fuzz/x86_instruction_emulator/fuzz-emul.c

#include <assert.h>
#include <errno.h>
#include <fcntl.h>
#include <inttypes.h>
#include <limits.h>
#include <stdbool.h>
#include <stddef.h>
#include <stdint.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <unistd.h>
#include <xen/xen.h>

#include "x86-emulate.h"
/*
 * include "x86-emulate.h" prior to <stdio.h> and <string.h>:
 * x86-emulate.h disables use of SSE registers, while <stdio.h> and <string.h>
 * declare functions that may be always_inline and use those registers
 * unless they have been disabled earlier, which can fail to compile.
 */
#include <stdio.h>
#include <string.h>
#include "fuzz-emul.h"

#define MSR_INDEX_MAX 16

#define SEG_NUM x86_seg_none

/* Layout of data expected as fuzzing input. */
struct fuzz_corpus
{
    unsigned long cr[5];
    uint64_t msr[MSR_INDEX_MAX];
    struct cpu_user_regs regs;
    struct segment_register segments[SEG_NUM];
    unsigned long options;
    unsigned char data[INPUT_SIZE];
} input;
#define DATA_OFFSET offsetof(struct fuzz_corpus, data)
#define FUZZ_CORPUS_SIZE (sizeof(struct fuzz_corpus))

/*
 * Internal state of the fuzzing harness.  Calculated initially from the input
 * corpus, and later mutates by the emulation callbacks.
 */
struct fuzz_state
{
    /* Fuzzer's input data. */
    struct fuzz_corpus *corpus;

    /* Real amount of data backing corpus->data[]. */
    size_t data_num;

    /* Amount of corpus->data[] consumed thus far. */
    size_t data_index;

    /* Emulation ops, some of which are disabled based on corpus->options. */
    struct x86_emulate_ops ops;
};

static inline bool input_avail(const struct fuzz_state *s, size_t size)
{
    return s->data_index + size <= s->data_num;
}

static inline bool input_read(struct fuzz_state *s, void *dst, size_t size)
{
    if ( !input_avail(s, size) )
        return false;

    memcpy(dst, &s->corpus->data[s->data_index], size);
    s->data_index += size;

    return true;
}

static bool check_state(struct x86_emulate_ctxt *ctxt);

static const char* const x86emul_return_string[] = {
    [X86EMUL_OKAY] = "X86EMUL_OKAY",
    [X86EMUL_UNHANDLEABLE] = "X86EMUL_UNHANDLEABLE",
    [X86EMUL_EXCEPTION] = "X86EMUL_EXCEPTION",
    [X86EMUL_RETRY] = "X86EMUL_RETRY",
    [X86EMUL_DONE] = "X86EMUL_DONE",
};

/*
 * Randomly return success or failure when processing data.  If
 * `exception` is false, this function turns _EXCEPTION to _OKAY.
 */
static int maybe_fail(struct x86_emulate_ctxt *ctxt,
                      const char *why, bool exception)
{
    struct fuzz_state *s = ctxt->data;
    unsigned char c;
    int rc;

    if ( !input_read(s, &c, sizeof(c)) )
        rc = X86EMUL_EXCEPTION;
    else
    {
        /* Randomly returns value:
         * 50% okay
         * 25% unhandlable
         * 25% exception
         */
        if ( c > 0xc0 )
            rc = X86EMUL_EXCEPTION;
        else if ( c > 0x80 )
            rc = X86EMUL_UNHANDLEABLE;
        else
            rc = X86EMUL_OKAY;
    }

    if ( rc == X86EMUL_EXCEPTION && !exception )
        rc = X86EMUL_OKAY;

    printf("maybe_fail %s: %s\n", why, x86emul_return_string[rc]);

    if ( rc == X86EMUL_EXCEPTION )
        /* Fake up a pagefault. */
        x86_emul_pagefault(0, 0, ctxt);

    return rc;
}

static int data_read(struct x86_emulate_ctxt *ctxt,
                     enum x86_segment seg,
                     const char *why, void *dst, unsigned int bytes)
{
    struct fuzz_state *s = ctxt->data;
    unsigned int i;
    int rc;

