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SubjectRe: [PATCH v4 2/3] security: Expose a mechanism to load lsm hooks dynamically at runtime
On Wed, Mar 7, 2018 at 9:59 AM, Casey Schaufler <casey@schaufler-ca.com> wrote:
> 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.
>
We can change this. Any suggestion?

>> 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?
>
Because xfrm_stat_pol_flow_match is very perf-sensitive, and it's not
designed to work with multiple LSMs (as the comments say), similarly
with inode_getsecurity / inode_setsecurity. We can block other hooks,
but these were the ones in the code that had explicit comments.

>> +/**
>> + * 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.
>
This isn't a "lock" -- it's rcu, so it should just do a ptr
dereference and per cpu counter inc / dec.

>> +})
>> +
>> +#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?
>
What do you mean? In order to avoid any perf overhead, this uses
static keys -- we have to check those.

>> + 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.
>
I can split this up.

>> #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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