// SPDX-License-Identifier: GPL-2.0-or-later
/*
 * transition.c - Kernel Live Patching transition functions
 *
 * Copyright (C) 2015-2016 Josh Poimboeuf <jpoimboe@redhat.com>
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/cpu.h>
#include <linux/stacktrace.h>
#include <linux/static_call.h>
#include "core.h"
#include "patch.h"
#include "transition.h"

#define MAX_STACK_ENTRIES  100
static DEFINE_PER_CPU(unsigned long[MAX_STACK_ENTRIES], klp_stack_entries);

#define STACK_ERR_BUF_SIZE 128

#define SIGNALS_TIMEOUT 15

struct klp_patch *klp_transition_patch;

static int klp_target_state = KLP_TRANSITION_IDLE;

static unsigned int klp_signals_cnt;

/*
 * When a livepatch is in progress, enable klp stack checking in
 * schedule().  This helps CPU-bound kthreads get patched.
 */

DEFINE_STATIC_KEY_FALSE(klp_sched_try_switch_key);

#define klp_resched_enable() static_branch_enable(&klp_sched_try_switch_key)
#define klp_resched_disable() static_branch_disable(&klp_sched_try_switch_key)

/*
 * This work can be performed periodically to finish patching or unpatching any
 * "straggler" tasks which failed to transition in the first attempt.
 */
static void klp_transition_work_fn(struct work_struct *work)
{
        mutex_lock(&klp_mutex);

        if (klp_transition_patch)
                klp_try_complete_transition();

        mutex_unlock(&klp_mutex);
}
static DECLARE_DELAYED_WORK(klp_transition_work, klp_transition_work_fn);

/*
 * This function is just a stub to implement a hard force
 * of synchronize_rcu(). This requires synchronizing
 * tasks even in userspace and idle.
 */
static void klp_sync(struct work_struct *work)
{
}

/*
 * We allow to patch also functions where RCU is not watching,
 * e.g. before user_exit(). We can not rely on the RCU infrastructure
 * to do the synchronization. Instead hard force the sched synchronization.
 *
 * This approach allows to use RCU functions for manipulating func_stack
 * safely.
 */
static void klp_synchronize_transition(void)
{
        schedule_on_each_cpu(klp_sync);
}

/*
 * The transition to the target patch state is complete.  Clean up the data
 * structures.
 */
static void klp_complete_transition(void)
{
        struct klp_object *obj;
        struct klp_func *func;
        struct task_struct *g, *task;
        unsigned int cpu;

        pr_debug("'%s': completing %s transition\n",
                 klp_transition_patch->mod->name,
                 klp_target_state == KLP_TRANSITION_PATCHED ? "patching" : "unpatching");

        if (klp_transition_patch->replace && klp_target_state == KLP_TRANSITION_PATCHED) {
                klp_unpatch_replaced_patches(klp_transition_patch);
                klp_discard_nops(klp_transition_patch);
        }

        if (klp_target_state == KLP_TRANSITION_UNPATCHED) {
                /*
                 * All tasks have transitioned to KLP_TRANSITION_UNPATCHED so we can now
                 * remove the new functions from the func_stack.
                 */
                klp_unpatch_objects(klp_transition_patch);

                /*
                 * Make sure klp_ftrace_handler() can no longer see functions
                 * from this patch on the ops->func_stack.  Otherwise, after
                 * func->transition gets cleared, the handler may choose a
                 * removed function.
                 */
                klp_synchronize_transition();
        }

        klp_for_each_object(klp_transition_patch, obj)
                klp_for_each_func(obj, func)
                        func->transition = false;

        /* Prevent klp_ftrace_handler() from seeing KLP_TRANSITION_IDLE state */
        if (klp_target_state == KLP_TRANSITION_PATCHED)
                klp_synchronize_transition();

        read_lock(&tasklist_lock);
        for_each_process_thread(g, task) {
                WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING));
                task->patch_state = KLP_TRANSITION_IDLE;
        }
        read_unlock(&tasklist_lock);

        for_each_possible_cpu(cpu) {
                task = idle_task(cpu);
                WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING));
                task->patch_state = KLP_TRANSITION_IDLE;
        }

        klp_for_each_object(klp_transition_patch, obj) {
                if (!klp_is_object_loaded(obj))
                        continue;
                if (klp_target_state == KLP_TRANSITION_PATCHED)
                        klp_post_patch_callback(obj);
                else if (klp_target_state == KLP_TRANSITION_UNPATCHED)
                        klp_post_unpatch_callback(obj);
        }