    if ( !input_avail(s, bytes) )
    {
        /*
         * Fake up a segment limit violation.  System segment limit volations
         * are reported by X86EMUL_EXCEPTION alone, so the emulator can fill
         * in the correct context.
         */
        if ( !is_x86_system_segment(seg) )
            x86_emul_hw_exception(13, 0, ctxt);

        rc = X86EMUL_EXCEPTION;
        printf("data_read %s: X86EMUL_EXCEPTION (end of input)\n", why);
    }
    else
        rc = maybe_fail(ctxt, why, true);

    if ( rc == X86EMUL_OKAY )
    {
        input_read(s, dst, bytes);

        printf("%s: ", why);
        for ( i = 0; i < bytes; i++ )
            printf(" %02x", *(unsigned char *)(dst + i));
        printf("\n");
    }

    return rc;
}

static int fuzz_read(
    enum x86_segment seg,
    unsigned long offset,
    void *p_data,
    unsigned int bytes,
    struct x86_emulate_ctxt *ctxt)
{
    /* Reads expected for all user and system segments. */
    if ( is_x86_user_segment(seg) )
        assert(ctxt->addr_size == 64 || !(offset >> 32));
    else if ( seg == x86_seg_tr )
        /*
         * The TSS is special in that accesses below the segment base are
         * possible, as the Interrupt Redirection Bitmap starts 32 bytes
         * ahead of the I/O Bitmap, regardless of the value of the latter.
         */
        assert((long)offset < 0 ? (long)offset > -32 : !(offset >> 17));
    else
        assert(is_x86_system_segment(seg) &&
               (ctxt->lma ? offset <= 0x10007 : !(offset >> 16)));

    return data_read(ctxt, seg, "read", p_data, bytes);
}

static int fuzz_read_io(
    unsigned int port,
    unsigned int bytes,
    unsigned long *val,
    struct x86_emulate_ctxt *ctxt)
{
    return data_read(ctxt, x86_seg_none, "read_io", val, bytes);
}

static int fuzz_insn_fetch(
    unsigned long offset,
    void *p_data,
    unsigned int bytes,
    struct x86_emulate_ctxt *ctxt)
{
    /* Minimal segment limit checking, until full one is being put in place. */
    if ( ctxt->addr_size < 64 && (offset >> 32) )
    {
        x86_emul_hw_exception(13, 0, ctxt);
        return X86EMUL_EXCEPTION;
    }

    /*
     * Zero-length instruction fetches are made at the destination of jumps,
     * to perform segmentation checks.  No data needs returning.
     */
    if ( bytes == 0 )
    {
        assert(p_data == NULL);
        return maybe_fail(ctxt, "insn_fetch", true);
    }

    return data_read(ctxt, x86_seg_cs, "insn_fetch", p_data, bytes);
}

static int _fuzz_rep_read(struct x86_emulate_ctxt *ctxt,
                          const char *why, unsigned long *reps)
{
    int rc;
    unsigned long bytes_read = 0;

    rc = data_read(ctxt, x86_seg_none, why, &bytes_read, sizeof(bytes_read));

    if ( bytes_read <= *reps )
        *reps = bytes_read;

    switch ( rc )
    {
    case X86EMUL_UNHANDLEABLE:
        /* No work is done in this case */
        *reps = 0;
        break;
    case X86EMUL_EXCEPTION:
    case X86EMUL_RETRY:
        /* Halve the amount in this case */
        *reps /= 2;
        break;
    }

    return rc;
}

static int _fuzz_rep_write(struct x86_emulate_ctxt *ctxt,
                           const char *why, unsigned long *reps)
{
    int rc = maybe_fail(ctxt, why, true);

    switch ( rc )
    {
    case X86EMUL_UNHANDLEABLE:
        /* No work is done in this case */
        *reps = 0;
        break;
    case X86EMUL_EXCEPTION:
    case X86EMUL_RETRY:
        /* Halve the amount in this case */
        *reps /= 2;
        break;
    }

    return rc;
}

static int fuzz_rep_ins(
    uint16_t src_port,
    enum x86_segment dst_seg,
    unsigned long dst_offset,
    unsigned int bytes_per_rep,
    unsigned long *reps,
    struct x86_emulate_ctxt *ctxt)
{
    assert(dst_seg == x86_seg_es);
    assert(ctxt->addr_size == 64 || !(dst_offset >> 32));

