// SPDX-License-Identifier: GPL-2.0
/*
 * Copyright (C) 2000 - 2007 Jeff Dike (jdike@{addtoit,linux.intel}.com)
 */

#include <linux/mm.h>
#include <linux/sched/signal.h>
#include <linux/hardirq.h>
#include <linux/module.h>
#include <linux/uaccess.h>
#include <linux/sched/debug.h>
#include <asm/current.h>
#include <asm/tlbflush.h>
#include <arch.h>
#include <as-layout.h>
#include <kern_util.h>
#include <os.h>
#include <skas.h>

/*
 * NOTE: UML does not have exception tables. As such, this is almost a copy
 * of the code in mm/memory.c, only adjusting the logic to simply check whether
 * we are coming from the kernel instead of doing an additional lookup in the
 * exception table.
 * We can do this simplification because we never get here if the exception was
 * fixable.
 */
static inline bool get_mmap_lock_carefully(struct mm_struct *mm, bool is_user)
{
        if (likely(mmap_read_trylock(mm)))
                return true;

        if (!is_user)
                return false;

        return !mmap_read_lock_killable(mm);
}

static inline bool mmap_upgrade_trylock(struct mm_struct *mm)
{
        /*
         * We don't have this operation yet.
         *
         * It should be easy enough to do: it's basically a
         *    atomic_long_try_cmpxchg_acquire()
         * from RWSEM_READER_BIAS -> RWSEM_WRITER_LOCKED, but
         * it also needs the proper lockdep magic etc.
         */
        return false;
}

static inline bool upgrade_mmap_lock_carefully(struct mm_struct *mm, bool is_user)
{
        mmap_read_unlock(mm);
        if (!is_user)
                return false;

        return !mmap_write_lock_killable(mm);
}

/*
 * Helper for page fault handling.
 *
 * This is kind of equivalend to "mmap_read_lock()" followed
 * by "find_extend_vma()", except it's a lot more careful about
 * the locking (and will drop the lock on failure).
 *
 * For example, if we have a kernel bug that causes a page
 * fault, we don't want to just use mmap_read_lock() to get
 * the mm lock, because that would deadlock if the bug were
 * to happen while we're holding the mm lock for writing.
 *
 * So this checks the exception tables on kernel faults in
 * order to only do this all for instructions that are actually
 * expected to fault.
 *
 * We can also actually take the mm lock for writing if we
 * need to extend the vma, which helps the VM layer a lot.
 */
static struct vm_area_struct *
um_lock_mm_and_find_vma(struct mm_struct *mm,
                        unsigned long addr, bool is_user)
{
        struct vm_area_struct *vma;

        if (!get_mmap_lock_carefully(mm, is_user))
                return NULL;

        vma = find_vma(mm, addr);
        if (likely(vma && (vma->vm_start <= addr)))
                return vma;

        /*
         * Well, dang. We might still be successful, but only
         * if we can extend a vma to do so.
         */
        if (!vma || !(vma->vm_flags & VM_GROWSDOWN)) {
                mmap_read_unlock(mm);
                return NULL;
        }

        /*
         * We can try to upgrade the mmap lock atomically,
         * in which case we can continue to use the vma
         * we already looked up.
         *
         * Otherwise we'll have to drop the mmap lock and
         * re-take it, and also look up the vma again,
         * re-checking it.
         */
        if (!mmap_upgrade_trylock(mm)) {
                if (!upgrade_mmap_lock_carefully(mm, is_user))
                        return NULL;

                vma = find_vma(mm, addr);
                if (!vma)
                        goto fail;
                if (vma->vm_start <= addr)
                        goto success;
                if (!(vma->vm_flags & VM_GROWSDOWN))
                        goto fail;
        }

        if (expand_stack_locked(vma, addr))
                goto fail;

success:
        mmap_write_downgrade(mm);
        return vma;

fail:
        mmap_write_unlock(mm);
        return NULL;
}

/*
 * Note this is constrained to return 0, -EFAULT, -EACCES, -ENOMEM by
 * segv().
 */
int handle_page_fault(unsigned long address, unsigned long ip,
                      int is_write, int is_user, int *code_out)
{
        struct mm_struct *mm = current->mm;
        struct vm_area_struct *vma;
        pmd_t *pmd;
        pte_t *pte;
        int err = -EFAULT;
        unsigned int flags = FAULT_FLAG_DEFAULT;

        *code_out = SEGV_MAPERR;

