Re: [PATCH 02/12] fuzz: Add general virtual-device fuzzer

[Alexander Bulekov]

On 200722 2339, Alexander Bulekov wrote:
> This is a generic fuzzer designed to fuzz a virtual device's
> MemoryRegions, as long as they exist within the Memory or Port IO (if it
> exists) AddressSpaces. The fuzzer's input is interpreted into a sequence
> of qtest commands (outb, readw, etc). The interpreted commands are
> separated by a magic seaparator, which should be easy for the fuzzer to
> guess. Without ASan, the separator can be specified as a "dictionary
> value" using the -dict argument (see libFuzzer documentation).
> 
> Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
> ---
>  tests/qtest/fuzz/Makefile.include |   1 +
>  tests/qtest/fuzz/general_fuzz.c   | 467 ++++++++++++++++++++++++++++++
>  2 files changed, 468 insertions(+)
>  create mode 100644 tests/qtest/fuzz/general_fuzz.c
> 
> diff --git a/tests/qtest/fuzz/Makefile.include 
> b/tests/qtest/fuzz/Makefile.include
> index 5bde793bf2..854322efb6 100644
> --- a/tests/qtest/fuzz/Makefile.include
> +++ b/tests/qtest/fuzz/Makefile.include
> @@ -11,6 +11,7 @@ fuzz-obj-y += tests/qtest/fuzz/qtest_wrappers.o
>  fuzz-obj-$(CONFIG_PCI_I440FX) += tests/qtest/fuzz/i440fx_fuzz.o
>  fuzz-obj-$(CONFIG_VIRTIO_NET) += tests/qtest/fuzz/virtio_net_fuzz.o
>  fuzz-obj-$(CONFIG_SCSI) += tests/qtest/fuzz/virtio_scsi_fuzz.o
> +fuzz-obj-y += tests/qtest/fuzz/general_fuzz.o
>  
>  FUZZ_CFLAGS += -I$(SRC_PATH)/tests -I$(SRC_PATH)/tests/qtest
>  
> diff --git a/tests/qtest/fuzz/general_fuzz.c b/tests/qtest/fuzz/general_fuzz.c
> new file mode 100644
> index 0000000000..fd92cc5bdf
> --- /dev/null
> +++ b/tests/qtest/fuzz/general_fuzz.c
> @@ -0,0 +1,467 @@
> +/*
> + * General Virtual-Device Fuzzing Target
> + *
> + * Copyright Red Hat Inc., 2020
> + *
> + * Authors:
> + *  Alexander Bulekov   <alxndr@bu.edu>
> + *
> + * This work is licensed under the terms of the GNU GPL, version 2 or later.
> + * See the COPYING file in the top-level directory.
> + */
> +
> +#include "qemu/osdep.h"
> +
> +#include <wordexp.h>
> +
> +#include "cpu.h"
> +#include "tests/qtest/libqtest.h"
> +#include "fuzz.h"
> +#include "fork_fuzz.h"
> +#include "exec/address-spaces.h"
> +#include "string.h"
> +#include "exec/memory.h"
> +#include "exec/ramblock.h"
> +#include "exec/address-spaces.h"
> +#include "hw/qdev-core.h"
> +
> +/*
> + * CMD_SEP is a random 32-bit value used to separate "commands" in the fuzz
> + * input
> + */
> +#define CMD_SEP "\x84\x05\x5C\x5E"
> +#define DEFAULT_TIMEOUT_US 100000
> +
> +typedef struct {
> +    size_t addr;
> +    size_t len; /* The number of bytes until the end of the I/O region */
> +} address_range;
> +
> +static useconds_t timeout = 100000;
> +/*
> + * List of memory regions that are children of QOM objects specified by the
> + * user for fuzzing.
> + */
> +static GPtrArray *fuzzable_memoryregions;
> +/*
> + * Here we want to convert a fuzzer-provided [io-region-index, offset] to
> + * a physical address. To do this, we iterate over all of the matched
> + * MemoryRegions. Check whether each region exists within the particular io
> + * space. Return the absolute address of the offset within the index'th 
> region
> + * that is a subregion of the io_space and the distance until the end of the
> + * memory region.