    return _fuzz_rep_read(ctxt, "rep_ins", reps);
}

static int fuzz_rep_movs(
    enum x86_segment src_seg,
    unsigned long src_offset,
    enum x86_segment dst_seg,
    unsigned long dst_offset,
    unsigned int bytes_per_rep,
    unsigned long *reps,
    struct x86_emulate_ctxt *ctxt)
{
    assert(is_x86_user_segment(src_seg));
    assert(dst_seg == x86_seg_es);
    assert(ctxt->addr_size == 64 || !((src_offset | dst_offset) >> 32));

    return _fuzz_rep_read(ctxt, "rep_movs", reps);
}

static int fuzz_rep_outs(
    enum x86_segment src_seg,
    unsigned long src_offset,
    uint16_t dst_port,
    unsigned int bytes_per_rep,
    unsigned long *reps,
    struct x86_emulate_ctxt *ctxt)
{
    assert(is_x86_user_segment(src_seg));
    assert(ctxt->addr_size == 64 || !(src_offset >> 32));

    return _fuzz_rep_write(ctxt, "rep_outs", reps);
}

static int fuzz_rep_stos(
    void *p_data,
    enum x86_segment seg,
    unsigned long offset,
    unsigned int bytes_per_rep,
    unsigned long *reps,
    struct x86_emulate_ctxt *ctxt)
{
    /*
     * STOS itself may only have an %es segment, but the stos() hook is reused
     * for CLZERO.
     */
    assert(is_x86_user_segment(seg));
    assert(ctxt->addr_size == 64 || !(offset >> 32));

    return _fuzz_rep_write(ctxt, "rep_stos", reps);
}

static int fuzz_write(
    enum x86_segment seg,
    unsigned long offset,
    void *p_data,
    unsigned int bytes,
    struct x86_emulate_ctxt *ctxt)
{
    /* Writes not expected for any system segments. */
    assert(is_x86_user_segment(seg));
    assert(ctxt->addr_size == 64 || !(offset >> 32));

    return maybe_fail(ctxt, "write", true);
}

static int fuzz_cmpxchg(
    enum x86_segment seg,
    unsigned long offset,
    void *old,
    void *new,
    unsigned int bytes,
    bool lock,
    struct x86_emulate_ctxt *ctxt)
{
    /*
     * Cmpxchg expected for user segments, and setting accessed/busy bits in
     * GDT/LDT enties, but not expected for any IDT or TR accesses.
     */
    if ( is_x86_user_segment(seg) )
        assert(ctxt->addr_size == 64 || !(offset >> 32));
    else
        assert((seg == x86_seg_gdtr || seg == x86_seg_ldtr) && !(offset >> 16));

    return maybe_fail(ctxt, "cmpxchg", true);
}

static int fuzz_tlb_op(
    enum x86emul_tlb_op op,
    unsigned long addr,
    unsigned long aux,
    struct x86_emulate_ctxt *ctxt)
{
    switch ( op )
    {
    case x86emul_invlpg:
        assert(is_x86_user_segment(aux));
        /* fall through */
    case x86emul_invlpga:
    case x86emul_invpcid:
        assert(ctxt->addr_size == 64 || !(addr >> 32));
        break;
    }

    return maybe_fail(ctxt, "TLB-management", false);
}

static int fuzz_cache_op(
    enum x86emul_cache_op op,
    enum x86_segment seg,
    unsigned long offset,
    struct x86_emulate_ctxt *ctxt)
{
    return maybe_fail(ctxt, "cache-management", true);
}

static int fuzz_write_io(
    unsigned int port,
    unsigned int bytes,
    unsigned long val,
    struct x86_emulate_ctxt *ctxt)
{
    return maybe_fail(ctxt, "write_io", true);
}

static int fuzz_read_segment(
    enum x86_segment seg,
    struct segment_register *reg,
    struct x86_emulate_ctxt *ctxt)
{
    const struct fuzz_state *s = ctxt->data;
    const struct fuzz_corpus *c = s->corpus;

    assert(is_x86_user_segment(seg) || is_x86_system_segment(seg));

