/*
 * Copyright (c) 2012-2014 Andy Lutomirski <luto@amacapital.net>
 *
 * Based on the original implementation which is:
 *  Copyright (C) 2001 Andrea Arcangeli <andrea@suse.de> SuSE
 *  Copyright 2003 Andi Kleen, SuSE Labs.
 *
 *  Parts of the original code have been moved to arch/x86/vdso/vma.c
 *
 * This file implements vsyscall emulation.  vsyscalls are a legacy ABI:
 * Userspace can request certain kernel services by calling fixed
 * addresses.  This concept is problematic:
 *
 * - It interferes with ASLR.
 * - It's awkward to write code that lives in kernel addresses but is
 *   callable by userspace at fixed addresses.
 * - The whole concept is impossible for 32-bit compat userspace.
 * - UML cannot easily virtualize a vsyscall.
 *
 * As of mid-2014, I believe that there is no new userspace code that
 * will use a vsyscall if the vDSO is present.  I hope that there will
 * soon be no new userspace code that will ever use a vsyscall.
 *
 * The code in this file emulates vsyscalls when notified of a page
 * fault to a vsyscall address.
 */

#include <linux/kernel.h>
#include <linux/timer.h>
#include <linux/sched/signal.h>
#include <linux/mm_types.h>
#include <linux/syscalls.h>
#include <linux/ratelimit.h>

#include <asm/vsyscall.h>
#include <asm/unistd.h>
#include <asm/fixmap.h>
#include <asm/traps.h>

#define CREATE_TRACE_POINTS
#include "vsyscall_trace.h"

static enum { EMULATE, NATIVE, NONE } vsyscall_mode =
#if defined(CONFIG_LEGACY_VSYSCALL_NATIVE)
        NATIVE;
#elif defined(CONFIG_LEGACY_VSYSCALL_NONE)
        NONE;
#else
        EMULATE;
#endif

static int __init vsyscall_setup(char *str)
{
        if (str) {
                if (!strcmp("emulate", str))
                        vsyscall_mode = EMULATE;
                else if (!strcmp("native", str))
                        vsyscall_mode = NATIVE;
                else if (!strcmp("none", str))
                        vsyscall_mode = NONE;
                else
                        return -EINVAL;

                return 0;
        }

        return -EINVAL;
}
early_param("vsyscall", vsyscall_setup);

static void warn_bad_vsyscall(const char *level, struct pt_regs *regs,
                              const char *message)
{
        if (!show_unhandled_signals)
                return;

        printk_ratelimited("%s%s[%d] %s ip:%lx cs:%lx sp:%lx ax:%lx si:%lx di:%lx\n",
                           level, current->comm, task_pid_nr(current),
                           message, regs->ip, regs->cs,
                           regs->sp, regs->ax, regs->si, regs->di);
}

static int addr_to_vsyscall_nr(unsigned long addr)
{
        int nr;

        if ((addr & ~0xC00UL) != VSYSCALL_ADDR)
                return -EINVAL;

        nr = (addr & 0xC00UL) >> 10;
        if (nr >= 3)
                return -EINVAL;

        return nr;
}

static bool write_ok_or_segv(unsigned long ptr, size_t size)
{
        /*
         * XXX: if access_ok, get_user, and put_user handled
         * sig_on_uaccess_err, this could go away.
         */

        if (!access_ok(VERIFY_WRITE, (void __user *)ptr, size)) {
                siginfo_t info;
                struct thread_struct *thread = &current->thread;

                thread->error_code      = 6;  /* user fault, no page, write */
                thread->cr2             = ptr;
                thread->trap_nr         = X86_TRAP_PF;

                memset(&info, 0, sizeof(info));
                info.si_signo           = SIGSEGV;
                info.si_errno           = 0;
                info.si_code            = SEGV_MAPERR;
                info.si_addr            = (void __user *)ptr;

                force_sig_info(SIGSEGV, &info, current);
                return false;
        } else {
                return true;
        }
}

bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
{
        struct task_struct *tsk;
        unsigned long caller;
        int vsyscall_nr, syscall_nr, tmp;
        int prev_sig_on_uaccess_err;
        long ret;

        /*
         * No point in checking CS -- the only way to get here is a user mode
         * trap to a high address, which means that we're in 64-bit user code.
         */

        WARN_ON_ONCE(address != regs->ip);

        if (vsyscall_mode == NONE) {
                warn_bad_vsyscall(KERN_INFO, regs,
                                  "vsyscall attempted with vsyscall=none");
                return false;
        }

        vsyscall_nr = addr_to_vsyscall_nr(address);

        trace_emulate_vsyscall(vsyscall_nr);

        if (vsyscall_nr < 0) {
                warn_bad_vsyscall(KERN_WARNING, regs,
                                  "misaligned vsyscall (exploit attempt or buggy program) -- look up the vsyscall kernel parameter if you need a workaround");
                goto sigsegv;
        }

        if (get_user(caller, (unsigned long __user *)regs->sp) != 0) {
                warn_bad_vsyscall(KERN_WARNING, regs,
                                  "vsyscall with bad stack (exploit attempt?)");
                goto sigsegv;
        }

        tsk = current;