        pr_notice("'%s': %s complete\n", klp_transition_patch->mod->name,
                  klp_target_state == KLP_TRANSITION_PATCHED ? "patching" : "unpatching");

        klp_target_state = KLP_TRANSITION_IDLE;
        klp_transition_patch = NULL;
}

/*
 * This is called in the error path, to cancel a transition before it has
 * started, i.e. klp_init_transition() has been called but
 * klp_start_transition() hasn't.  If the transition *has* been started,
 * klp_reverse_transition() should be used instead.
 */
void klp_cancel_transition(void)
{
        if (WARN_ON_ONCE(klp_target_state != KLP_TRANSITION_PATCHED))
                return;

        pr_debug("'%s': canceling patching transition, going to unpatch\n",
                 klp_transition_patch->mod->name);

        klp_target_state = KLP_TRANSITION_UNPATCHED;
        klp_complete_transition();
}

/*
 * Switch the patched state of the task to the set of functions in the target
 * patch state.
 *
 * NOTE: If task is not 'current', the caller must ensure the task is inactive.
 * Otherwise klp_ftrace_handler() might read the wrong 'patch_state' value.
 */
void klp_update_patch_state(struct task_struct *task)
{
        /*
         * A variant of synchronize_rcu() is used to allow patching functions
         * where RCU is not watching, see klp_synchronize_transition().
         */
        preempt_disable_notrace();

        /*
         * This test_and_clear_tsk_thread_flag() call also serves as a read
         * barrier (smp_rmb) for two cases:
         *
         * 1) Enforce the order of the TIF_PATCH_PENDING read and the
         *    klp_target_state read.  The corresponding write barriers are in
         *    klp_init_transition() and klp_reverse_transition().
         *
         * 2) Enforce the order of the TIF_PATCH_PENDING read and a future read
         *    of func->transition, if klp_ftrace_handler() is called later on
         *    the same CPU.  See __klp_disable_patch().
         */
        if (test_and_clear_tsk_thread_flag(task, TIF_PATCH_PENDING))
                task->patch_state = READ_ONCE(klp_target_state);

        preempt_enable_notrace();
}

/*
 * Determine whether the given stack trace includes any references to a
 * to-be-patched or to-be-unpatched function.
 */
static int klp_check_stack_func(struct klp_func *func, unsigned long *entries,
                                unsigned int nr_entries)
{
        unsigned long func_addr, func_size, address;
        struct klp_ops *ops;
        int i;

        if (klp_target_state == KLP_TRANSITION_UNPATCHED) {
                 /*
                  * Check for the to-be-unpatched function
                  * (the func itself).
                  */
                func_addr = (unsigned long)func->new_func;
                func_size = func->new_size;
        } else {
                /*
                 * Check for the to-be-patched function
                 * (the previous func).
                 */
                ops = klp_find_ops(func->old_func);

                if (list_is_singular(&ops->func_stack)) {
                        /* original function */
                        func_addr = (unsigned long)func->old_func;
                        func_size = func->old_size;
                } else {
                        /* previously patched function */
                        struct klp_func *prev;

                        prev = list_next_entry(func, stack_node);
                        func_addr = (unsigned long)prev->new_func;
                        func_size = prev->new_size;
                }
        }

        for (i = 0; i < nr_entries; i++) {
                address = entries[i];

                if (address >= func_addr && address < func_addr + func_size)
                        return -EAGAIN;
        }

        return 0;
}

/*
 * Determine whether it's safe to transition the task to the target patch state
 * by looking for any to-be-patched or to-be-unpatched functions on its stack.
 */
static int klp_check_stack(struct task_struct *task, const char **oldname)
{
        unsigned long *entries = this_cpu_ptr(klp_stack_entries);
        struct klp_object *obj;
        struct klp_func *func;
        int ret, nr_entries;

        /* Protect 'klp_stack_entries' */
        lockdep_assert_preemption_disabled();

        ret = stack_trace_save_tsk_reliable(task, entries, MAX_STACK_ENTRIES);
        if (ret < 0)
                return -EINVAL;
        nr_entries = ret;

        klp_for_each_object(klp_transition_patch, obj) {
                if (!obj->patched)
                        continue;
                klp_for_each_func(obj, func) {
                        ret = klp_check_stack_func(func, entries, nr_entries);
                        if (ret) {
                                *oldname = func->old_name;
                                return -EADDRINUSE;
                        }
                }
        }

        return 0;
}

static int klp_check_and_switch_task(struct task_struct *task, void *arg)
{
        int ret;

        if (task_curr(task) && task != current)
                return -EBUSY;

        ret = klp_check_stack(task, arg);
        if (ret)
                return ret;

        clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
        task->patch_state = klp_target_state;
        return 0;
}

