fuzz: Add DMA support to the generic-fuzzer

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 <alxndr@bu.edu>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Message-Id: <20201023150746.107063-5-alxndr@bu.edu>
[thuth: Reformatted one comment according to the QEMU coding style]
Signed-off-by: Thomas Huth <thuth@redhat.com>

include/exec/memory.h

@@ -42,6 +42,13 @@ typedef struct IOMMUMemoryRegionClass IOMMUMemoryRegionClass;
 DECLARE_OBJ_CHECKERS(IOMMUMemoryRegion, IOMMUMemoryRegionClass,
                      IOMMU_MEMORY_REGION, TYPE_IOMMU_MEMORY_REGION)
 
+#ifdef CONFIG_FUZZ
+void fuzz_dma_read_cb(size_t addr,
+                      size_t len,
+                      MemoryRegion *mr,
+                      bool is_write);
+#endif
+
 extern bool global_dirty_log;
 
 typedef struct MemoryRegionOps MemoryRegionOps;

tests/qtest/fuzz/generic_fuzz.c

@@ -25,6 +25,7 @@
 #include "exec/address-spaces.h"
 #include "hw/qdev-core.h"
 #include "hw/pci/pci.h"
+#include "hw/boards.h"
 
 /*
  * SEPARATOR is used to separate "operations" in the fuzz input
@@ -38,12 +39,16 @@ enum cmds {
     OP_WRITE,
     OP_PCI_READ,
     OP_PCI_WRITE,
+    OP_ADD_DMA_PATTERN,
+    OP_CLEAR_DMA_PATTERNS,
     OP_CLOCK_STEP,
 };
 
 #define DEFAULT_TIMEOUT_US 100000
 #define USEC_IN_SEC 1000000000
 
+#define MAX_DMA_FILL_SIZE 0x10000
+
 #define PCI_HOST_BRIDGE_CFG 0xcf8
 #define PCI_HOST_BRIDGE_DATA 0xcfc
 
@@ -56,6 +61,24 @@ static useconds_t timeout = DEFAULT_TIMEOUT_US;
 
 static bool qtest_log_enabled;
 
+/*
+ * 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 02   00 05 02   00 07 02 ...
+ */
+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.
@@ -84,6 +107,169 @@ static int get_io_address_cb(Int128 start, Int128 size,
     return 0;
 }
 
+/*
+ * 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;
+static 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;
+}
+
+static int memory_access_size(MemoryRegion *mr, unsigned l, hwaddr addr)
+{
+    unsigned access_size_max = mr->ops->valid.max_access_size;
+
+    /*
+     * Regions are assumed to support 1-4 byte accesses unless
+     * otherwise specified.
+     */
+    if (access_size_max == 0) {
+        access_size_max = 4;
+    }
+
+    /* Bound the maximum access by the alignment of the address.  */
+    if (!mr->ops->impl.unaligned) {
+        unsigned align_size_max = addr & -addr;
+        if (align_size_max != 0 && align_size_max < access_size_max) {
+            access_size_max = align_size_max;
+        }
+    }
+
+    /* Don't attempt accesses larger than the maximum.  */
+    if (l > access_size_max) {
+        l = access_size_max;
+    }
+    l = pow2floor(l);
+
+    return l;
+}
+
+/*
+ * 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
+ * generic_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 generic-fuzzer or are we using another fuzz-target? */
+    if (!qts_global) {
+        return;
+    }
+
+    /*
+     * 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.
+     */
+    address_range region;
+    bool double_fetch = false;
+    for (int i = 0;
+         i < dma_regions->len && (avoid_double_fetches || qtest_log_enabled);
+         ++i) {
+        region = g_array_index(dma_regions, address_range, i);
+        if (addr < region.addr + region.size && addr + len > region.addr) {
+            double_fetch = true;
+            if (addr < region.addr
+                && avoid_double_fetches) {
+                fuzz_dma_read_cb(addr, region.addr - addr, mr, is_write);
+            }
+            if (addr + len > region.addr + region.size
+                && avoid_double_fetches) {
+                fuzz_dma_read_cb(region.addr + region.size,
+                        addr + len - (region.addr + region.size), 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.size);
+    hwaddr l, addr1;
+    MemoryRegion *mr1;
+    uint8_t *ram_ptr;
+    while (len > 0) {
+        l = len;
+        mr1 = address_space_translate(first_cpu->as,
+                                      addr, &addr1, &l, true,
+                                      MEMTXATTRS_UNSPECIFIED);
+
+        if (!(memory_region_is_ram(mr1) ||
+              memory_region_is_romd(mr1))) {
+            l = memory_access_size(mr1, l, addr1);
+        } else {
+            /* ROM/RAM case */
+            ram_ptr = qemu_map_ram_ptr(mr1->ram_block, addr1);
+            memcpy(ram_ptr, buf, l);
+            break;
+        }
+        len -= l;
+        buf += l;
+        addr += l;
+
+    }
+    if (qtest_log_enabled) {
+        /*
+         * 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] ");
+        if (double_fetch) {
+            fprintf(stderr, "[DOUBLE-FETCH] ");
+        }
+        fflush(stderr);
+    }
+    qtest_memwrite(qts_global, ar.addr, buf, ar.size);
+    g_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
@@ -349,6 +535,35 @@ 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);
@@ -409,6 +624,8 @@ static void generic_fuzz(QTestState *s, const unsigned char *Data, size_t Size)
         [OP_WRITE]              = op_write,
         [OP_PCI_READ]           = op_pci_read,
         [OP_PCI_WRITE]          = op_pci_write,
+        [OP_ADD_DMA_PATTERN]    = op_add_dma_pattern,
+        [OP_CLEAR_DMA_PATTERNS] = op_clear_dma_patterns,
         [OP_CLOCK_STEP]         = op_clock_step,
     };
     const unsigned char *cmd = Data;
@@ -438,6 +655,8 @@ static void generic_fuzz(QTestState *s, const unsigned char *Data, size_t Size)
             setitimer(ITIMER_VIRTUAL, &timer, NULL);
         }
 
+        op_clear_dma_patterns(s, NULL, 0);
+
         while (cmd && Size) {
             /* Get the length until the next command or end of input */
             nextcmd = memmem(cmd, Size, SEPARATOR, strlen(SEPARATOR));
@@ -454,6 +673,7 @@ static void generic_fuzz(QTestState *s, const unsigned char *Data, size_t Size)
             /* Advance to the next command */
             cmd = nextcmd ? nextcmd + sizeof(SEPARATOR) - 1 : nextcmd;
             Size = Size - (cmd_len + sizeof(SEPARATOR) - 1);
+            g_array_set_size(dma_regions, 0);
         }
         _Exit(0);
     } else {
@@ -468,6 +688,9 @@ 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);
@@ -539,9 +762,16 @@ static void generic_pre_fuzz(QTestState *s)
     if (getenv("QTEST_LOG")) {
         qtest_log_enabled = 1;
     }
+    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_hash_table_new(NULL, NULL);
     fuzzable_pci_devices   = g_ptr_array_new();