// SPDX-License-Identifier: GPL-2.0
/*
 * 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>
#include <asm/paravirt.h>

#define CREATE_TRACE_POINTS
#include "vsyscall_trace.h"

static enum { EMULATE, XONLY, NONE } vsyscall_mode __ro_after_init =
#ifdef CONFIG_LEGACY_VSYSCALL_NONE
        NONE;
#elif defined(CONFIG_LEGACY_VSYSCALL_XONLY)
        XONLY;
#else
        EMULATE;
#endif

static int __init vsyscall_setup(char *str)
{
        if (str) {
                if (!strcmp("emulate", str))
                        vsyscall_mode = EMULATE;
                else if (!strcmp("xonly", str))
                        vsyscall_mode = XONLY;
                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((void __user *)ptr, size)) {
                struct thread_struct *thread = &current->thread;

                thread->error_code      = X86_PF_USER | X86_PF_WRITE;
                thread->cr2             = ptr;
                thread->trap_nr         = X86_TRAP_PF;

                force_sig_fault(SIGSEGV, SEGV_MAPERR, (void __user *)ptr);
                return false;
        } else {
                return true;
        }
}

bool emulate_vsyscall(unsigned long error_code,
                      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;
        unsigned long orig_dx;

        /* Write faults or kernel-privilege faults never get fixed up. */
        if ((error_code & (X86_PF_WRITE | X86_PF_USER)) != X86_PF_USER)
                return false;

        if (!(error_code & X86_PF_INSTR)) {
                /* Failed vsyscall read */
                if (vsyscall_mode == EMULATE)
                        return false;

                /*
                 * User code tried and failed to read the vsyscall page.
                 */
                warn_bad_vsyscall(KERN_INFO, regs, "vsyscall read attempt denied -- look up the vsyscall kernel parameter if you need a workaround");
                return false;
        }

        /*
         * 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 __kernel_old_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(__kernel_old_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/userspace-api/seccomp_filter.rst.
         *
         * We could optimize the seccomp disabled case, but performance
         * here doesn't matter.
         */
        regs->orig_ax = syscall_nr;
        regs->ax = -ENOSYS;
        tmp = secure_computing();
        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:
                /* this decodes regs->di and regs->si on its own */
                ret = __x64_sys_gettimeofday(regs);
                break;

        case 1:
                /* this decodes regs->di on its own */
                ret = __x64_sys_time(regs);
                break;

        case 2:
                /* while we could clobber regs->dx, we didn't in the past... */
                orig_dx = regs->dx;
                regs->dx = 0;
                /* this decodes regs->di, regs->si and regs->dx on its own */
                ret = __x64_sys_getcpu(regs);
                regs->dx = orig_dx;
                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);
        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 __ro_after_init = {
        .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.flags & MM_CONTEXT_HAS_VSYSCALL))
                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;
}

/*
 * The VSYSCALL page is the only user-accessible page in the kernel address
 * range.  Normally, the kernel page tables can have _PAGE_USER clear, but
 * the tables covering VSYSCALL_ADDR need _PAGE_USER set if vsyscalls
 * are enabled.
 *
 * Some day we may create a "minimal" vsyscall mode in which we emulate
 * vsyscalls but leave the page not present.  If so, we skip calling
 * this.
 */
void __init set_vsyscall_pgtable_user_bits(pgd_t *root)
{
        pgd_t *pgd;
        p4d_t *p4d;
        pud_t *pud;
        pmd_t *pmd;

        pgd = pgd_offset_pgd(root, VSYSCALL_ADDR);
        set_pgd(pgd, __pgd(pgd_val(*pgd) | _PAGE_USER));
        p4d = p4d_offset(pgd, VSYSCALL_ADDR);
#if CONFIG_PGTABLE_LEVELS >= 5
        set_p4d(p4d, __p4d(p4d_val(*p4d) | _PAGE_USER));
#endif
        pud = pud_offset(p4d, VSYSCALL_ADDR);
        set_pud(pud, __pud(pud_val(*pud) | _PAGE_USER));
        pmd = pmd_offset(pud, VSYSCALL_ADDR);
        set_pmd(pmd, __pmd(pmd_val(*pmd) | _PAGE_USER));
}

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

        /*
         * For full emulation, the page needs to exist for real.  In
         * execute-only mode, there is no PTE at all backing the vsyscall
         * page.
         */
        if (vsyscall_mode == EMULATE) {
                __set_fixmap(VSYSCALL_PAGE, physaddr_vsyscall,
                             PAGE_KERNEL_VVAR);
                set_vsyscall_pgtable_user_bits(swapper_pg_dir);
        }

        if (vsyscall_mode == XONLY)
                gate_vma.vm_flags = VM_EXEC;

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