    *reg = c->segments[seg];

    return X86EMUL_OKAY;
}

static int fuzz_write_segment(
    enum x86_segment seg,
    const struct segment_register *reg,
    struct x86_emulate_ctxt *ctxt)
{
    struct fuzz_state *s = ctxt->data;
    struct fuzz_corpus *c = s->corpus;
    int rc;

    assert(is_x86_user_segment(seg) || is_x86_system_segment(seg));

    rc = maybe_fail(ctxt, "write_segment", true);

    if ( rc == X86EMUL_OKAY )
    {
        struct segment_register old = c->segments[seg];

        c->segments[seg] = *reg;

        if ( !check_state(ctxt) )
        {
            c->segments[seg] = old;
            x86_emul_hw_exception(13 /* #GP */, 0, ctxt);
            rc = X86EMUL_EXCEPTION;
        }
    }

    return rc;
}

static int fuzz_read_cr(
    unsigned int reg,
    unsigned long *val,
    struct x86_emulate_ctxt *ctxt)
{
    const struct fuzz_state *s = ctxt->data;
    const struct fuzz_corpus *c = s->corpus;

    if ( reg >= ARRAY_SIZE(c->cr) )
        return X86EMUL_UNHANDLEABLE;

    *val = c->cr[reg];

    return X86EMUL_OKAY;
}

static int fuzz_write_cr(
    unsigned int reg,
    unsigned long val,
    struct x86_emulate_ctxt *ctxt)
{
    struct fuzz_state *s = ctxt->data;
    struct fuzz_corpus *c = s->corpus;
    unsigned long old;
    int rc;

    if ( reg >= ARRAY_SIZE(c->cr) )
        return X86EMUL_UNHANDLEABLE;

    rc = maybe_fail(ctxt, "write_cr", true);
    if ( rc != X86EMUL_OKAY )
        return rc;

    old = c->cr[reg];
    c->cr[reg] = val;

    if ( !check_state(ctxt) )
    {
        c->cr[reg] = old;
        x86_emul_hw_exception(13 /* #GP */, 0, ctxt);
        rc = X86EMUL_EXCEPTION;
    }

    return rc;
}

#define fuzz_read_xcr emul_test_read_xcr

enum {
    MSRI_IA32_SYSENTER_CS,
    MSRI_IA32_SYSENTER_ESP,
    MSRI_IA32_SYSENTER_EIP,
    MSRI_EFER,
    MSRI_STAR,
    MSRI_LSTAR,
    MSRI_CSTAR,
    MSRI_SYSCALL_MASK,
    MSRI_IA32_DEBUGCTLMSR,
};

static const unsigned int msr_index[MSR_INDEX_MAX] = {
    [MSRI_IA32_SYSENTER_CS]  = MSR_IA32_SYSENTER_CS,
    [MSRI_IA32_SYSENTER_ESP] = MSR_IA32_SYSENTER_ESP,
    [MSRI_IA32_SYSENTER_EIP] = MSR_IA32_SYSENTER_EIP,
    [MSRI_EFER]              = MSR_EFER,
    [MSRI_STAR]              = MSR_STAR,
    [MSRI_LSTAR]             = MSR_LSTAR,
    [MSRI_CSTAR]             = MSR_CSTAR,
    [MSRI_SYSCALL_MASK]      = MSR_SYSCALL_MASK,
    [MSRI_IA32_DEBUGCTLMSR]  = MSR_IA32_DEBUGCTLMSR,
};

static int fuzz_read_msr(
    unsigned int reg,
    uint64_t *val,
    struct x86_emulate_ctxt *ctxt)
{
    const struct fuzz_state *s = ctxt->data;
    const struct fuzz_corpus *c = s->corpus;
    unsigned int idx;

    switch ( reg )
    {
    case MSR_TSC_AUX:
    case MSR_IA32_TSC:
        /*
         * TSC should return monotonically increasing values, TSC_AUX
         * should preferably return consistent values, but returning
         * random values is fine in fuzzer.
         */
        return data_read(ctxt, x86_seg_none, "read_msr", val, sizeof(*val));
    case MSR_EFER:
        *val = c->msr[MSRI_EFER];
        *val &= ~EFER_LMA;
        if ( (*val & EFER_LME) && (c->cr[4] & X86_CR4_PAE) &&
             (c->cr[0] & X86_CR0_PG) )
        {
            printf("Setting EFER_LMA\n");
            *val |= EFER_LMA;
        }
        return X86EMUL_OKAY;
    }