> + */
> +static bool get_io_address(address_range *result,
> +                                    MemoryRegion *io_space,
> +                                    uint8_t index,
> +                                    uint32_t offset) {
> +    MemoryRegion *mr, *root;
> +    index = index % fuzzable_memoryregions->len;
> +    int candidate_regions = 0;
> +    int i = 0;
> +    int ind = index;
> +    size_t abs_addr;
> +
> +    while (ind >= 0 && fuzzable_memoryregions->len) {
> +        *result = (address_range){0, 0};
> +        mr = g_ptr_array_index(fuzzable_memoryregions, i);
> +        if (mr->enabled) {
> +            abs_addr = mr->addr;
> +            for (root = mr; root->container; ) {
> +                root = root->container;
> +                abs_addr += root->addr;
> +            }
> +            /*
> +             * Only consider the region if it is rooted at the io_space we 
> want
> +             */
> +            if (root == io_space) {

Theres a problem here. This finds an aboslute address for an index
+ offset in our fuzzable_memory_regions array, but doesn't check that
the MemoryRegion has the highest priority for that address.
I think the way to solve this is to do the opposite
address_space_translate and ensure that the MemoryRegion* we get back is
the same MemoryRegion* that is in our fuzzable_memory_regions array.

Only noticed this as I was trying to fuzz an audio device and saw that
by fuzzing the device's PCI space the fuzzer would set the BAR over an
existing higher-priority device and the fuzzer was exercising code for
that device.
-Alex

> +                ind--;
> +                candidate_regions++;
> +                result->addr = abs_addr + (offset % mr->size);
> +                result->len = mr->size - (offset % mr->size);
> +            }
> +        }
> +        ++i;
> +        /* Loop around */
> +        if (i == fuzzable_memoryregions->len) {
> +            /* No enabled regions in our io_space? */
> +            if (candidate_regions == 0) {
> +                break;
> +            }
> +            i = 0;
> +        }
> +    }
> +    return candidate_regions != 0;
> +}
> +static bool get_pio_address(address_range *result,
> +                                     uint8_t index, uint16_t offset)
> +{
> +    /*
> +     * PIO BARs can be set past the maximum port address (0xFFFF). Thus, 
> result
> +     * can contain an addr that extends past the PIO space. When we pass this
> +     * address to qtest_in/qtest_out, it is cast to a uint16_t, so we might 
> end
> +     * up fuzzing a completely different MemoryRegion/Device. Therefore, 
> check
> +     * that the address here is within the PIO space limits.
> +     */
> +
> +    bool success = get_io_address(result, get_system_io(), index, offset);
> +    return success && result->addr <= 0xFFFF;
> +}
> +static bool get_mmio_address(address_range *result,
> +                                      uint8_t index, uint32_t offset)
> +{
> +    return get_io_address(result, get_system_memory(), index, offset);
> +}
> +
> +static void op_in(QTestState *s, const unsigned char * data, size_t len)
> +{
> +    enum Sizes {Byte, Word, Long, end_sizes};
> +    struct {
> +        uint8_t size;
> +        uint8_t base;
> +        uint16_t offset;
> +    } a;
> +    address_range abs;
> +
> +    if (len < sizeof(a)) {
> +        return;
> +    }
> +    memcpy(&a, data, sizeof(a));
> +    if (get_pio_address(&abs, a.base, a.offset) == 0) {
> +        return;
> +    }
> +
> +    switch (a.size %= end_sizes) {
> +    case Byte:
> +        qtest_inb(s, abs.addr);
