> +static struct srcu_struct dynamic_hook_srcus[__MAX_LSM_HOOK];
> +#define DYNAMIC_HOOK_HEAD(NAME) (&dynamic_security_hook_heads[HOOK_IDX(NAME)])
> +#define DYNAMIC_HOOK_SRCU(NAME) (&dynamic_hook_srcus[HOOK_IDX(NAME)])
> +DEFINE_STATIC_KEY_ARRAY_FALSE(dynamic_hook_keys, __MAX_LSM_HOOK);
> +#endif
> +static ATOMIC_NOTIFIER_HEAD(lsm_notifier_chain);
> +
> +DEFINE_MUTEX(lsm_lock);
> char *lsm_names;
> /* Boot-time LSM user choice */
> static __initdata char chosen_lsm[SECURITY_NAME_MAX + 1] =
> @@ -55,6 +64,23 @@ static void __init do_security_initcalls(void)
> }
> }
>
> +#ifdef CONFIG_SECURITY_DYNAMIC_HOOKS
> +static void security_init_dynamic_hooks(void)
> +{
> + int i, err;
> +
> + for (i = 0; i < ARRAY_SIZE(dynamic_security_hook_heads); i++) {
> + INIT_LIST_HEAD(&dynamic_security_hook_heads[i]);
> + err = init_srcu_struct(&dynamic_hook_srcus[i]);
> + if (err)
> + panic("%s: Could not initialize SRCU - %d\n",
> + __func__, err);
> + }
> +}
> +#else
> +static inline void security_init_dynamic_hooks(void) {};
> +#endif
> +
> /**
> * security_init - initializes the security framework
> *
> @@ -66,6 +92,7 @@ int __init security_init(void)
>
> for (i = 0; i < ARRAY_SIZE(security_hook_heads); i++)
> INIT_LIST_HEAD(&security_hook_heads[i]);
> + security_init_dynamic_hooks();
> pr_info("Security Framework initialized\n");
>
> /*
> @@ -172,6 +199,64 @@ void __init security_add_hooks(struct security_hook_list *hooks, int count,
> panic("%s - Cannot get early memory.\n", __func__);
> }
>
> +#ifdef CONFIG_SECURITY_DYNAMIC_HOOKS
> +static int hook_allowed(enum lsm_hook hook_idx)
> +{
> + switch (hook_idx) {
> + case HOOK_IDX(inode_getsecurity):
> + case HOOK_IDX(inode_setsecurity):
> + case HOOK_IDX(xfrm_state_pol_flow_match):
> + return -EPERM;
> + default:
> + return 0;
> + }
> +}

Where did this list come from? Why doesn't it include
secid_to_secctx? Why does it include xfrm_stat_pol_flow_match?

> +/**
> + * security_add_dynamic_hooks:
> + * Register a dynamically loadable module's security hooks.
> + *
> + * @hooks: the hooks to add
> + * @count: the number of hooks to add
> + * @lsm: the name of the security module
> + *
> + * Returns:
> + * 0 if successful
> + * else errno, and no hooks will be installed
> + */
> +int security_add_dynamic_hooks(struct security_hook_list *hooks, int count,
> + char *lsm)
> +{
> + enum lsm_hook hook_idx;
> + int ret = 0, i;
> +
> + for (i = 0; i < count; i++) {
> + ret = hook_allowed(hooks[i].head_idx);
> + if (ret)
> + return ret;
> + }
> +
> + mutex_lock(&lsm_lock);
> + ret = lsm_append(lsm, &lsm_names);
> + if (ret)
> + goto out;
> +
> + for (i = 0; i < count; i++) {
> + WARN_ON(!try_module_get(hooks[i].owner));
> + hooks[i].lsm = lsm;
> + hook_idx = hooks[i].head_idx;
> + list_add_tail_rcu(&hooks[i].list,
> + &dynamic_security_hook_heads[hook_idx]);
> + static_branch_enable(&dynamic_hook_keys[hook_idx]);
> + }
> +
> +out:
> + mutex_unlock(&lsm_lock);
> +
> + return ret;
> +}
> +EXPORT_SYMBOL_GPL(security_add_dynamic_hooks);
> +#endif
> +
> int call_lsm_notifier(enum lsm_event event, void *data)
> {
> return atomic_notifier_call_chain(&lsm_notifier_chain, event, data);
> @@ -200,14 +285,69 @@ EXPORT_SYMBOL(unregister_lsm_notifier);
> * This is a hook that returns a value.
> */
>
> -#define call_void_hook(FUNC, ...) \
> +#define call_void_hook_builtin(FUNC, ...) do { \
> + struct security_hook_list *P; \
> + list_for_each_entry(P, HOOK_HEAD(FUNC), list) \
> + P->hook.FUNC(__VA_ARGS__); \
> +} while (0)
> +
> +#ifdef CONFIG_SECURITY_DYNAMIC_HOOKS
> +#define IS_DYNAMIC_HOOK_ENABLED(FUNC) \
> + (static_branch_unlikely(&dynamic_hook_keys[HOOK_IDX(FUNC)]))
> +
> +#define call_void_hook_dynamic(FUNC, ...) ({ \
> + struct security_hook_list *P; \
> + int idx; \
> + \
> + idx = srcu_read_lock(DYNAMIC_HOOK_SRCU(FUNC)); \
> + list_for_each_entry_rcu(P, \
> + DYNAMIC_HOOK_HEAD(FUNC), \
> + list) { \
> + P->hook.FUNC(__VA_ARGS__); \
> + } \
> + srcu_read_unlock(DYNAMIC_HOOK_SRCU(FUNC), idx); \

There has got to be a better way to do this than setting a lock
for every hook call.