/*
 * Try to safely switch a task to the target patch state.  If it's currently
 * running, or it's sleeping on a to-be-patched or to-be-unpatched function, or
 * if the stack is unreliable, return false.
 */
static bool klp_try_switch_task(struct task_struct *task)
{
        const char *old_name;
        int ret;

        /* check if this task has already switched over */
        if (task->patch_state == klp_target_state)
                return true;

        /*
         * For arches which don't have reliable stack traces, we have to rely
         * on other methods (e.g., switching tasks at kernel exit).
         */
        if (!klp_have_reliable_stack())
                return false;

        /*
         * Now try to check the stack for any to-be-patched or to-be-unpatched
         * functions.  If all goes well, switch the task to the target patch
         * state.
         */
        if (task == current)
                ret = klp_check_and_switch_task(current, &old_name);
        else
                ret = task_call_func(task, klp_check_and_switch_task, &old_name);

        switch (ret) {
        case 0:         /* success */
                break;

        case -EBUSY:    /* klp_check_and_switch_task() */
                pr_debug("%s: %s:%d is running\n",
                         __func__, task->comm, task->pid);
                break;
        case -EINVAL:   /* klp_check_and_switch_task() */
                pr_debug("%s: %s:%d has an unreliable stack\n",
                         __func__, task->comm, task->pid);
                break;
        case -EADDRINUSE: /* klp_check_and_switch_task() */
                pr_debug("%s: %s:%d is sleeping on function %s\n",
                         __func__, task->comm, task->pid, old_name);
                break;

        default:
                pr_debug("%s: Unknown error code (%d) when trying to switch %s:%d\n",
                         __func__, ret, task->comm, task->pid);
                break;
        }

        return !ret;
}

void __klp_sched_try_switch(void)
{
        /*
         * This function is called from __schedule() while a context switch is
         * about to happen. Preemption is already disabled and klp_mutex
         * can't be acquired.
         * Disabled preemption is used to prevent racing with other callers of
         * klp_try_switch_task(). Thanks to task_call_func() they won't be
         * able to switch to this task while it's running.
         */
        lockdep_assert_preemption_disabled();

        if (likely(!klp_patch_pending(current)))
                return;

        /*
         * Enforce the order of the TIF_PATCH_PENDING read above and the
         * klp_target_state read in klp_try_switch_task().  The corresponding
         * write barriers are in klp_init_transition() and
         * klp_reverse_transition().
         */
        smp_rmb();

        klp_try_switch_task(current);
}

/*
 * Sends a fake signal to all non-kthread tasks with TIF_PATCH_PENDING set.
 * Kthreads with TIF_PATCH_PENDING set are woken up.
 */
static void klp_send_signals(void)
{
        struct task_struct *g, *task;

        if (klp_signals_cnt == SIGNALS_TIMEOUT)
                pr_notice("signaling remaining tasks\n");

        read_lock(&tasklist_lock);
        for_each_process_thread(g, task) {
                if (!klp_patch_pending(task))
                        continue;

                /*
                 * There is a small race here. We could see TIF_PATCH_PENDING
                 * set and decide to wake up a kthread or send a fake signal.
                 * Meanwhile the task could migrate itself and the action
                 * would be meaningless. It is not serious though.
                 */
                if (task->flags & PF_KTHREAD) {
                        /*
                         * Wake up a kthread which sleeps interruptedly and
                         * still has not been migrated.
                         */
                        wake_up_state(task, TASK_INTERRUPTIBLE);
                } else {
                        /*
                         * Send fake signal to all non-kthread tasks which are
                         * still not migrated.
                         */
                        set_notify_signal(task);
                }
        }
        read_unlock(&tasklist_lock);
}

/*
 * Try to switch all remaining tasks to the target patch state by walking the
 * stacks of sleeping tasks and looking for any to-be-patched or
 * to-be-unpatched functions.  If such functions are found, the task can't be
 * switched yet.
 *
 * If any tasks are still stuck in the initial patch state, schedule a retry.
 */
void klp_try_complete_transition(void)
{
        unsigned int cpu;
        struct task_struct *g, *task;
        struct klp_patch *patch;
        bool complete = true;

        WARN_ON_ONCE(klp_target_state == KLP_TRANSITION_IDLE);

        /*
         * 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
         * functions.  If such functions are found on a stack, or if the stack
         * is deemed unreliable, the task can't be switched yet.
         *
         * Usually this will transition most (or all) of the tasks on a system
         * unless the patch includes changes to a very common function.
         */
        read_lock(&tasklist_lock);
        for_each_process_thread(g, task)
                if (!klp_try_switch_task(task))
                        complete = false;
        read_unlock(&tasklist_lock);

