UserspaceEmulator: Catch use-after-frees by tracking malloc/free :^)

This patch introduces a "MallocTracer" to the UserspaceEmulator.
If this object is present on the Emulator, it can be notified whenever
the emulated program does a malloc() or free().

The notifications come in via a magic instruction sequence that we
embed in the LibC malloc() and free() functions. The sequence is:

    "salc x2, push reg32 x2, pop reg32 x3"

The data about the malloc/free operation is in the three pushes.
We make sure the sequence is harmless when running natively.

Memory accesses on MmapRegion are then audited to see if they fall
inside a known-to-be-freed malloc chunk. If so, we complain loud
and red in the debugger output. :^)

This is very, very cool! :^)

It's also a whole lot slower than before, since now we're auditing
memory accesses against a new set of metadata. This will need to be
optimized (and running in this mode should be opt-in, perhaps even
a separate program, etc.)

DevTools/UserspaceEmulator/MallocTracer.cpp

@@ -0,0 +1,110 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "MallocTracer.h"
+#include "Emulator.h"
+#include <AK/LogStream.h>
+
+namespace UserspaceEmulator {
+
+static pid_t s_pid = getpid();
+
+MallocTracer::MallocTracer()
+{
+}
+
+void MallocTracer::target_did_malloc(Badge<SoftCPU>, FlatPtr address, size_t size)
+{
+    if (auto* existing_mallocation = find_mallocation(address)) {
+        ASSERT(existing_mallocation->freed);
+        existing_mallocation->size = size;
+        existing_mallocation->freed = false;
+        return;
+    }
+    m_mallocations.append({ address, size });
+}
+
+void MallocTracer::target_did_free(Badge<SoftCPU>, FlatPtr address)
+{
+    for (auto& mallocation : m_mallocations) {
+        if (mallocation.address == address) {
+            mallocation.freed = true;
+            return;
+        }
+    }
+    ASSERT_NOT_REACHED();
+}
+
+MallocTracer::Mallocation* MallocTracer::find_mallocation(FlatPtr address)
+{
+    for (auto& mallocation : m_mallocations) {
+        if (mallocation.contains(address))
+            return &mallocation;
+    }
+    return nullptr;
+}
+
+void MallocTracer::audit_read(FlatPtr address, size_t size)
+{
+    if (Emulator::the().is_in_malloc_or_free())
+        return;
+
+    auto* mallocation = find_mallocation(address);
+    if (!mallocation)
+        return;
+
+    size_t offset_into_mallocation = address - mallocation->address;
+
+    if (mallocation->freed) {
+        dbgprintf("\n");
+        dbgprintf("==%d==  \033[31;1mUAF\033[0m, invalid %zu-byte read at address %p\n", s_pid, size, address);
+        dbgprintf("==%d==  Address is %zu bytes into freed block of size %zu\n", s_pid, offset_into_mallocation, mallocation->size);
+        Emulator::the().dump_backtrace();
+        return;
+    }
+}
+
+void MallocTracer::audit_write(FlatPtr address, size_t size)
+{
+    if (Emulator::the().is_in_malloc_or_free())
+        return;
+
+    auto* mallocation = find_mallocation(address);
+    if (!mallocation)
+        return;
+
+    size_t offset_into_mallocation = address - mallocation->address;
+
+    if (mallocation->freed) {
+        dbgprintf("\n");
+        dbgprintf("==%d==   \033[31;1mUAF\033[0m, invalid %zu-byte write at address %p\n", s_pid, size, address);
+        dbgprintf("==%d==   Address is %zu bytes into freed block of size %zu\n", s_pid, offset_into_mallocation, mallocation->size);
+        Emulator::the().dump_backtrace();
+        return;
+    }
+}
+
+}