        /*
         * If the fault was with pagefaults disabled, don't take the fault, just
         * fail.
         */
        if (faulthandler_disabled())
                goto out_nosemaphore;

        if (is_user)
                flags |= FAULT_FLAG_USER;
retry:
        vma = um_lock_mm_and_find_vma(mm, address, is_user);
        if (!vma)
                goto out_nosemaphore;

        *code_out = SEGV_ACCERR;
        if (is_write) {
                if (!(vma->vm_flags & VM_WRITE))
                        goto out;
                flags |= FAULT_FLAG_WRITE;
        } else {
                /* Don't require VM_READ|VM_EXEC for write faults! */
                if (!(vma->vm_flags & (VM_READ | VM_EXEC)))
                        goto out;
        }

        do {
                vm_fault_t fault;

                fault = handle_mm_fault(vma, address, flags, NULL);

                if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
                        goto out_nosemaphore;

                /* The fault is fully completed (including releasing mmap lock) */
                if (fault & VM_FAULT_COMPLETED)
                        return 0;

                if (unlikely(fault & VM_FAULT_ERROR)) {
                        if (fault & VM_FAULT_OOM) {
                                goto out_of_memory;
                        } else if (fault & VM_FAULT_SIGSEGV) {
                                goto out;
                        } else if (fault & VM_FAULT_SIGBUS) {
                                err = -EACCES;
                                goto out;
                        }
                        BUG();
                }
                if (fault & VM_FAULT_RETRY) {
                        flags |= FAULT_FLAG_TRIED;

                        goto retry;
                }

                pmd = pmd_off(mm, address);
                pte = pte_offset_kernel(pmd, address);
        } while (!pte_present(*pte));
        err = 0;
        /*
         * The below warning was added in place of
         *      pte_mkyoung(); if (is_write) pte_mkdirty();
         * If it's triggered, we'd see normally a hang here (a clean pte is
         * marked read-only to emulate the dirty bit).
         * However, the generic code can mark a PTE writable but clean on a
         * concurrent read fault, triggering this harmlessly. So comment it out.
         */
#if 0
        WARN_ON(!pte_young(*pte) || (is_write && !pte_dirty(*pte)));
#endif

out:
        mmap_read_unlock(mm);
out_nosemaphore:
        return err;

out_of_memory:
        /*
         * We ran out of memory, call the OOM killer, and return the userspace
         * (which will retry the fault, or kill us if we got oom-killed).
         */
        mmap_read_unlock(mm);
        if (!is_user)
                goto out_nosemaphore;
        pagefault_out_of_memory();
        return 0;
}

static void show_segv_info(struct uml_pt_regs *regs)
{
        struct task_struct *tsk = current;
        struct faultinfo *fi = UPT_FAULTINFO(regs);

        if (!unhandled_signal(tsk, SIGSEGV))
                return;

        if (!printk_ratelimit())
                return;

        printk("%s%s[%d]: segfault at %lx ip %px sp %px error %x",
                task_pid_nr(tsk) > 1 ? KERN_INFO : KERN_EMERG,
                tsk->comm, task_pid_nr(tsk), FAULT_ADDRESS(*fi),
                (void *)UPT_IP(regs), (void *)UPT_SP(regs),
                fi->error_code);

        print_vma_addr(KERN_CONT " in ", UPT_IP(regs));
        printk(KERN_CONT "\n");
}

static void bad_segv(struct faultinfo fi, unsigned long ip)
{
        current->thread.arch.faultinfo = fi;
        force_sig_fault(SIGSEGV, SEGV_ACCERR, (void __user *) FAULT_ADDRESS(fi));
}

void fatal_sigsegv(void)
{
        force_fatal_sig(SIGSEGV);
        do_signal(&current->thread.regs);
        /*
         * This is to tell gcc that we're not returning - do_signal
         * can, in general, return, but in this case, it's not, since
         * we just got a fatal SIGSEGV queued.
         */
        os_dump_core();
}

/**
 * segv_handler() - the SIGSEGV handler
 * @sig:        the signal number
 * @unused_si:  the signal info struct; unused in this handler
 * @regs:       the ptrace register information
 * @mc:         the mcontext of the signal
 *
 * The handler first extracts the faultinfo from the UML ptrace regs struct.
 * If the userfault did not happen in an UML userspace process, bad_segv is called.
 * Otherwise the signal did happen in a cloned userspace process, handle it.
 */
void segv_handler(int sig, struct siginfo *unused_si, struct uml_pt_regs *regs,
                  void *mc)
{
        struct faultinfo * fi = UPT_FAULTINFO(regs);

        if (UPT_IS_USER(regs) && !SEGV_IS_FIXABLE(fi)) {
                show_segv_info(regs);
                bad_segv(*fi, UPT_IP(regs));
                return;
        }
        segv(*fi, UPT_IP(regs), UPT_IS_USER(regs), regs, mc);
}