> +        break;
> +    case Word:
> +        if (abs.len >= 2) {
> +            qtest_inw(s, abs.addr);
> +        }
> +        break;
> +    case Long:
> +        if (abs.len >= 4) {
> +            qtest_inl(s, abs.addr);
> +        }
> +        break;
> +    }
> +}
> +
> +static void op_out(QTestState *s, const unsigned char * data, size_t len)
> +{
> +    enum Sizes {Byte, Word, Long, end_sizes};
> +    struct {
> +        uint8_t size;
> +        uint8_t base;
> +        uint16_t offset;
> +        uint32_t value;
> +    } a;
> +    address_range abs;
> +
> +    if (len < sizeof(a)) {
> +        return;
> +    }
> +    memcpy(&a, data, sizeof(a));
> +
> +    if (get_pio_address(&abs, a.base, a.offset) == 0) {
> +        return;
> +    }
> +
> +    switch (a.size %= end_sizes) {
> +    case Byte:
> +        qtest_outb(s, abs.addr, a.value & 0xFF);
> +        break;
> +    case Word:
> +        if (abs.len >= 2) {
> +            qtest_outw(s, abs.addr, a.value & 0xFFFF);
> +        }
> +        break;
> +    case Long:
> +        if (abs.len >= 4) {
> +            qtest_outl(s, abs.addr, a.value);
> +        }
> +        break;
> +    }
> +}
> +
> +static void op_read(QTestState *s, const unsigned char * data, size_t len)
> +{
> +    enum Sizes {Byte, Word, Long, Quad, end_sizes};
> +    struct {
> +        uint8_t size;
> +        uint8_t base;
> +        uint32_t offset;
> +    } a;
> +    address_range abs;
> +
> +    if (len < sizeof(a)) {
> +        return;
> +    }
> +    memcpy(&a, data, sizeof(a));
> +
> +    if (get_mmio_address(&abs, a.base, a.offset) == 0) {
> +        return;
> +    }
> +
> +    switch (a.size %= end_sizes) {
> +    case Byte:
> +        qtest_readb(s, abs.addr);
> +        break;
> +    case Word:
> +        if (abs.len >= 2) {
> +            qtest_readw(s, abs.addr);
> +        }
> +        break;
> +    case Long:
> +        if (abs.len >= 4) {
> +            qtest_readl(s, abs.addr);
> +        }
> +        break;
> +    case Quad:
> +        if (abs.len >= 8) {
> +            qtest_readq(s, abs.addr);
> +        }
> +        break;
> +    }
> +}
> +
> +static void op_write(QTestState *s, const unsigned char * data, size_t len)
> +{
> +    enum Sizes {Byte, Word, Long, Quad, end_sizes};
> +    struct {
> +        uint8_t size;
> +        uint8_t base;
> +        uint32_t offset;
> +        uint64_t value;
> +    } a;
> +    address_range abs;
> +
> +    if (len < sizeof(a)) {
> +        return;
> +    }
> +    memcpy(&a, data, sizeof(a));
> +
> +    if (get_mmio_address(&abs, a.base, a.offset) == 0) {
> +        return;
> +    }
> +
> +    switch (a.size %= end_sizes) {
> +    case Byte:
> +            qtest_writeb(s, abs.addr, a.value & 0xFF);
> +        break;
> +    case Word:
> +        if (abs.len >= 2) {
> +            qtest_writew(s, abs.addr, a.value & 0xFFFF);
> +        }
> +        break;
> +    case Long:
> +        if (abs.len >= 4) {
> +            qtest_writel(s, abs.addr, a.value & 0xFFFFFFFF);
> +        }
> +        break;
> +    case Quad:
> +        if (abs.len >= 8) {
> +            qtest_writeq(s, abs.addr, a.value);
> +        }
> +        break;
> +    }
> +}
> +static void op_clock_step(QTestState *s, const unsigned char *data, size_t 
> len)
> +{
> +    qtest_clock_step_next(s);
> +}
> +
> +static void handle_timeout(int sig)
> +{
> +    if (getenv("QTEST_LOG")) {
> +        fprintf(stderr, "[Timeout]\n");
> +        fflush(stderr);
> +    }
> +    _Exit(0);
> +}
> +
> +/*
> + * Here, we interpret random bytes from the fuzzer, as a sequence of 
> commands.