    for ( idx = 0; idx < MSR_INDEX_MAX; idx++ )
    {
        if ( msr_index[idx] == reg )
        {
            *val = c->msr[idx];
            return X86EMUL_OKAY;
        }
    }

    x86_emul_hw_exception(13, 0, ctxt);
    return X86EMUL_EXCEPTION;
}

static int fuzz_write_msr(
    unsigned int reg,
    uint64_t val,
    struct x86_emulate_ctxt *ctxt)
{
    struct fuzz_state *s = ctxt->data;
    struct fuzz_corpus *c = s->corpus;
    unsigned int idx;
    int rc;

    rc = maybe_fail(ctxt, "write_msr", true);
    if ( rc != X86EMUL_OKAY )
        return rc;

    switch ( reg )
    {
    case MSR_TSC_AUX:
    case MSR_IA32_TSC:
        return X86EMUL_OKAY;
    }

    for ( idx = 0; idx < MSR_INDEX_MAX; idx++ )
    {
        if ( msr_index[idx] == reg )
        {
            uint64_t old = c->msr[idx];

            c->msr[idx] = val;

            if ( !check_state(ctxt) )
            {
                c->msr[idx] = old;
                break;
            }

            return X86EMUL_OKAY;
        }
    }

    x86_emul_hw_exception(13, 0, ctxt);
    return X86EMUL_EXCEPTION;
}

#define SET(h) .h = fuzz_##h
static const struct x86_emulate_ops all_fuzzer_ops = {
    SET(read),
    SET(insn_fetch),
    SET(write),
    SET(cmpxchg),
    SET(rep_ins),
    SET(rep_outs),
    SET(rep_movs),
    SET(rep_stos),
    SET(read_segment),
    SET(write_segment),
    SET(read_io),
    SET(write_io),
    SET(read_cr),
    SET(write_cr),
    SET(read_xcr),
    SET(read_msr),
    SET(write_msr),
    SET(cache_op),
    SET(tlb_op),
    .get_fpu    = emul_test_get_fpu,
    .put_fpu    = emul_test_put_fpu,
    .cpuid      = emul_test_cpuid,
};
#undef SET

static void setup_fpu_exception_handler(void)
{
    /* FIXME - just disable exceptions for now */
    unsigned long a;

    asm volatile ( "fnclex");
    a = 0x37f; /* FCW_DEFAULT in Xen */
    asm volatile ( "fldcw %0" :: "m" (a));
    a = 0x1f80; /* MXCSR_DEFAULT in Xen */
    asm volatile ( "ldmxcsr %0" :: "m" (a) );
}

static void dump_state(struct x86_emulate_ctxt *ctxt)
{
    struct fuzz_state *s = ctxt->data;
    const struct fuzz_corpus *c = s->corpus;
    struct cpu_user_regs *regs = ctxt->regs;
    uint64_t val = 0;

    printf(" -- State -- \n");
    printf("addr / sp size: %d / %d\n", ctxt->addr_size, ctxt->sp_size);
    printf(" cr0: %lx\n", c->cr[0]);
    printf(" cr3: %lx\n", c->cr[3]);
    printf(" cr4: %lx\n", c->cr[4]);

    printf(" rip: %"PRIx64"\n", regs->rip);

    fuzz_read_msr(MSR_EFER, &val, ctxt);
    printf("EFER: %"PRIx64"\n", val);
}

static bool long_mode_active(struct x86_emulate_ctxt *ctxt)
{
    uint64_t val;

    if ( fuzz_read_msr(MSR_EFER, &val, ctxt) != X86EMUL_OKAY )
        return false;

    return val & EFER_LMA;
}

static bool in_longmode(struct x86_emulate_ctxt *ctxt)
{
    const struct fuzz_state *s = ctxt->data;
    const struct fuzz_corpus *c = s->corpus;

    return long_mode_active(ctxt) && c->segments[x86_seg_cs].l;
}

static void set_sizes(struct x86_emulate_ctxt *ctxt)
{
    struct fuzz_state *s = ctxt->data;
    const struct fuzz_corpus *c = s->corpus;

    ctxt->lma = long_mode_active(ctxt);

    if ( in_longmode(ctxt) )
        ctxt->addr_size = ctxt->sp_size = 64;
    else
    {
        ctxt->addr_size = c->segments[x86_seg_cs].db ? 32 : 16;
        ctxt->sp_size   = c->segments[x86_seg_ss].db ? 32 : 16;
    }
}

