/*
 * fsgsbase.c, an fsgsbase test
 * Copyright (c) 2014-2016 Andy Lutomirski
 * GPL v2
 */

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <string.h>
#include <sys/syscall.h>
#include <unistd.h>
#include <err.h>
#include <sys/user.h>
#include <asm/prctl.h>
#include <sys/prctl.h>
#include <signal.h>
#include <limits.h>
#include <sys/ucontext.h>
#include <sched.h>
#include <linux/futex.h>
#include <pthread.h>
#include <asm/ldt.h>
#include <sys/mman.h>

#ifndef __x86_64__
# error This test is 64-bit only
#endif

static volatile sig_atomic_t want_segv;
static volatile unsigned long segv_addr;

static int nerrs;

static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
                       int flags)
{
        struct sigaction sa;
        memset(&sa, 0, sizeof(sa));
        sa.sa_sigaction = handler;
        sa.sa_flags = SA_SIGINFO | flags;
        sigemptyset(&sa.sa_mask);
        if (sigaction(sig, &sa, 0))
                err(1, "sigaction");
}

static void clearhandler(int sig)
{
        struct sigaction sa;
        memset(&sa, 0, sizeof(sa));
        sa.sa_handler = SIG_DFL;
        sigemptyset(&sa.sa_mask);
        if (sigaction(sig, &sa, 0))
                err(1, "sigaction");
}

static void sigsegv(int sig, siginfo_t *si, void *ctx_void)
{
        ucontext_t *ctx = (ucontext_t*)ctx_void;

        if (!want_segv) {
                clearhandler(SIGSEGV);
                return;  /* Crash cleanly. */
        }

        want_segv = false;
        segv_addr = (unsigned long)si->si_addr;

        ctx->uc_mcontext.gregs[REG_RIP] += 4;   /* Skip the faulting mov */

}

enum which_base { FS, GS };

static unsigned long read_base(enum which_base which)
{
        unsigned long offset;
        /*
         * Unless we have FSGSBASE, there's no direct way to do this from
         * user mode.  We can get at it indirectly using signals, though.
         */

        want_segv = true;

        offset = 0;
        if (which == FS) {
                /* Use a constant-length instruction here. */
                asm volatile ("mov %%fs:(%%rcx), %%rax" : : "c" (offset) : "rax");
        } else {
                asm volatile ("mov %%gs:(%%rcx), %%rax" : : "c" (offset) : "rax");
        }
        if (!want_segv)
                return segv_addr + offset;

        /*
         * If that didn't segfault, try the other end of the address space.
         * Unless we get really unlucky and run into the vsyscall page, this
         * is guaranteed to segfault.
         */

        offset = (ULONG_MAX >> 1) + 1;
        if (which == FS) {
                asm volatile ("mov %%fs:(%%rcx), %%rax"
                              : : "c" (offset) : "rax");
        } else {
                asm volatile ("mov %%gs:(%%rcx), %%rax"
                              : : "c" (offset) : "rax");
        }
        if (!want_segv)
                return segv_addr + offset;

        abort();
}

static void check_gs_value(unsigned long value)
{
        unsigned long base;
        unsigned short sel;

        printf("[RUN]\tARCH_SET_GS to 0x%lx\n", value);
        if (syscall(SYS_arch_prctl, ARCH_SET_GS, value) != 0)
                err(1, "ARCH_SET_GS");

        asm volatile ("mov %%gs, %0" : "=rm" (sel));
        base = read_base(GS);
        if (base == value) {
                printf("[OK]\tGSBASE was set as expected (selector 0x%hx)\n",
                       sel);
        } else {
                nerrs++;
                printf("[FAIL]\tGSBASE was not as expected: got 0x%lx (selector 0x%hx)\n",
                       base, sel);
        }

        if (syscall(SYS_arch_prctl, ARCH_GET_GS, &base) != 0)
                err(1, "ARCH_GET_GS");
        if (base == value) {
                printf("[OK]\tARCH_GET_GS worked as expected (selector 0x%hx)\n",
                       sel);
        } else {
                nerrs++;
                printf("[FAIL]\tARCH_GET_GS was not as expected: got 0x%lx (selector 0x%hx)\n",
                       base, sel);
        }
}

static void mov_0_gs(unsigned long initial_base, bool schedule)
{
        unsigned long base, arch_base;