        /*
         * Check for access_ok violations and find the syscall nr.
         *
         * NULL is a valid user pointer (in the access_ok sense) on 32-bit and
         * 64-bit, so we don't need to special-case it here.  For all the
         * vsyscalls, NULL means "don't write anything" not "write it at
         * address 0".
         */
        switch (vsyscall_nr) {
        case 0:
                if (!write_ok_or_segv(regs->di, sizeof(struct timeval)) ||
                    !write_ok_or_segv(regs->si, sizeof(struct timezone))) {
                        ret = -EFAULT;
                        goto check_fault;
                }

                syscall_nr = __NR_gettimeofday;
                break;

        case 1:
                if (!write_ok_or_segv(regs->di, sizeof(time_t))) {
                        ret = -EFAULT;
                        goto check_fault;
                }

                syscall_nr = __NR_time;
                break;

        case 2:
                if (!write_ok_or_segv(regs->di, sizeof(unsigned)) ||
                    !write_ok_or_segv(regs->si, sizeof(unsigned))) {
                        ret = -EFAULT;
                        goto check_fault;
                }

                syscall_nr = __NR_getcpu;
                break;
        }

        /*
         * Handle seccomp.  regs->ip must be the original value.
         * See seccomp_send_sigsys and Documentation/prctl/seccomp_filter.txt.
         *
         * We could optimize the seccomp disabled case, but performance
         * here doesn't matter.
         */
        regs->orig_ax = syscall_nr;
        regs->ax = -ENOSYS;
        tmp = secure_computing(NULL);
        if ((!tmp && regs->orig_ax != syscall_nr) || regs->ip != address) {
                warn_bad_vsyscall(KERN_DEBUG, regs,
                                  "seccomp tried to change syscall nr or ip");
                do_exit(SIGSYS);
        }
        regs->orig_ax = -1;
        if (tmp)
                goto do_ret;  /* skip requested */

        /*
         * With a real vsyscall, page faults cause SIGSEGV.  We want to
         * preserve that behavior to make writing exploits harder.
         */
        prev_sig_on_uaccess_err = current->thread.sig_on_uaccess_err;
        current->thread.sig_on_uaccess_err = 1;

        ret = -EFAULT;
        switch (vsyscall_nr) {
        case 0:
                ret = sys_gettimeofday(
                        (struct timeval __user *)regs->di,
                        (struct timezone __user *)regs->si);
                break;

        case 1:
                ret = sys_time((time_t __user *)regs->di);
                break;

        case 2:
                ret = sys_getcpu((unsigned __user *)regs->di,
                                 (unsigned __user *)regs->si,
                                 NULL);
                break;
        }

        current->thread.sig_on_uaccess_err = prev_sig_on_uaccess_err;

check_fault:
        if (ret == -EFAULT) {
                /* Bad news -- userspace fed a bad pointer to a vsyscall. */
                warn_bad_vsyscall(KERN_INFO, regs,
                                  "vsyscall fault (exploit attempt?)");

                /*
                 * If we failed to generate a signal for any reason,
                 * generate one here.  (This should be impossible.)
                 */
                if (WARN_ON_ONCE(!sigismember(&tsk->pending.signal, SIGBUS) &&
                                 !sigismember(&tsk->pending.signal, SIGSEGV)))
                        goto sigsegv;

                return true;  /* Don't emulate the ret. */
        }

        regs->ax = ret;

do_ret:
        /* Emulate a ret instruction. */
        regs->ip = caller;
        regs->sp += 8;
        return true;

sigsegv:
        force_sig(SIGSEGV, current);
        return true;
}

/*
 * A pseudo VMA to allow ptrace access for the vsyscall page.  This only
 * covers the 64bit vsyscall page now. 32bit has a real VMA now and does
 * not need special handling anymore:
 */
static const char *gate_vma_name(struct vm_area_struct *vma)
{
        return "[vsyscall]";
}
static const struct vm_operations_struct gate_vma_ops = {
        .name = gate_vma_name,
};
static struct vm_area_struct gate_vma = {
        .vm_start       = VSYSCALL_ADDR,
        .vm_end         = VSYSCALL_ADDR + PAGE_SIZE,
        .vm_page_prot   = PAGE_READONLY_EXEC,
        .vm_flags       = VM_READ | VM_EXEC,
        .vm_ops         = &gate_vma_ops,
};

struct vm_area_struct *get_gate_vma(struct mm_struct *mm)
{
#ifdef CONFIG_COMPAT
        if (!mm || mm->context.ia32_compat)
                return NULL;
#endif
        if (vsyscall_mode == NONE)
                return NULL;
        return &gate_vma;
}

int in_gate_area(struct mm_struct *mm, unsigned long addr)
{
        struct vm_area_struct *vma = get_gate_vma(mm);

        if (!vma)
                return 0;

        return (addr >= vma->vm_start) && (addr < vma->vm_end);
}

/*
 * Use this when you have no reliable mm, typically from interrupt
 * context. It is less reliable than using a task's mm and may give
 * false positives.
 */
int in_gate_area_no_mm(unsigned long addr)
{
        return vsyscall_mode != NONE && (addr & PAGE_MASK) == VSYSCALL_ADDR;
}

void __init map_vsyscall(void)
{
        extern char __vsyscall_page;
        unsigned long physaddr_vsyscall = __pa_symbol(&__vsyscall_page);

        if (vsyscall_mode != NONE)
                __set_fixmap(VSYSCALL_PAGE, physaddr_vsyscall,
                             vsyscall_mode == NATIVE
                             ? PAGE_KERNEL_VSYSCALL
                             : PAGE_KERNEL_VVAR);

        BUILD_BUG_ON((unsigned long)__fix_to_virt(VSYSCALL_PAGE) !=
                     (unsigned long)VSYSCALL_ADDR);
}