        /*
         * Ditto for the idle "swapper" tasks.
         */
        cpus_read_lock();
        for_each_possible_cpu(cpu) {
                task = idle_task(cpu);
                if (cpu_online(cpu)) {
                        if (!klp_try_switch_task(task)) {
                                complete = false;
                                /* Make idle task go through the main loop. */
                                wake_up_if_idle(cpu);
                        }
                } else if (task->patch_state != klp_target_state) {
                        /* offline idle tasks can be switched immediately */
                        clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
                        task->patch_state = klp_target_state;
                }
        }
        cpus_read_unlock();

        if (!complete) {
                if (klp_signals_cnt && !(klp_signals_cnt % SIGNALS_TIMEOUT))
                        klp_send_signals();
                klp_signals_cnt++;

                /*
                 * Some tasks weren't able to be switched over.  Try again
                 * later and/or wait for other methods like kernel exit
                 * switching.
                 */
                schedule_delayed_work(&klp_transition_work,
                                      round_jiffies_relative(HZ));
                return;
        }

        /* Done!  Now cleanup the data structures. */
        klp_resched_disable();
        patch = klp_transition_patch;
        klp_complete_transition();

        /*
         * It would make more sense to free the unused patches in
         * klp_complete_transition() but it is called also
         * from klp_cancel_transition().
         */
        if (!patch->enabled)
                klp_free_patch_async(patch);
        else if (patch->replace)
                klp_free_replaced_patches_async(patch);
}

/*
 * Start the transition to the specified target patch state so tasks can begin
 * switching to it.
 */
void klp_start_transition(void)
{
        struct task_struct *g, *task;
        unsigned int cpu;

        WARN_ON_ONCE(klp_target_state == KLP_TRANSITION_IDLE);

        pr_notice("'%s': starting %s transition\n",
                  klp_transition_patch->mod->name,
                  klp_target_state == KLP_TRANSITION_PATCHED ? "patching" : "unpatching");

        /*
         * Mark all normal tasks as needing a patch state update.  They'll
         * switch either in klp_try_complete_transition() or as they exit the
         * kernel.
         */
        read_lock(&tasklist_lock);
        for_each_process_thread(g, task)
                if (task->patch_state != klp_target_state)
                        set_tsk_thread_flag(task, TIF_PATCH_PENDING);
        read_unlock(&tasklist_lock);

        /*
         * Mark all idle tasks as needing a patch state update.  They'll switch
         * either in klp_try_complete_transition() or at the idle loop switch
         * point.
         */
        for_each_possible_cpu(cpu) {
                task = idle_task(cpu);
                if (task->patch_state != klp_target_state)
                        set_tsk_thread_flag(task, TIF_PATCH_PENDING);
        }

        klp_resched_enable();

        klp_signals_cnt = 0;
}

/*
 * Initialize the global target patch state and all tasks to the initial patch
 * state, and initialize all function transition states to true in preparation
 * for patching or unpatching.
 */
void klp_init_transition(struct klp_patch *patch, int state)
{
        struct task_struct *g, *task;
        unsigned int cpu;
        struct klp_object *obj;
        struct klp_func *func;
        int initial_state = !state;

        WARN_ON_ONCE(klp_target_state != KLP_TRANSITION_IDLE);

        klp_transition_patch = patch;

        /*
         * Set the global target patch state which tasks will switch to.  This
         * has no effect until the TIF_PATCH_PENDING flags get set later.
         */
        klp_target_state = state;

        pr_debug("'%s': initializing %s transition\n", patch->mod->name,
                 klp_target_state == KLP_TRANSITION_PATCHED ? "patching" : "unpatching");

        /*
         * Initialize all tasks to the initial patch state to prepare them for
         * switching to the target state.
         */
        read_lock(&tasklist_lock);
        for_each_process_thread(g, task) {
                WARN_ON_ONCE(task->patch_state != KLP_TRANSITION_IDLE);
                task->patch_state = initial_state;
        }
        read_unlock(&tasklist_lock);

        /*
         * Ditto for the idle "swapper" tasks.
         */
        for_each_possible_cpu(cpu) {
                task = idle_task(cpu);
                WARN_ON_ONCE(task->patch_state != KLP_TRANSITION_IDLE);
                task->patch_state = initial_state;
        }