        printf("[RUN]\tARCH_SET_GS to 0x%lx then mov 0 to %%gs%s\n", initial_base, schedule ? " and schedule " : "");
        if (syscall(SYS_arch_prctl, ARCH_SET_GS, initial_base) != 0)
                err(1, "ARCH_SET_GS");

        if (schedule)
                usleep(10);

        asm volatile ("mov %0, %%gs" : : "rm" (0));
        base = read_base(GS);
        if (syscall(SYS_arch_prctl, ARCH_GET_GS, &arch_base) != 0)
                err(1, "ARCH_GET_GS");
        if (base == arch_base) {
                printf("[OK]\tGSBASE is 0x%lx\n", base);
        } else {
                nerrs++;
                printf("[FAIL]\tGSBASE changed to 0x%lx but kernel reports 0x%lx\n", base, arch_base);
        }
}

static volatile unsigned long remote_base;
static volatile bool remote_hard_zero;
static volatile unsigned int ftx;

/*
 * ARCH_SET_FS/GS(0) may or may not program a selector of zero.  HARD_ZERO
 * means to force the selector to zero to improve test coverage.
 */
#define HARD_ZERO 0xa1fa5f343cb85fa4

static void do_remote_base()
{
        unsigned long to_set = remote_base;
        bool hard_zero = false;
        if (to_set == HARD_ZERO) {
                to_set = 0;
                hard_zero = true;
        }

        if (syscall(SYS_arch_prctl, ARCH_SET_GS, to_set) != 0)
                err(1, "ARCH_SET_GS");

        if (hard_zero)
                asm volatile ("mov %0, %%gs" : : "rm" ((unsigned short)0));

        unsigned short sel;
        asm volatile ("mov %%gs, %0" : "=rm" (sel));
        printf("\tother thread: ARCH_SET_GS(0x%lx)%s -- sel is 0x%hx\n",
               to_set, hard_zero ? " and clear gs" : "", sel);
}

void do_unexpected_base(void)
{
        /*
         * The goal here is to try to arrange for GS == 0, GSBASE !=
         * 0, and for the the kernel the think that GSBASE == 0.
         *
         * To make the test as reliable as possible, this uses
         * explicit descriptorss.  (This is not the only way.  This
         * could use ARCH_SET_GS with a low, nonzero base, but the
         * relevant side effect of ARCH_SET_GS could change.)
         */

        /* Step 1: tell the kernel that we have GSBASE == 0. */
        if (syscall(SYS_arch_prctl, ARCH_SET_GS, 0) != 0)
                err(1, "ARCH_SET_GS");

        /* Step 2: change GSBASE without telling the kernel. */
        struct user_desc desc = {
                .entry_number    = 0,
                .base_addr       = 0xBAADF00D,
                .limit           = 0xfffff,
                .seg_32bit       = 1,
                .contents        = 0, /* Data, grow-up */
                .read_exec_only  = 0,
                .limit_in_pages  = 1,
                .seg_not_present = 0,
                .useable         = 0
        };
        if (syscall(SYS_modify_ldt, 1, &desc, sizeof(desc)) == 0) {
                printf("\tother thread: using LDT slot 0\n");
                asm volatile ("mov %0, %%gs" : : "rm" ((unsigned short)0x7));
        } else {
                /* No modify_ldt for us (configured out, perhaps) */

                struct user_desc *low_desc = mmap(
                        NULL, sizeof(desc),
                        PROT_READ | PROT_WRITE,
                        MAP_PRIVATE | MAP_ANONYMOUS | MAP_32BIT, -1, 0);
                memcpy(low_desc, &desc, sizeof(desc));

                low_desc->entry_number = -1;

                /* 32-bit set_thread_area */
                long ret;
                asm volatile ("int $0x80"
                              : "=a" (ret) : "a" (243), "b" (low_desc)
                              : "r8", "r9", "r10", "r11");
                memcpy(&desc, low_desc, sizeof(desc));
                munmap(low_desc, sizeof(desc));

                if (ret != 0) {
                        printf("[NOTE]\tcould not create a segment -- test won't do anything\n");
                        return;
                }
                printf("\tother thread: using GDT slot %d\n", desc.entry_number);
                asm volatile ("mov %0, %%gs" : : "rm" ((unsigned short)((desc.entry_number << 3) | 0x3)));
        }