#define CANONICALIZE(x, bits)                             \
    do {                                                  \
        uint64_t _y = (x);                                \
        if ( _y & (1ULL << ((bits) - 1)) )                \
            _y |= (~0ULL) << (bits);                      \
        else                                              \
            _y &= (1ULL << (bits)) - 1;                   \
        printf("Canonicalized %" PRIx64 " to %" PRIx64 "\n", x, _y);    \
        (x) = _y;                                       \
    } while( 0 )

/* Expects bitmap, regs, and c to be defined */
#define CANONICALIZE_MAYBE(reg)                       \
    if ( !(bitmap & (1 << CANONICALIZE_##reg)) )      \
        CANONICALIZE(regs->reg, c->cr[4] & X86_CR4_LA57 ? 57 : 48); \

enum {
    HOOK_read,
    HOOK_insn_fetch,
    HOOK_write,
    HOOK_cmpxchg,
    HOOK_rep_ins,
    HOOK_rep_outs,
    HOOK_rep_movs,
    HOOK_rep_stos,
    HOOK_read_segment,
    HOOK_write_segment,
    HOOK_read_io,
    HOOK_write_io,
    HOOK_read_cr,
    HOOK_write_cr,
    HOOK_read_dr,
    HOOK_write_dr,
    HOOK_read_xcr,
    HOOK_read_msr,
    HOOK_write_msr,
    HOOK_cache_op,
    HOOK_tlb_op,
    HOOK_cpuid,
    HOOK_inject_hw_exception,
    HOOK_inject_sw_interrupt,
    HOOK_get_fpu,
    HOOK_put_fpu,
    HOOK_vmfunc,
    CANONICALIZE_rip,
    CANONICALIZE_rsp,
};

/* Expects bitmap to be defined */
#define MAYBE_DISABLE_HOOK(h)                          \
    if ( bitmap & (1 << HOOK_##h) )                    \
    {                                                  \
        s->ops.h = NULL;                               \
        printf("Disabling hook "#h"\n");               \
    }

static void disable_hooks(struct x86_emulate_ctxt *ctxt)
{
    struct fuzz_state *s = ctxt->data;
    const struct fuzz_corpus *c = s->corpus;
    unsigned long bitmap = c->options;

    /* See also sanitize_input, some hooks can't be disabled. */
    MAYBE_DISABLE_HOOK(read);
    MAYBE_DISABLE_HOOK(insn_fetch);
    MAYBE_DISABLE_HOOK(write);
    MAYBE_DISABLE_HOOK(cmpxchg);
    MAYBE_DISABLE_HOOK(rep_ins);
    MAYBE_DISABLE_HOOK(rep_outs);
    MAYBE_DISABLE_HOOK(rep_movs);
    MAYBE_DISABLE_HOOK(rep_stos);
    MAYBE_DISABLE_HOOK(read_segment);
    MAYBE_DISABLE_HOOK(write_segment);
    MAYBE_DISABLE_HOOK(read_io);
    MAYBE_DISABLE_HOOK(write_io);
    MAYBE_DISABLE_HOOK(read_cr);
    MAYBE_DISABLE_HOOK(write_cr);
    MAYBE_DISABLE_HOOK(read_xcr);
    MAYBE_DISABLE_HOOK(read_msr);
    MAYBE_DISABLE_HOOK(write_msr);
    MAYBE_DISABLE_HOOK(cache_op);
    MAYBE_DISABLE_HOOK(tlb_op);
    MAYBE_DISABLE_HOOK(cpuid);
    MAYBE_DISABLE_HOOK(get_fpu);
}

/*
 * Constrain input to architecturally-possible states where
 * the emulator relies on these
 *
 * In general we want the emulator to be as absolutely robust as
 * possible; which means that we want to minimize the number of things
 * it assumes about the input state.  Tesing this means minimizing and
 * removing as much of the input constraints as possible.
 *
 * So we only add constraints that (in general) have been proven to
 * cause crashes in the emulator.
 *
 * For future reference: other constraints which might be necessary at
 * some point:
 *
 * - EFER.LMA => !EFLAGS.NT
 * - In VM86 mode, force segment...
 *  - ...access rights to 0xf3
 *  - ...limits to 0xffff
 *  - ...bases to below 1Mb, 16-byte aligned
 *  - ...selectors to (base >> 4)
 */
static void sanitize_input(struct x86_emulate_ctxt *ctxt)
{
    struct fuzz_state *s = ctxt->data;
    struct fuzz_corpus *c = s->corpus;
    struct cpu_user_regs *regs = &c->regs;
    unsigned long bitmap = c->options;