        /*
         * Enforce the order of the task->patch_state initializations and the
         * func->transition updates to ensure that klp_ftrace_handler() doesn't
         * see a func in transition with a task->patch_state of KLP_TRANSITION_IDLE.
         *
         * Also enforce the order of the klp_target_state write and future
         * TIF_PATCH_PENDING writes to ensure klp_update_patch_state() and
         * __klp_sched_try_switch() don't set a task->patch_state to
         * KLP_TRANSITION_IDLE.
         */
        smp_wmb();

        /*
         * Set the func transition states so klp_ftrace_handler() will know to
         * switch to the transition logic.
         *
         * When patching, the funcs aren't yet in the func_stack and will be
         * made visible to the ftrace handler shortly by the calls to
         * klp_patch_object().
         *
         * When unpatching, the funcs are already in the func_stack and so are
         * already visible to the ftrace handler.
         */
        klp_for_each_object(patch, obj)
                klp_for_each_func(obj, func)
                        func->transition = true;
}

/*
 * This function can be called in the middle of an existing transition to
 * reverse the direction of the target patch state.  This can be done to
 * effectively cancel an existing enable or disable operation if there are any
 * tasks which are stuck in the initial patch state.
 */
void klp_reverse_transition(void)
{
        unsigned int cpu;
        struct task_struct *g, *task;

        pr_debug("'%s': reversing transition from %s\n",
                 klp_transition_patch->mod->name,
                 klp_target_state == KLP_TRANSITION_PATCHED ? "patching to unpatching" :
                                                   "unpatching to patching");

        /*
         * Clear all TIF_PATCH_PENDING flags to prevent races caused by
         * klp_update_patch_state() or __klp_sched_try_switch() running in
         * parallel with the reverse transition.
         */
        read_lock(&tasklist_lock);
        for_each_process_thread(g, task)
                clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
        read_unlock(&tasklist_lock);

        for_each_possible_cpu(cpu)
                clear_tsk_thread_flag(idle_task(cpu), TIF_PATCH_PENDING);

        /*
         * Make sure all existing invocations of klp_update_patch_state() and
         * __klp_sched_try_switch() see the cleared TIF_PATCH_PENDING before
         * starting the reverse transition.
         */
        klp_synchronize_transition();

        /*
         * All patching has stopped, now re-initialize the global variables to
         * prepare for the reverse transition.
         */
        klp_transition_patch->enabled = !klp_transition_patch->enabled;
        klp_target_state = !klp_target_state;

        /*
         * Enforce the order of the klp_target_state write and the
         * TIF_PATCH_PENDING writes in klp_start_transition() to ensure
         * klp_update_patch_state() and __klp_sched_try_switch() don't set
         * task->patch_state to the wrong value.
         */
        smp_wmb();

        klp_start_transition();
}

/* Called from copy_process() during fork */
void klp_copy_process(struct task_struct *child)
{

        /*
         * The parent process may have gone through a KLP transition since
         * the thread flag was copied in setup_thread_stack earlier. Bring
         * the task flag up to date with the parent here.
         *
         * The operation is serialized against all klp_*_transition()
         * operations by the tasklist_lock. The only exceptions are
         * klp_update_patch_state(current) and __klp_sched_try_switch(), but we
         * cannot race with them because we are current.
         */
        if (test_tsk_thread_flag(current, TIF_PATCH_PENDING))
                set_tsk_thread_flag(child, TIF_PATCH_PENDING);
        else
                clear_tsk_thread_flag(child, TIF_PATCH_PENDING);

        child->patch_state = current->patch_state;
}

/*
 * Drop TIF_PATCH_PENDING of all tasks on admin's request. This forces an
 * existing transition to finish.
 *
 * NOTE: klp_update_patch_state(task) requires the task to be inactive or
 * 'current'. This is not the case here and the consistency model could be
 * broken. Administrator, who is the only one to execute the
 * klp_force_transitions(), has to be aware of this.
 */
void klp_force_transition(void)
{
        struct klp_patch *patch;
        struct task_struct *g, *task;
        unsigned int cpu;

        pr_warn("forcing remaining tasks to the patched state\n");

        read_lock(&tasklist_lock);
        for_each_process_thread(g, task)
                klp_update_patch_state(task);
        read_unlock(&tasklist_lock);

        for_each_possible_cpu(cpu)
                klp_update_patch_state(idle_task(cpu));

        /* Set forced flag for patches being removed. */
        if (klp_target_state == KLP_TRANSITION_UNPATCHED)
                klp_transition_patch->forced = true;
        else if (klp_transition_patch->replace) {
                klp_for_each_patch(patch) {
                        if (patch != klp_transition_patch)
                                patch->forced = true;
                }
        }
}