        /*
         * Step 3: set the selector back to zero.  On AMD chips, this will
         * preserve GSBASE.
         */

        asm volatile ("mov %0, %%gs" : : "rm" ((unsigned short)0));
}

static void *threadproc(void *ctx)
{
        while (1) {
                while (ftx == 0)
                        syscall(SYS_futex, &ftx, FUTEX_WAIT, 0, NULL, NULL, 0);
                if (ftx == 3)
                        return NULL;

                if (ftx == 1)
                        do_remote_base();
                else if (ftx == 2)
                        do_unexpected_base();
                else
                        errx(1, "helper thread got bad command");

                ftx = 0;
                syscall(SYS_futex, &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);
        }
}

static void set_gs_and_switch_to(unsigned long local, unsigned long remote)
{
        unsigned long base;

        bool hard_zero = false;
        if (local == HARD_ZERO) {
                hard_zero = true;
                local = 0;
        }

        printf("[RUN]\tARCH_SET_GS(0x%lx)%s, then schedule to 0x%lx\n",
               local, hard_zero ? " and clear gs" : "", remote);
        if (syscall(SYS_arch_prctl, ARCH_SET_GS, local) != 0)
                err(1, "ARCH_SET_GS");
        if (hard_zero)
                asm volatile ("mov %0, %%gs" : : "rm" ((unsigned short)0));

        if (read_base(GS) != local) {
                nerrs++;
                printf("[FAIL]\tGSBASE wasn't set as expected\n");
        }

        remote_base = remote;
        ftx = 1;
        syscall(SYS_futex, &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);
        while (ftx != 0)
                syscall(SYS_futex, &ftx, FUTEX_WAIT, 1, NULL, NULL, 0);

        base = read_base(GS);
        if (base == local) {
                printf("[OK]\tGSBASE remained 0x%lx\n", local);
        } else {
                nerrs++;
                printf("[FAIL]\tGSBASE changed to 0x%lx\n", base);
        }
}

static void test_unexpected_base(void)
{
        unsigned long base;

        printf("[RUN]\tARCH_SET_GS(0), clear gs, then manipulate GSBASE in a different thread\n");
        if (syscall(SYS_arch_prctl, ARCH_SET_GS, 0) != 0)
                err(1, "ARCH_SET_GS");
        asm volatile ("mov %0, %%gs" : : "rm" ((unsigned short)0));

        ftx = 2;
        syscall(SYS_futex, &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);
        while (ftx != 0)
                syscall(SYS_futex, &ftx, FUTEX_WAIT, 1, NULL, NULL, 0);

        base = read_base(GS);
        if (base == 0) {
                printf("[OK]\tGSBASE remained 0\n");
        } else {
                nerrs++;
                printf("[FAIL]\tGSBASE changed to 0x%lx\n", base);
        }
}

int main()
{
        pthread_t thread;

        sethandler(SIGSEGV, sigsegv, 0);

        check_gs_value(0);
        check_gs_value(1);
        check_gs_value(0x200000000);
        check_gs_value(0);
        check_gs_value(0x200000000);
        check_gs_value(1);

        for (int sched = 0; sched < 2; sched++) {
                mov_0_gs(0, !!sched);
                mov_0_gs(1, !!sched);
                mov_0_gs(0x200000000, !!sched);
        }

        /* Set up for multithreading. */

        cpu_set_t cpuset;
        CPU_ZERO(&cpuset);
        CPU_SET(0, &cpuset);
        if (sched_setaffinity(0, sizeof(cpuset), &cpuset) != 0)
                err(1, "sched_setaffinity to CPU 0");   /* should never fail */

        if (pthread_create(&thread, 0, threadproc, 0) != 0)
                err(1, "pthread_create");

        static unsigned long bases_with_hard_zero[] = {
                0, HARD_ZERO, 1, 0x200000000,
        };

        for (int local = 0; local < 4; local++) {
                for (int remote = 0; remote < 4; remote++) {
                        set_gs_and_switch_to(bases_with_hard_zero[local],
                                             bases_with_hard_zero[remote]);
                }
        }

        test_unexpected_base();

        ftx = 3;  /* Kill the thread. */
        syscall(SYS_futex, &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);

        if (pthread_join(thread, NULL) != 0)
                err(1, "pthread_join");

        return nerrs == 0 ? 0 : 1;
}