/*
 * We give a *copy* of the faultinfo in the regs to segv.
 * This must be done, since nesting SEGVs could overwrite
 * the info in the regs. A pointer to the info then would
 * give us bad data!
 */
unsigned long segv(struct faultinfo fi, unsigned long ip, int is_user,
                   struct uml_pt_regs *regs, void *mc)
{
        int si_code;
        int err;
        int is_write = FAULT_WRITE(fi);
        unsigned long address = FAULT_ADDRESS(fi);

        if (!is_user && regs)
                current->thread.segv_regs = container_of(regs, struct pt_regs, regs);

        if (!is_user && address >= start_vm && address < end_vm) {
                /*
                 * Kernel has pending updates from set_ptes that were not
                 * flushed yet. Syncing them should fix the pagefault (if not
                 * we'll get here again and panic).
                 */
                err = um_tlb_sync(&init_mm);
                if (err == -ENOMEM)
                        report_enomem();
                if (err)
                        panic("Failed to sync kernel TLBs: %d", err);
                goto out;
        }
        else if (current->pagefault_disabled) {
                if (!mc) {
                        show_regs(container_of(regs, struct pt_regs, regs));
                        panic("Segfault with pagefaults disabled but no mcontext");
                }
                if (!current->thread.segv_continue) {
                        show_regs(container_of(regs, struct pt_regs, regs));
                        panic("Segfault without recovery target");
                }
                mc_set_rip(mc, current->thread.segv_continue);
                current->thread.segv_continue = NULL;
                goto out;
        }
        else if (current->mm == NULL) {
                show_regs(container_of(regs, struct pt_regs, regs));
                panic("Segfault with no mm");
        }
        else if (!is_user && address > PAGE_SIZE && address < TASK_SIZE) {
                show_regs(container_of(regs, struct pt_regs, regs));
                panic("Kernel tried to access user memory at addr 0x%lx, ip 0x%lx",
                       address, ip);
        }

        if (SEGV_IS_FIXABLE(&fi))
                err = handle_page_fault(address, ip, is_write, is_user,
                                        &si_code);
        else {
                err = -EFAULT;
                /*
                 * A thread accessed NULL, we get a fault, but CR2 is invalid.
                 * This code is used in __do_copy_from_user() of TT mode.
                 * XXX tt mode is gone, so maybe this isn't needed any more
                 */
                address = 0;
        }

        if (!err)
                goto out;
        else if (!is_user && arch_fixup(ip, regs))
                goto out;

        if (!is_user) {
                show_regs(container_of(regs, struct pt_regs, regs));
                panic("Kernel mode fault at addr 0x%lx, ip 0x%lx",
                      address, ip);
        }

        show_segv_info(regs);

        if (err == -EACCES) {
                current->thread.arch.faultinfo = fi;
                force_sig_fault(SIGBUS, BUS_ADRERR, (void __user *)address);
        } else {
                BUG_ON(err != -EFAULT);
                current->thread.arch.faultinfo = fi;
                force_sig_fault(SIGSEGV, si_code, (void __user *) address);
        }

out:
        if (regs)
                current->thread.segv_regs = NULL;

        return 0;
}

void relay_signal(int sig, struct siginfo *si, struct uml_pt_regs *regs,
                  void *mc)
{
        int code, err;
        if (!UPT_IS_USER(regs)) {
                if (sig == SIGBUS)
                        printk(KERN_ERR "Bus error - the host /dev/shm or /tmp "
                               "mount likely just ran out of space\n");
                panic("Kernel mode signal %d", sig);
        }

        arch_examine_signal(sig, regs);

        /* Is the signal layout for the signal known?
         * Signal data must be scrubbed to prevent information leaks.
         */
        code = si->si_code;
        err = si->si_errno;
        if ((err == 0) && (siginfo_layout(sig, code) == SIL_FAULT)) {
                struct faultinfo *fi = UPT_FAULTINFO(regs);
                current->thread.arch.faultinfo = *fi;
                force_sig_fault(sig, code, (void __user *)FAULT_ADDRESS(*fi));
        } else {
                printk(KERN_ERR "Attempted to relay unknown signal %d (si_code = %d) with errno %d\n",
                       sig, code, err);
                force_sig(sig);
        }
}

void winch(int sig, struct siginfo *unused_si, struct uml_pt_regs *regs,
           void *mc)
{
        do_IRQ(WINCH_IRQ, regs);
}