> +})
> +
> +#define call_void_hook(FUNC, ...) \
> + do { \
> + call_void_hook_builtin(FUNC, __VA_ARGS__); \
> + if (!IS_DYNAMIC_HOOK_ENABLED(FUNC)) \
> + break; \

Why not let the list macros do their job?

> + call_void_hook_dynamic(FUNC, __VA_ARGS__); \
> + } while (0)
> +
> +#define call_int_hook(FUNC, IRC, ...) ({ \
> + bool continue_iteration = true; \
> + int RC = IRC, idx; \
> do { \
> struct security_hook_list *P; \
> \
> - list_for_each_entry(P, HOOK_HEAD(FUNC), list) \
> - P->hook.FUNC(__VA_ARGS__); \
> - } while (0)
> + list_for_each_entry(P, HOOK_HEAD(FUNC), list) { \
> + RC = P->hook.FUNC(__VA_ARGS__); \
> + if (RC != 0) { \
> + continue_iteration = false; \
> + break; \
> + } \
> + } \
> + if (!IS_DYNAMIC_HOOK_ENABLED(FUNC)) \
> + break; \
> + if (!continue_iteration) \
> + break; \
> + idx = srcu_read_lock(DYNAMIC_HOOK_SRCU(FUNC)); \
> + list_for_each_entry_rcu(P, \
> + DYNAMIC_HOOK_HEAD(FUNC), \
> + list) { \
> + RC = P->hook.FUNC(__VA_ARGS__); \
> + if (RC != 0) \
> + break; \
> + } \
> + srcu_read_unlock(DYNAMIC_HOOK_SRCU(FUNC), idx); \
> + } while (0); \
> + RC; \
> +})
>
> +#else
> +#define call_void_hook call_void_hook_builtin

I think this is hideous.

> #define call_int_hook(FUNC, IRC, ...) ({ \
> int RC = IRC; \
> do { \
> @@ -221,6 +361,7 @@ EXPORT_SYMBOL(unregister_lsm_notifier);
> } while (0); \
> RC; \
> })
> +#endif
>
> /* Security operations */
>
> @@ -312,6 +453,9 @@ int security_vm_enough_memory_mm(struct mm_struct *mm, long pages)
> struct security_hook_list *hp;
> int cap_sys_admin = 1;
> int rc;
> +#ifdef CONFIG_SECURITY_DYNAMIC_HOOKS
> + int idx;
> +#endif
>
> /*
> * The module will respond with a positive value if
> @@ -324,9 +468,25 @@ int security_vm_enough_memory_mm(struct mm_struct *mm, long pages)
> rc = hp->hook.vm_enough_memory(mm, pages);
> if (rc <= 0) {
> cap_sys_admin = 0;
> - break;
> + goto out;
> }
> }
> +
> +#ifdef CONFIG_SECURITY_DYNAMIC_HOOKS
> + if (!IS_DYNAMIC_HOOK_ENABLED(vm_enough_memory))
> + goto out;
> + idx = srcu_read_lock(DYNAMIC_HOOK_SRCU(vm_enough_memory));
> + list_for_each_entry_rcu(hp, DYNAMIC_HOOK_HEAD(vm_enough_memory),
> + list) {
> + rc = hp->hook.vm_enough_memory(mm, pages);
> + if (rc <= 0) {
> + cap_sys_admin = 0;
> + goto out;
> + }
> + }
> + srcu_read_unlock(DYNAMIC_HOOK_SRCU(vm_enough_memory), idx);
> +#endif
> +out:
> return __vm_enough_memory(mm, pages, cap_sys_admin);
> }
>
> @@ -1128,15 +1288,36 @@ int security_task_prctl(int option, unsigned long arg2, unsigned long arg3,
> int thisrc;
> int rc = -ENOSYS;
> struct security_hook_list *hp;
> +#ifdef CONFIG_SECURITY_DYNAMIC_HOOKS
> + int idx;
> +#endif
>
> list_for_each_entry(hp, HOOK_HEAD(task_prctl), list) {
> thisrc = hp->hook.task_prctl(option, arg2, arg3, arg4, arg5);
> if (thisrc != -ENOSYS) {
> rc = thisrc;
> if (thisrc != 0)
> - break;
> + goto out;
> }
> }
> +
> +#ifdef CONFIG_SECURITY_DYNAMIC_HOOKS
> + if (!IS_DYNAMIC_HOOK_ENABLED(task_prctl))
> + goto out;
> + idx = srcu_read_lock(DYNAMIC_HOOK_SRCU(task_prctl));
> + list_for_each_entry_rcu(hp, DYNAMIC_HOOK_HEAD(task_prctl),
> + list) {
> + thisrc = hp->hook.task_prctl(option, arg2, arg3, arg4, arg5);
> + if (thisrc != -ENOSYS) {
> + rc = thisrc;
> + if (thisrc != 0)
> + goto out_unlock;
> + }
> + }
> +out_unlock:
> + srcu_read_unlock(DYNAMIC_HOOK_SRCU(task_prctl), idx);
> +#endif
> +out:
> return rc;
> }
>
> @@ -1636,6 +1817,7 @@ int security_xfrm_state_pol_flow_match(struct xfrm_state *x,
> rc = hp->hook.xfrm_state_pol_flow_match(x, xp, fl);
> break;
> }
> +
> return rc;
> }
>

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