livepatch: Remove immediate feature

a missing memory barrier, or add some locking around a critical section.

Most of these changes are self contained and the function presents itself

the same way to the rest of the system. In this case, the functions might

be updated independently one by one.  (This can be done by setting the

'immediate' flag in the klp_patch struct.)

be updated independently one by one.

But there are more complex fixes. For example, a patch might change

ordering of locking in multiple functions at the same time. Or a patch

   b) Patching CPU-bound user tasks.  If the task is highly CPU-bound

      then it will get patched the next time it gets interrupted by an

      IRQ.

   c) In the future it could be useful for applying patches for

      architectures which don't yet have HAVE_RELIABLE_STACKTRACE.  In

      this case you would have to signal most of the tasks on the

      system.  However this isn't supported yet because there's

      currently no way to patch kthreads without

      HAVE_RELIABLE_STACKTRACE.

3. For idle "swapper" tasks, since they don't ever exit the kernel, they

   instead have a klp_update_patch_state() call in the idle loop which

   (Note there's not yet such an approach for kthreads.)

All the above approaches may be skipped by setting the 'immediate' flag

in the 'klp_patch' struct, which will disable per-task consistency and

patch all tasks immediately.  This can be useful if the patch doesn't

change any function or data semantics.  Note that, even with this flag

set, it's possible that some tasks may still be running with an old

version of the function, until that function returns.

Architectures which don't have HAVE_RELIABLE_STACKTRACE solely rely on

the second approach. It's highly likely that some tasks may still be

running with an old version of the function, until that function

returns. In this case you would have to signal the tasks. This

especially applies to kthreads. They may not be woken up and would need

to be forced. See below for more information.

There's also an 'immediate' flag in the 'klp_func' struct which allows

you to specify that certain functions in the patch can be applied

without per-task consistency.  This might be useful if you want to patch

a common function like schedule(), and the function change doesn't need

consistency but the rest of the patch does.

For architectures which don't have HAVE_RELIABLE_STACKTRACE, the user

must set patch->immediate which causes all tasks to be patched

immediately.  This option should be used with care, only when the patch

doesn't change any function or data semantics.

In the future, architectures which don't have HAVE_RELIABLE_STACKTRACE

may be allowed to use per-task consistency if we can come up with

another way to patch kthreads.

Unless we can come up with another way to patch kthreads, architectures

without HAVE_RELIABLE_STACKTRACE are not considered fully supported by

the kernel livepatching.

The /sys/kernel/livepatch/<patch>/transition file shows whether a patch

is in transition.  Only a single patch (the topmost patch on the stack)

guaranteed there is no task sleeping in such module. It implies unbounded

reference count if a patch module is disabled and enabled in a loop.

Moreover, the usage of force may also affect future applications of live

patches and cause even more harm to the system. Administrator should first

consider to simply cancel a transition (see above). If force is used, reboot

should be planned and no more live patches applied.

3.1 Adding consistency model support to new architectures

---------------------------------------------------------

   a good backup option for those architectures which don't have

   reliable stack traces yet.

In the meantime, patches for such architectures can bypass the

consistency model by setting klp_patch.immediate to true.  This option

is perfectly fine for patches which don't change the semantics of the

patched functions.  In practice, this is usable for ~90% of security

fixes.  Use of this option also means the patch can't be unloaded after

it has been disabled.

4. Livepatch module

===================

    only for a particular object ( vmlinux or a kernel module ). Note that

    kallsyms allows for searching symbols according to the object name.

    There's also an 'immediate' flag which, when set, patches the

    function immediately, bypassing the consistency model safety checks.

  + struct klp_object defines an array of patched functions (struct

    klp_func) in the same object. Where the object is either vmlinux

    (NULL) or a module name.

    symbols are found. The only exception are symbols from objects

    (kernel modules) that have not been loaded yet.

    Setting the 'immediate' flag applies the patch to all tasks

    immediately, bypassing the consistency model safety checks.

    For more details on how the patch is applied on a per-task basis,

    see the "Consistency model" section.

two operations.

Module removal is only safe when there are no users of the underlying

functions. The immediate consistency model is not able to detect this. The

code just redirects the functions at the very beginning and it does not

check if the functions are in use. In other words, it knows when the

functions get called but it does not know when the functions return.

Therefore it cannot be decided when the livepatch module can be safely

removed. This is solved by a hybrid consistency model. When the system is

transitioned to a new patch state (patched/unpatched) it is guaranteed that

no task sleeps or runs in the old code.

functions. This is the reason why the force feature permanently disables

the removal. The forced tasks entered the functions but we cannot say

that they returned back.  Therefore it cannot be decided when the

livepatch module can be safely removed. When the system is successfully

transitioned to a new patch state (patched/unpatched) without being

forced it is guaranteed that no task sleeps or runs in the old code.

5. Livepatch life-cycle

loaded objects are found. The error handling is much easier if this

check is done before particular functions get redirected.

Second, the immediate consistency model does not guarantee that anyone is not

sleeping in the new code after the patch is reverted. This means that the new

code needs to stay around "forever". If the code is there, one could apply it

again. Therefore it makes sense to separate the operations that might be done

once and those that need to be repeated when the patch is enabled (applied)

again.

Third, it might take some time until the entire system is migrated

when a more complex consistency model is used. The patch revert might

block the livepatch module removal for too long. Therefore it is useful

to revert the patch using a separate operation that might be called

explicitly. But it does not make sense to remove all information

until the livepatch module is really removed.

Second, it might take some time until the entire system is migrated with

the hybrid consistency model being used. The patch revert might block

the livepatch module removal for too long. Therefore it is useful to

revert the patch using a separate operation that might be called

explicitly. But it does not make sense to remove all information until

the livepatch module is really removed.