> + * Our commands are variable-width, so we use a separator, CMD_SEP, to 
> specify
> + * the boundaries between commands. This is just a random 32-bit value, which
> + * is easily identified by libfuzzer+AddressSanitizer, as long as we use
> + * memmem. It can also be included in the fuzzer's dictionary. More details
> + * here:
> + * https://github.com/google/fuzzing/blob/master/docs/split-inputs.md
> + *
> + * As a result, the stream of bytes is converted into a sequence of commands.
> + * In a simplified example where CMD_SEP is 0xFF:
> + * 00 01 02 FF 03 04 05 06 FF 01 FF ...
> + * becomes this sequence of commands:
> + * 00 01 02    -> op00 (0102)   -> in (0102, 2)
> + * 03 04 05 06 -> op03 (040506) -> write (040506, 3)
> + * 01          -> op01 (-,0)    -> out (-,0)
> + * ...
> + *
> + * Note here that it is the job of the individual opcode functions to check
> + * that enough data was provided. I.e. in the last command out (,0), out 
> needs
> + * to check that there is not enough data provided to select an address/value
> + * for the operation.
> + */
> +static void general_fuzz(QTestState *s, const unsigned char *Data, size_t 
> Size)
> +{
> +    void (*ops[]) (QTestState *s, const unsigned char* , size_t) = {
> +        op_in,
> +        op_out,
> +        op_read,
> +        op_write,
> +        op_clock_step,
> +    };
> +    const unsigned char *cmd = Data;
> +    const unsigned char *nextcmd;
> +    size_t cmd_len;
> +    uint8_t op;
> +
> +    if (fork() == 0) {
> +        /*
> +         * Sometimes the fuzzer will find inputs that take quite a long time 
> to
> +         * process. Often times, these inputs do not result in new coverage.
> +         * Even if these inputs might be interesting, they can slow down the
> +         * fuzzer, overall. Set a timeout to avoid hurting performance, too 
> much
> +         */
> +        if (timeout) {
> +            struct sigaction sact;
> +            sigemptyset(&sact.sa_mask);
> +            sact.sa_flags = 0;
> +            sact.sa_handler = handle_timeout;
> +            sigaction(SIGALRM, &sact, NULL);
> +            ualarm(timeout, 0);
> +        }
> +
> +        while (cmd && Size) {
> +            /* Get the length until the next command or end of input */
> +            nextcmd = memmem(cmd, Size, CMD_SEP, strlen(CMD_SEP));
> +            cmd_len = nextcmd ? nextcmd - cmd : Size;
> +
> +            if (cmd_len > 0) {
> +                /* Interpret the first byte of the command as an opcode */
> +                op = *cmd % (sizeof(ops) / sizeof((ops)[0]));
> +                ops[op](s, cmd + 1, cmd_len - 1);
> +
> +                /* Run the main loop */
> +                flush_events(s);
> +            }
> +            /* Advance to the next command */
> +            cmd = nextcmd ? nextcmd + sizeof(CMD_SEP) - 1 : nextcmd;
> +            Size = Size - (cmd_len + sizeof(CMD_SEP) - 1);
> +        }
> +        _Exit(0);
> +    } else {
> +        flush_events(s);
> +        wait(NULL);
> +    }
> +}
> +
> +static void usage(void)
> +{
> +    printf("Please specify the following environment variables:\n");
> +    printf("QEMU_FUZZ_ARGS= the command line arguments passed to qemu\n");
> +    printf("QEMU_FUZZ_OBJECTS= "
> +            "a space separated list of QOM type names for objects to 
> fuzz\n");
> +    printf("Optionally: QEMU_FUZZ_TIMEOUT= Specify a custom timeout (us). "
> +            "0 to disable. %d by default\n", timeout);
> +    exit(0);
> +}
> +
> +static int locate_fuzz_memory_regions(Object *child, void *opaque)
> +{
> +    const char *name;
> +    MemoryRegion *mr;
> +    if (object_dynamic_cast(child, TYPE_MEMORY_REGION)) {
> +        mr = MEMORY_REGION(child);
> +        if ((memory_region_is_ram(mr) ||
> +            memory_region_is_ram_device(mr) ||
> +            memory_region_is_rom(mr) ||
> +            memory_region_is_romd(mr)) == false) {
> +            name = object_get_canonical_path_component(child);
> +            /*
> +             * We don't want duplicate pointers to the same MemoryRegion, so
> +             * try to remove copies of the pointer, before adding it.
