// SPDX-License-Identifier: GPL-2.0-only
/*
 * fsgsbase.c, an fsgsbase test
 * Copyright (c) 2014-2016 Andy Lutomirski
 */

#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>
#include <stddef.h>
#include <sys/ptrace.h>
#include <sys/wait.h>
#include <setjmp.h>

#include "helpers.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 unsigned short *shared_scratch;

static int nerrs;

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 */

}

static jmp_buf jmpbuf;

static void sigill(int sig, siginfo_t *si, void *ctx_void)
{
        siglongjmp(jmpbuf, 1);
}

static bool have_fsgsbase;

static inline unsigned long rdgsbase(void)
{
        unsigned long gsbase;

        asm volatile("rdgsbase %0" : "=r" (gsbase) :: "memory");

        return gsbase;
}

static inline unsigned long rdfsbase(void)
{
        unsigned long fsbase;

        asm volatile("rdfsbase %0" : "=r" (fsbase) :: "memory");

        return fsbase;
}

static inline void wrgsbase(unsigned long gsbase)
{
        asm volatile("wrgsbase %0" :: "r" (gsbase) : "memory");
}

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 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);
}

static __thread int set_thread_area_entry_number = -1;

static unsigned short load_gs(void)
{
        /*
         * Sets GS != 0 and GSBASE != 0 but arranges for the kernel to think
         * that GSBASE == 0 (i.e. thread.gsbase == 0).
         */

        /* 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("\tusing LDT slot 0\n");
                asm volatile ("mov %0, %%gs" : : "rm" ((unsigned short)0x7));
                return 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 = set_thread_area_entry_number;

                /* 32-bit set_thread_area */
                long ret;
                asm volatile ("int $0x80"
                              : "=a" (ret), "+m" (*low_desc)
                              : "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 0;
                }
                printf("\tusing GDT slot %d\n", desc.entry_number);
                set_thread_area_entry_number = desc.entry_number;

                unsigned short gs = (unsigned short)((desc.entry_number << 3) | 0x3);
                asm volatile ("mov %0, %%gs" : : "rm" (gs));
                return gs;
        }
}

void test_wrbase(unsigned short index, unsigned long base)
{
        unsigned short newindex;
        unsigned long newbase;

        printf("[RUN]\tGS = 0x%hx, GSBASE = 0x%lx\n", index, base);

        asm volatile ("mov %0, %%gs" : : "rm" (index));
        wrgsbase(base);

        remote_base = 0;
        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);

        asm volatile ("mov %%gs, %0" : "=rm" (newindex));
        newbase = rdgsbase();

        if (newindex == index && newbase == base) {
                printf("[OK]\tIndex and base were preserved\n");
        } else {
                printf("[FAIL]\tAfter switch, GS = 0x%hx and GSBASE = 0x%lx\n",
                       newindex, newbase);
                nerrs++;
        }
}

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) {
                        /*
                         * On AMD chips, this causes GSBASE != 0, GS == 0, and
                         * thread.gsbase == 0.
                         */

                        load_gs();
                        asm volatile ("mov %0, %%gs" : : "rm" ((unsigned short)0));
                } 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 short force_sel,
                                 unsigned long remote)
{
        unsigned long base;
        unsigned short sel_pre_sched, sel_post_sched;

        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 (force_sel)
                printf("\tBefore schedule, set selector to 0x%hx\n", force_sel);
        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");
        }

        if (force_sel) {
                asm volatile ("mov %0, %%gs" : : "rm" (force_sel));
                sel_pre_sched = force_sel;
                local = read_base(GS);

                /*
                 * Signal delivery is quite likely to change a selector
                 * of 1, 2, or 3 back to 0 due to IRET being defective.
                 */
                asm volatile ("mov %0, %%gs" : : "rm" (force_sel));
        } else {
                asm volatile ("mov %%gs, %0" : "=rm" (sel_pre_sched));
        }

        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);

        asm volatile ("mov %%gs, %0" : "=rm" (sel_post_sched));
        base = read_base(GS);
        if (base == local && sel_pre_sched == sel_post_sched) {
                printf("[OK]\tGS/BASE remained 0x%hx/0x%lx\n",
                       sel_pre_sched, local);
        } else if (base == local && sel_pre_sched >= 1 && sel_pre_sched <= 3 &&
                   sel_post_sched == 0) {
                /*
                 * IRET is misdesigned and will squash selectors 1, 2, or 3
                 * to zero.  Don't fail the test just because this happened.
                 */
                printf("[OK]\tGS/BASE changed from 0x%hx/0x%lx to 0x%hx/0x%lx because IRET is defective\n",
                       sel_pre_sched, local, sel_post_sched, base);
        } else {
                nerrs++;
                printf("[FAIL]\tGS/BASE changed from 0x%hx/0x%lx to 0x%hx/0x%lx\n",
                       sel_pre_sched, local, sel_post_sched, 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);
        }
}

#define USER_REGS_OFFSET(r) offsetof(struct user_regs_struct, r)

static void test_ptrace_write_gs_read_base(void)
{
        int status;
        pid_t child = fork();

        if (child < 0)
                err(1, "fork");

        if (child == 0) {
                printf("[RUN]\tPTRACE_POKE GS, read GSBASE back\n");

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

                if (ptrace(PTRACE_TRACEME, 0, NULL, NULL) != 0)
                        err(1, "PTRACE_TRACEME");

                raise(SIGTRAP);
                _exit(0);
        }

        wait(&status);

        if (WSTOPSIG(status) == SIGTRAP) {
                unsigned long base;
                unsigned long gs_offset = USER_REGS_OFFSET(gs);
                unsigned long base_offset = USER_REGS_OFFSET(gs_base);

