When a virtual-device tries to access some buffer in memory over DMA, we
add call-backs into the fuzzer(next commit). The fuzzer checks verifies
that the DMA request maps to a physical RAM address and fills the memory
with fuzzer-provided data. The patterns that we use to fill this memory
are specified using add_dma_pattern and clear_dma_patterns operations.
Signed-off-by: Alexander Bulekov <alx...@bu.edu>
---
tests/qtest/fuzz/general_fuzz.c | 177 ++++++++++++++++++++++++++++++++
1 file changed, 177 insertions(+)
diff --git a/tests/qtest/fuzz/general_fuzz.c b/tests/qtest/fuzz/general_fuzz.c
index e715b77d59..4b6967c5d2 100644
--- a/tests/qtest/fuzz/general_fuzz.c
+++ b/tests/qtest/fuzz/general_fuzz.c
@@ -27,6 +27,7 @@
#include "tests/qtest/libqos/pci.h"
#include "tests/qtest/libqos/pci-pc.h"
#include "hw/pci/pci.h"
+#include "hw/boards.h"
/*
* CMD_SEP is a random 32-bit value used to separate "commands" in the fuzz
@@ -34,6 +35,7 @@
*/
#define CMD_SEP "\x84\x05\x5C\x5E"
#define DEFAULT_TIMEOUT_US 100000
+#define MAX_DMA_FILL_SIZE 0x10000
#define PCI_HOST_BRIDGE_CFG 0xcf8
#define PCI_HOST_BRIDGE_DATA 0xcfc
@@ -44,6 +46,24 @@ typedef struct {
} address_range;
static useconds_t timeout = 100000;
+/*
+ * A pattern used to populate a DMA region or perform a memwrite. This is
+ * useful for e.g. populating tables of unique addresses.
+ * Example {.index = 1; .stride = 2; .len = 3; .data = "\x00\x01\x02"}
+ * Renders as: 00 01 02 00 03 03 00 05 03 00 07 03 ...
+ */
+typedef struct {
+ uint8_t index; /* Index of a byte to increment by stride */
+ uint8_t stride; /* Increment each index'th byte by this amount */
+ size_t len;
+ const uint8_t *data;
+} pattern;
+
+/* Avoid filling the same DMA region between MMIO/PIO commands ? */
+static bool avoid_double_fetches;
+
+static QTestState *qts_global; /* Need a global for the DMA callback */
+
/*
* List of memory regions that are children of QOM objects specified by the
* user for fuzzing.
@@ -51,6 +71,122 @@ static useconds_t timeout = 100000;
static GPtrArray *fuzzable_memoryregions;
static GPtrArray *fuzzable_pci_devices;
+/*
+ * List of dma regions populated since the last fuzzing command. Used to ensure
+ * that we only write to each DMA address once, to avoid race conditions when
+ * building reproducers.
+ */
+static GArray *dma_regions;
+
+static GArray *dma_patterns;
+int dma_pattern_index;
+
+/*
+ * Allocate a block of memory and populate it with a pattern.
+ */
+static void *pattern_alloc(pattern p, size_t len)
+{
+ int i;
+ uint8_t *buf = g_malloc(len);
+ uint8_t sum = 0;
+
+ for (i = 0; i < len; ++i) {
+ buf[i] = p.data[i % p.len];
+ if ((i % p.len) == p.index) {
+ buf[i] += sum;
+ sum += p.stride;
+ }
+ }
+ return buf;
+}
+
+/*
+ * Call-back for functions that perform DMA reads from guest memory. Confirm
+ * that the region has not already been populated since the last loop in
+ * general_fuzz(), avoiding potential race-conditions, which we don't have
+ * a good way for reproducing right now.
+ */
+void fuzz_dma_read_cb(size_t addr, size_t len, MemoryRegion *mr, bool is_write)
+{
+ /* Are we in the general-fuzzer or are we using another fuzz-target? */
+ if (!qts_global) {
+ return;
+ }
+
+ /*
+ * If the device is trying to read from a ROM, exit early. We do not want
+ * to fuzz devices using data that we have no control over.
+ */
+ if (mr->readonly) {
+ _Exit(0);
+ }
+
+ /*
+ * Return immediately if:
+ * - We have no DMA patterns defined
+ * - The length of the DMA read request is zero
+ * - The DMA read is hitting an MR other than the machine's main RAM
+ * - The DMA request is not a read (what happens for a address_space_map
+ * with is_write=True? Can the device use the same pointer to do reads?)
+ * - The DMA request hits past the bounds of our RAM
+ */
+ if (dma_patterns->len == 0
+ || len == 0
+ || mr != MACHINE(qdev_get_machine())->ram
+ || is_write
+ || addr > current_machine->ram_size) {
+ return;
+ }
+
+ /*
+ * If we overlap with any existing dma_regions, split the range and only
+ * populate the non-overlapping parts.