    /* Some hooks can't be disabled. */
    c->options &= ~((1<<HOOK_read)|(1<<HOOK_insn_fetch));

    /* Zero 'private' entries */
    regs->error_code = 0;
    regs->entry_vector = 0;

    /*
     * For both RIP and RSP make sure we test with canonical values in at
     * least a fair number of cases. As all other registers aren't tied to
     * special addressing purposes, leave everything else alone.
     */
    CANONICALIZE_MAYBE(rip);
    CANONICALIZE_MAYBE(rsp);

    /*
     * CR0.PG can't be set if CR0.PE isn't set.  Set is more interesting, so
     * set PE if PG is set.
     */
    if ( c->cr[0] & X86_CR0_PG )
        c->cr[0] |= X86_CR0_PE;

    /* EFLAGS.VM not available in long mode */
    if ( long_mode_active(ctxt) )
        regs->rflags &= ~X86_EFLAGS_VM;

    /* EFLAGS.VM implies 16-bit mode */
    if ( regs->rflags & X86_EFLAGS_VM )
    {
        c->segments[x86_seg_cs].db = 0;
        c->segments[x86_seg_ss].db = 0;
    }
}

/*
 * Call this function from hooks potentially altering machine state into
 * something that's not architecturally valid, yet which - as per above -
 * the emulator relies on.
 */
static bool check_state(struct x86_emulate_ctxt *ctxt)
{
    const struct fuzz_state *s = ctxt->data;
    const struct fuzz_corpus *c = s->corpus;
    const struct cpu_user_regs *regs = &c->regs;

    if ( long_mode_active(ctxt) && !(c->cr[0] & X86_CR0_PG) )
        return false;

    if ( (c->cr[0] & X86_CR0_PG) && !(c->cr[0] & X86_CR0_PE) )
        return false;

    if ( (regs->rflags & X86_EFLAGS_VM) &&
         (c->segments[x86_seg_cs].db || c->segments[x86_seg_ss].db) )
        return false;

    return true;
}

int LLVMFuzzerInitialize(int *argc, char ***argv)
{
    if ( !emul_test_init() )
    {
        printf("Warning: Stack could not be made executable (%d).\n", errno);
        return 1;
    }

    return 0;
}

int LLVMFuzzerTestOneInput(const uint8_t *data_p, size_t size)
{
    struct fuzz_state state = {
        .ops = all_fuzzer_ops,
    };
    struct x86_emulate_ctxt ctxt = {
        .data = &state,
        .regs = &input.regs,
        .addr_size = 8 * sizeof(void *),
        .sp_size = 8 * sizeof(void *),
    };
    int rc;

    /* Not part of the initializer, for old gcc to cope. */
    ctxt.cpu_policy = &cp;

    /* Reset all global state variables */
    memset(&input, 0, sizeof(input));

    if ( size <= DATA_OFFSET )
    {
        printf("Input too small\n");
        return 1;
    }

    if ( size > FUZZ_CORPUS_SIZE )
    {
        printf("Input too large\n");
        return 1;
    }

    memcpy(&input, data_p, size);

    state.corpus = &input;
    state.data_num = size - DATA_OFFSET;

    sanitize_input(&ctxt);

    disable_hooks(&ctxt);

    do {
        /* FIXME: Until we actually implement SIGFPE handling properly */
        setup_fpu_exception_handler();

        set_sizes(&ctxt);
        dump_state(&ctxt);

        rc = x86_emulate(&ctxt, &state.ops);
        printf("Emulation result: %d\n", rc);
    } while ( rc == X86EMUL_OKAY );

    return 0;
}

unsigned int fuzz_minimal_input_size(void)
{
    return DATA_OFFSET + 1;
}

/*
 * Local variables:
 * mode: C
 * c-file-style: "BSD"
 * c-basic-offset: 4
 * indent-tabs-mode: nil
 * End:
 */