                /* Read the initial base.  It should be 1. */
                base = ptrace(PTRACE_PEEKUSER, child, base_offset, NULL);
                if (base == 1) {
                        printf("[OK]\tGSBASE started at 1\n");
                } else {
                        nerrs++;
                        printf("[FAIL]\tGSBASE started at 0x%lx\n", base);
                }

                printf("[RUN]\tSet GS = 0x7, read GSBASE\n");

                /* Poke an LDT selector into GS. */
                if (ptrace(PTRACE_POKEUSER, child, gs_offset, 0x7) != 0)
                        err(1, "PTRACE_POKEUSER");

                /* And read the base. */
                base = ptrace(PTRACE_PEEKUSER, child, base_offset, NULL);

                if (base == 0 || base == 1) {
                        printf("[OK]\tGSBASE reads as 0x%lx with invalid GS\n", base);
                } else {
                        nerrs++;
                        printf("[FAIL]\tGSBASE=0x%lx (should be 0 or 1)\n", base);
                }
        }

        ptrace(PTRACE_CONT, child, NULL, NULL);

        wait(&status);
        if (!WIFEXITED(status))
                printf("[WARN]\tChild didn't exit cleanly.\n");
}

static void test_ptrace_write_gsbase(void)
{
        int status;
        pid_t child = fork();

        if (child < 0)
                err(1, "fork");

        if (child == 0) {
                printf("[RUN]\tPTRACE_POKE(), write GSBASE from ptracer\n");

                *shared_scratch = load_gs();

                if (ptrace(PTRACE_TRACEME, 0, NULL, NULL) != 0)
                        err(1, "PTRACE_TRACEME");

                raise(SIGTRAP);
                _exit(0);
        }

        wait(&status);

        if (WSTOPSIG(status) == SIGTRAP) {
                unsigned long gs, base;
                unsigned long gs_offset = USER_REGS_OFFSET(gs);
                unsigned long base_offset = USER_REGS_OFFSET(gs_base);

                gs = ptrace(PTRACE_PEEKUSER, child, gs_offset, NULL);

                if (gs != *shared_scratch) {
                        nerrs++;
                        printf("[FAIL]\tGS is not prepared with nonzero\n");
                        goto END;
                }

                if (ptrace(PTRACE_POKEUSER, child, base_offset, 0xFF) != 0)
                        err(1, "PTRACE_POKEUSER");

                gs = ptrace(PTRACE_PEEKUSER, child, gs_offset, NULL);
                base = ptrace(PTRACE_PEEKUSER, child, base_offset, NULL);

                /*
                 * In a non-FSGSBASE system, the nonzero selector will load
                 * GSBASE (again). But what is tested here is whether the
                 * selector value is changed or not by the GSBASE write in
                 * a ptracer.
                 */
                if (gs != *shared_scratch) {
                        nerrs++;
                        printf("[FAIL]\tGS changed to %lx\n", gs);

                        /*
                         * On older kernels, poking a nonzero value into the
                         * base would zero the selector.  On newer kernels,
                         * this behavior has changed -- poking the base
                         * changes only the base and, if FSGSBASE is not
                         * available, this may have no effect once the tracee
                         * is resumed.
                         */
                        if (gs == 0)
                                printf("\tNote: this is expected behavior on older kernels.\n");
                } else if (have_fsgsbase && (base != 0xFF)) {
                        nerrs++;
                        printf("[FAIL]\tGSBASE changed to %lx\n", base);
                } else {
                        printf("[OK]\tGS remained 0x%hx", *shared_scratch);
                        if (have_fsgsbase)
                                printf(" and GSBASE changed to 0xFF");
                        printf("\n");
                }
        }

END:
        ptrace(PTRACE_CONT, child, NULL, NULL);
        wait(&status);
        if (!WIFEXITED(status))
                printf("[WARN]\tChild didn't exit cleanly.\n");
}

int main()
{
        pthread_t thread;

        shared_scratch = mmap(NULL, 4096, PROT_READ | PROT_WRITE,
                              MAP_ANONYMOUS | MAP_SHARED, -1, 0);

        /* Do these tests before we have an LDT. */
        test_ptrace_write_gs_read_base();

        /* Probe FSGSBASE */
        sethandler(SIGILL, sigill, 0);
        if (sigsetjmp(jmpbuf, 1) == 0) {
                rdfsbase();
                have_fsgsbase = true;
                printf("\tFSGSBASE instructions are enabled\n");
        } else {
                printf("\tFSGSBASE instructions are disabled\n");
        }
        clearhandler(SIGILL);

        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++) {
                        for (unsigned short s = 0; s < 5; s++) {
                                unsigned short sel = s;
                                if (s == 4)
                                        asm ("mov %%ss, %0" : "=rm" (sel));
                                set_gs_and_switch_to(
                                        bases_with_hard_zero[local],
                                        sel,
                                        bases_with_hard_zero[remote]);
                        }
                }
        }

        test_unexpected_base();

        if (have_fsgsbase) {
                unsigned short ss;

                asm volatile ("mov %%ss, %0" : "=rm" (ss));

                test_wrbase(0, 0);
                test_wrbase(0, 1);
                test_wrbase(0, 0x200000000);
                test_wrbase(0, 0xffffffffffffffff);
                test_wrbase(ss, 0);
                test_wrbase(ss, 1);
                test_wrbase(ss, 0x200000000);
                test_wrbase(ss, 0xffffffffffffffff);
        }

        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");

        test_ptrace_write_gsbase();

        return nerrs == 0 ? 0 : 1;
}