+ */
+ for (int i = 0; i < dma_regions->len && !avoid_double_fetches; ++i) {
+ address_range region = g_array_index(dma_regions, address_range, i);
+ if (addr < region.addr + region.len && addr + len > region.addr) {
+ if (addr < region.addr) {
+ fuzz_dma_read_cb(addr, region.addr - addr, mr, is_write);
+ }
+ if (addr + len > region.addr + region.len) {
+ fuzz_dma_read_cb(region.addr + region.len,
+ addr + len - (region.addr + region.len), mr, is_write);
+ }
+ return;
+ }
+ }
+
+ /* Cap the length of the DMA access to something reasonable */
+ len = MIN(len, MAX_DMA_FILL_SIZE);
+
+ address_range ar = {addr, len};
+ g_array_append_val(dma_regions, ar);
+ pattern p = g_array_index(dma_patterns, pattern, dma_pattern_index);
+ void *buf = pattern_alloc(p, ar.len);
+ if (getenv("QTEST_LOG")) {
+ /*
+ * With QTEST_LOG, use a normal, slow QTest memwrite. Prefix the log
+ * that will be written by qtest.c with a DMA tag, so we can reorder
+ * the resulting QTest trace so the DMA fills precede the last PIO/MMIO
+ * command.
+ */
+ fprintf(stderr, "[DMA] ");
+ fflush(stderr);
+ qtest_memwrite(qts_global, ar.addr, buf, ar.len);
+ } else {
+ /*
+ * Populate the region using address_space_write_rom to avoid writing to
+ * any IO MemoryRegions
+ */
+ address_space_write_rom(first_cpu->as, ar.addr, MEMTXATTRS_UNSPECIFIED,
+ buf, ar.len);
+ }
+ free(buf);
+
+ /* Increment the index of the pattern for the next DMA access */
+ dma_pattern_index = (dma_pattern_index + 1) % dma_patterns->len;
+}
+
/*
* 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
@@ -334,6 +470,35 @@ static void op_pci_write(QTestState *s, const unsigned
char * data, size_t len)
}
}
+static void op_add_dma_pattern(QTestState *s,
+ const unsigned char *data, size_t len)
+{
+ struct {
+ /*
+ * index and stride can be used to increment the index-th byte of the
+ * pattern by the value stride, for each loop of the pattern.
+ */
+ uint8_t index;
+ uint8_t stride;
+ } a;
+
+ if (len < sizeof(a) + 1) {
+ return;
+ }
+ memcpy(&a, data, sizeof(a));
+ pattern p = {a.index, a.stride, len - sizeof(a), data + sizeof(a)};
+ p.index = a.index % p.len;
+ g_array_append_val(dma_patterns, p);
+ return;
+}
+
+static void op_clear_dma_patterns(QTestState *s,
+ const unsigned char *data, size_t len)
+{
+ g_array_set_size(dma_patterns, 0);
+ dma_pattern_index = 0;
+}
+
static void op_clock_step(QTestState *s, const unsigned char *data, size_t len)
{
qtest_clock_step_next(s);
@@ -380,6 +545,8 @@ static void general_fuzz(QTestState *s, const unsigned char
*Data, size_t Size)
op_write,
op_pci_read,
op_pci_write,
+ op_add_dma_pattern,
+ op_clear_dma_patterns,
op_clock_step,
};
const unsigned char *cmd = Data;
@@ -433,6 +600,9 @@ static void usage(void)
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_AVOID_DOUBLE_FETCH= "
+ "Try to avoid racy DMA double fetch bugs? %d by default\n",
+ avoid_double_fetches);
printf("Optionally: QEMU_FUZZ_TIMEOUT= Specify a custom timeout (us). "
"0 to disable. %d by default\n", timeout);
exit(0);
@@ -502,9 +672,16 @@ static void general_pre_fuzz(QTestState *s)
if (!getenv("QEMU_FUZZ_OBJECTS")) {
usage();
}
+ if (getenv("QEMU_AVOID_DOUBLE_FETCH")) {
+ avoid_double_fetches = 1;
+ }
if (getenv("QEMU_FUZZ_TIMEOUT")) {
timeout = g_ascii_strtoll(getenv("QEMU_FUZZ_TIMEOUT"), NULL, 0);
}
+ qts_global = s;
+
+ dma_regions = g_array_new(false, false, sizeof(address_range));
+ dma_patterns = g_array_new(false, false, sizeof(pattern));
fuzzable_memoryregions = g_ptr_array_new();
fuzzable_pci_devices = g_ptr_array_new();
--
2.27.0