5.1. Registration

 * @new_func:	pointer to the patched function code

 * @old_sympos: a hint indicating which symbol position the old function

 *		can be found (optional)

 * @immediate:  patch the func immediately, bypassing safety mechanisms

 * @old_addr:	the address of the function being patched

 * @kobj:	kobject for sysfs resources

 * @stack_node:	list node for klp_ops func_stack list

	 * in kallsyms for the given object is used.

	 */

	unsigned long old_sympos;

	bool immediate;

	/* internal */

	unsigned long old_addr;

 * struct klp_patch - patch structure for live patching

 * @mod:	reference to the live patch module

 * @objs:	object entries for kernel objects to be patched

 * @immediate:  patch all funcs immediately, bypassing safety mechanisms

 * @list:	list node for global list of registered patches

 * @kobj:	kobject for sysfs resources

 * @enabled:	the patch is enabled (but operation may be incomplete)

	/* external */

	struct module *mod;

	struct klp_object *objs;

	bool immediate;

	/* internal */

	struct list_head list;

	/*

	 * A reference is taken on the patch module to prevent it from being

	 * unloaded.

	 *

	 * Note: For immediate (no consistency model) patches we don't allow

	 * patch modules to unload since there is no safe/sane method to

	 * determine if a thread is still running in the patched code contained

	 * in the patch module once the ftrace registration is successful.

	 */

	if (!try_module_get(patch->mod))

		return -ENODEV;

	if (!klp_initialized())

		return -ENODEV;

	/*

	 * Architectures without reliable stack traces have to set

	 * patch->immediate because there's currently no way to patch kthreads

	 * with the consistency model.

	 */

	if (!klp_have_reliable_stack() && !patch->immediate) {

	if (!klp_have_reliable_stack()) {

		pr_err("This architecture doesn't have support for the livepatch consistency model.\n");

		return -ENOSYS;

	}

	struct klp_func *func;

	struct task_struct *g, *task;

	unsigned int cpu;

	bool immediate_func = false;

	pr_debug("'%s': completing %s transition\n",

		 klp_transition_patch->mod->name,

		klp_synchronize_transition();

	}

	if (klp_transition_patch->immediate)

		goto done;

	klp_for_each_object(klp_transition_patch, obj) {

		klp_for_each_func(obj, func) {

	klp_for_each_object(klp_transition_patch, obj)

		klp_for_each_func(obj, func)

			func->transition = false;

			if (func->immediate)

				immediate_func = true;

		}

	}

	/* Prevent klp_ftrace_handler() from seeing KLP_UNDEFINED state */

	if (klp_target_state == KLP_PATCHED)

		task->patch_state = KLP_UNDEFINED;

	}

done:

	klp_for_each_object(klp_transition_patch, obj) {

		if (!klp_is_object_loaded(obj))

			continue;

		  klp_target_state == KLP_PATCHED ? "patching" : "unpatching");

	/*

	 * See complementary comment in __klp_enable_patch() for why we

	 * keep the module reference for immediate patches.

	 *

	 * klp_forced or immediate_func set implies unbounded increase of

	 * module's ref count if the module is disabled/enabled in a loop.

	 * klp_forced set implies unbounded increase of module's ref count if

	 * the module is disabled/enabled in a loop.

	 */

	if (!klp_forced && !klp_transition_patch->immediate &&

		!immediate_func && klp_target_state == KLP_UNPATCHED) {

	if (!klp_forced && klp_target_state == KLP_UNPATCHED)

		module_put(klp_transition_patch->mod);

	}

	klp_target_state = KLP_UNDEFINED;

	klp_transition_patch = NULL;

	struct klp_ops *ops;

	int i;

	if (func->immediate)

		return 0;

	for (i = 0; i < trace->nr_entries; i++) {

		address = trace->entries[i];

	WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED);

	/*

	 * If the patch can be applied or reverted immediately, skip the

	 * per-task transitions.

	 */

	if (klp_transition_patch->immediate)

		goto success;

	/*

	 * Try to switch the tasks to the target patch state by walking their

	 * stacks and looking for any to-be-patched or to-be-unpatched

		return;

	}

success:

	/* we're done, now cleanup the data structures */

	klp_complete_transition();

}

		  klp_transition_patch->mod->name,

		  klp_target_state == KLP_PATCHED ? "patching" : "unpatching");

	/*

	 * If the patch can be applied or reverted immediately, skip the

	 * per-task transitions.

	 */

	if (klp_transition_patch->immediate)

		return;

	/*

	 * Mark all normal tasks as needing a patch state update.  They'll

	 * switch either in klp_try_complete_transition() or as they exit the

	pr_debug("'%s': initializing %s transition\n", patch->mod->name,

		 klp_target_state == KLP_PATCHED ? "patching" : "unpatching");

	/*

	 * If the patch can be applied or reverted immediately, skip the

	 * per-task transitions.

	 */

	if (patch->immediate)

		return;

	/*

	 * Initialize all tasks to the initial patch state to prepare them for

	 * switching to the target state.

{

	int ret;

	if (!klp_have_reliable_stack() && !patch.immediate) {

		/*

		 * WARNING: Be very careful when using 'patch.immediate' in

		 * your patches.  It's ok to use it for simple patches like

		 * this, but for more complex patches which change function

		 * semantics, locking semantics, or data structures, it may not

		 * be safe.  Use of this option will also prevent removal of

		 * the patch.

		 *

		 * See Documentation/livepatch/livepatch.txt for more details.

		 */

		patch.immediate = true;

		pr_notice("The consistency model isn't supported for your architecture.  Bypassing safety mechanisms and applying the patch immediately.\n");

	}

	ret = klp_register_patch(&patch);

	if (ret)

		return ret;