> +             */
> +            g_ptr_array_remove_fast(fuzzable_memoryregions, mr);
> +            g_ptr_array_add(fuzzable_memoryregions, mr);
> +        }
> +    }
> +    return 0;
> +}
> +static int locate_fuzz_objects(Object *child, void *opaque)
> +{
> +    char *pattern = opaque;
> +    if (g_pattern_match_simple(pattern, object_get_typename(child))) {
> +        printf("Matched Object by Type: %s\n", object_get_typename(child));
> +        /* Find and save ptrs to any child MemoryRegions */
> +        object_child_foreach_recursive(child, locate_fuzz_memory_regions, 
> NULL);
> +    } else if (object_dynamic_cast(OBJECT(child), TYPE_MEMORY_REGION)) {
> +        if (g_pattern_match_simple(pattern,
> +            object_get_canonical_path_component(child))) {
> +            MemoryRegion *mr;
> +            mr = MEMORY_REGION(child);
> +            if ((memory_region_is_ram(mr) ||
> +                 memory_region_is_ram_device(mr) ||
> +                 memory_region_is_rom(mr) ||
> +                 memory_region_is_romd(mr)) == false) {
> +                g_ptr_array_remove_fast(fuzzable_memoryregions, mr);
> +                g_ptr_array_add(fuzzable_memoryregions, mr);
> +            }
> +        }
> +    }
> +    return 0;
> +}
> +
> +static void general_pre_fuzz(QTestState *s)
> +{
> +    if (!getenv("QEMU_FUZZ_OBJECTS")) {
> +        usage();
> +    }
> +    if (getenv("QEMU_FUZZ_TIMEOUT")) {
> +        timeout = g_ascii_strtoll(getenv("QEMU_FUZZ_TIMEOUT"), NULL, 0);
> +    }
> +
> +    fuzzable_memoryregions = g_ptr_array_new();
> +    wordexp_t result;
> +    wordexp(getenv("QEMU_FUZZ_OBJECTS"), &result, 0);
> +    for (int i = 0; i < result.we_wordc; i++) {
> +        object_child_foreach_recursive(qdev_get_machine(),
> +                                    locate_fuzz_objects,
> +                                    result.we_wordv[i]);
> +    }
> +
> +    printf("This process will try to fuzz the following MemoryRegions:\n");
> +    for (int i = 0; i < fuzzable_memoryregions->len; i++) {
> +        MemoryRegion *mr;
> +        mr = g_ptr_array_index(fuzzable_memoryregions, i);
> +        printf("  * %s (size %lx)\n",
> +               object_get_canonical_path_component(&(mr->parent_obj)),
> +               mr->addr);
> +    }
> +    counter_shm_init();
> +}
> +static GString *general_fuzz_cmdline(FuzzTarget *t)
> +{
> +    GString *cmd_line = g_string_new(TARGET_NAME);
> +    if (!getenv("QEMU_FUZZ_ARGS")) {
> +        usage();
> +    }
> +    g_string_append_printf(cmd_line, " -display none \
> +                                      -machine accel=qtest, \
> +                                      -m 64 %s ", getenv("QEMU_FUZZ_ARGS"));
> +    return cmd_line;
> +}
> +
> +static void register_general_fuzz_targets(void)
> +{
> +    fuzz_add_target(&(FuzzTarget){
> +            .name = "general-fuzz",
> +            .description = "Fuzz based on any qemu command-line args. ",
> +            .get_init_cmdline = general_fuzz_cmdline,
> +            .pre_fuzz = general_pre_fuzz,
> +            .fuzz = general_fuzz});
> +}
> +
> +fuzz_target_init(register_general_fuzz_targets);
> -- 
> 2.27.0
>