Prevent invalid SaveData writes from damaging guest memory (#444)

Add an optional write monitor so the team can find future memory damage on each supported desktop system.
2026-07-31 23:19:44 +08:00 · 2026-07-19 12:27:17 -05:00
parent d7f6e3f578
commit bc51cc2c4d
8 changed files with 437 additions and 21 deletions
@@ -0,0 +1,203 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+using System.Buffers.Binary;
+using System.Globalization;
+using System.Threading;
+
+namespace SharpEmu.HLE;
+
+// This tool monitors guest-memory writes only when a watch mode is active.
+public static class GuestWriteWatch
+{
+    private const ulong WatchBytes = 8;
+    private const int MaxBulkReports = 64;
+
+    private static readonly ulong WatchBase = Parse(
+        Environment.GetEnvironmentVariable("SHARPEMU_WATCH_WRITE"));
+
+    private static readonly bool WatchPoolHeaders = IsEnabled("SHARPEMU_WATCH_POOL_HEADER");
+
+    private static readonly ulong[] PoolSlots = new ulong[64];
+    private static int _poolSlotCount;
+
+    private static readonly bool WatchValuePattern = IsEnabled("SHARPEMU_WATCH_VALUE_PATTERN");
+
+    private static readonly bool WatchValue1 = IsEnabled("SHARPEMU_WATCH_VALUE1");
+
+    private const ulong DirectBandLow = 0x100_0000_0000;
+    private const ulong DirectBandHigh = 0x1000_0000_0000;
+    private static int _value1Reports;
+
+    private static readonly bool WatchBulkTorn = IsEnabled("SHARPEMU_WATCH_BULK_TORN");
+
+    private static readonly ulong BulkDestHigh = Parse(
+        Environment.GetEnvironmentVariable("SHARPEMU_WATCH_BULK_DEST_HI"));
+    private static int _bulkTornReports;
+    private static int _bulkShiftReports;
+
+    public static bool Armed =>
+        WatchBase != 0 || WatchPoolHeaders || WatchValuePattern || WatchValue1 || WatchBulkTorn;
+
+    public static void OnDirectMapping(ulong mappedAddress, ulong length, int protection)
+    {
+        if (!WatchPoolHeaders || !IsPoolMapping(length, protection))
+        {
+            return;
+        }
+
+        var index = Interlocked.Increment(ref _poolSlotCount) - 1;
+        if (index < PoolSlots.Length)
+        {
+            Volatile.Write(ref PoolSlots[index], mappedAddress + 0x40);
+            Console.Error.WriteLine(
+                $"[LOADER][WARN] watch_write armed on pool header slot 0x{mappedAddress + 0x40:X16}");
+        }
+    }
+
+    public static void Check(ulong address, ReadOnlySpan<byte> data)
+    {
+        if (WatchBulkTorn &&
+            data.Length >= 8 &&
+            (BulkDestHigh != 0
+                ? (address >> 32) == BulkDestHigh
+                : address >= DirectBandLow && address < DirectBandHigh))
+        {
+            for (var offset = FirstAlignedOffset(address); offset + 8 <= data.Length; offset += 8)
+            {
+                var qword = BinaryPrimitives.ReadUInt64LittleEndian(data.Slice(offset, 8));
+                var kind = ClassifyBulkValue(qword);
+                if (kind is not null && ReserveBulkReport(kind))
+                {
+                    Console.Error.WriteLine(
+                        $"[LOADER][WARN] watch_bulk_torn HIT ({kind}) " +
+                        $"dest=0x{address + (ulong)offset:X16} (base=0x{address:X16}+0x{offset:X}) " +
+                        $"len={data.Length} qword=0x{qword:X16}{Environment.NewLine}{Environment.StackTrace}");
+                    Console.Error.Flush();
+                    return;
+                }
+            }
+        }
+
+        if (WatchValue1 &&
+            address >= DirectBandLow && address < DirectBandHigh &&
+            data.Length is >= 1 and <= 8 &&
+            LittleEndianValue(data) == 1 &&
+            Interlocked.Increment(ref _value1Reports) <= 128)
+        {
+            Report(address, data);
+            return;
+        }
+
+        if (WatchValuePattern && data.Length == 8)
+        {
+            var value = BinaryPrimitives.ReadUInt64LittleEndian(data);
+            if ((value & 0xFFFFFFFF) == 1 && value >> 32 is > 0 and <= 0xFFFF)
+            {
+                Report(address, data);
+                return;
+            }
+        }
+
+        if (WatchBase != 0 && Overlaps(address, data.Length, WatchBase))
+        {
+            Report(address, data);
+            return;
+        }
+
+        var slots = Math.Min(Volatile.Read(ref _poolSlotCount), PoolSlots.Length);
+        for (var i = 0; i < slots; i++)
+        {
+            var slot = Volatile.Read(ref PoolSlots[i]);
+            if (slot != 0 && Overlaps(address, data.Length, slot))
+            {
+                Report(address, data);
+                return;
+            }
+        }
+    }
+
+    internal static string? ClassifyBulkValue(ulong qword)
+    {
+        var low32 = qword & 0xFFFFFFFF;
+        var high32 = qword >> 32;
+        if (low32 == 1 && high32 is > 0 and <= 0xFFFF)
+        {
+            return "torn";
+        }
+
+        var prefix = low32 & 0xFF00_0000;
+        var hasShiftedPointerPrefix = prefix is 0x0800_0000 or 0x8000_0000;
+        return high32 == 0 && hasShiftedPointerPrefix && (low32 & 0xFF) == 0
+            ? "shift"
+            : null;
+    }
+
+    internal static int FirstAlignedOffset(ulong address) =>
+        (int)((8 - (address & 7)) & 7);
+
+    internal static bool IsPoolMapping(ulong length, int protection) =>
+        length == 0x10000 && protection == 0xF2;
+
+    internal static bool Overlaps(ulong address, int length, ulong slot)
+    {
+        if (length <= 0)
+        {
+            return false;
+        }
+
+        var writeLength = (ulong)length - 1;
+        var writeEnd = address > ulong.MaxValue - writeLength
+            ? ulong.MaxValue
+            : address + writeLength;
+        var slotEnd = slot > ulong.MaxValue - (WatchBytes - 1)
+            ? ulong.MaxValue
+            : slot + WatchBytes - 1;
+        return address <= slotEnd && slot <= writeEnd;
+    }
+
+    private static ulong LittleEndianValue(ReadOnlySpan<byte> data)
+    {
+        ulong value = 0;
+        for (var i = 0; i < data.Length; i++)
+        {
+            value |= (ulong)data[i] << (i * 8);
+        }
+
+        return value;
+    }
+
+    private static void Report(ulong address, ReadOnlySpan<byte> data)
+    {
+        Console.Error.WriteLine(
+            $"[LOADER][WARN] watch_write HIT addr=0x{address:X16} len={data.Length} " +
+            $"first_qword=0x{LittleEndianValue(data):X16}{Environment.NewLine}{Environment.StackTrace}");
+        Console.Error.Flush();
+    }
+
+    private static bool IsEnabled(string name) =>
+        string.Equals(Environment.GetEnvironmentVariable(name), "1", StringComparison.Ordinal);
+
+    private static bool ReserveBulkReport(string kind) =>
+        kind == "torn"
+            ? Interlocked.Increment(ref _bulkTornReports) <= MaxBulkReports
+            : Interlocked.Increment(ref _bulkShiftReports) <= MaxBulkReports;
+
+    internal static ulong Parse(string? text)
+    {
+        if (string.IsNullOrWhiteSpace(text))
+        {
+            return 0;
+        }
+
+        text = text.Trim();
+        if (text.StartsWith("0x", StringComparison.OrdinalIgnoreCase))
+        {
+            text = text[2..];
+        }
+
+        return ulong.TryParse(text, NumberStyles.HexNumber, CultureInfo.InvariantCulture, out var value)
+            ? value
+            : 0;
+    }
+}