memory: avoid redundant Windows view protection

src/kernel/memory.cpp

@@ -3621,6 +3621,36 @@ uint64_t TestGuestBackingSize() {
 	return g_guest_address_space->GetBackingSize();
 }
 
+#if KYTY_PLATFORM == KYTY_PLATFORM_WINDOWS
+bool TestWindowsBackingViewModes() {
+	constexpr uint64_t size = 0x10000;
+	GuestBackingStore backing(size);
+	void* placeholder = VirtualAlloc2(GetCurrentProcess(), nullptr, size,
+	                                  MEM_RESERVE | MEM_RESERVE_PLACEHOLDER, PAGE_NOACCESS,
+	                                  nullptr, 0);
+	if (placeholder == nullptr) {
+		return false;
+	}
+	const auto address = reinterpret_cast<uint64_t>(placeholder);
+	bool valid = true;
+	for (auto [mode, protection]: std::array {
+	         std::pair {VirtualMemory::Mode::NoAccess, DWORD {PAGE_NOACCESS}},
+	         std::pair {VirtualMemory::Mode::Read, DWORD {PAGE_READONLY}},
+	         std::pair {VirtualMemory::Mode::ReadWrite, DWORD {PAGE_READWRITE}},
+	         std::pair {VirtualMemory::Mode::ExecuteReadWrite, DWORD {PAGE_EXECUTE_READWRITE}}}) {
+		if (!backing.MapFixed(address, size, 0, mode)) {
+			valid = false;
+			break;
+		}
+		MEMORY_BASIC_INFORMATION info {};
+		valid = VirtualQuery(placeholder, &info, sizeof(info)) != 0 &&
+		        info.Type == MEM_MAPPED && info.Protect == protection && valid;
+		EXIT_IF(!backing.Unmap(address, size));
+	}
+	return VirtualFree(placeholder, 0, MEM_RELEASE) != 0 && valid;
+}
+#endif
+
 bool TestGuestFreeRangeBounds() {
 	return GuestFreeRangeContains(0x10000, 0x20000, 0x18000, 0x4000) &&
 	       !GuestFreeRangeContains(0x10000, 0x20000, 0x40000, 0x4000) &&

src/kernel/memoryAddressSpace.inc

@@ -597,7 +597,7 @@ private:
 			return nullptr;
 		}
 
-		if (!ApplyProtection(ptr, size, mode)) {
+		if (mode == VirtualMemory::Mode::NoAccess && !ApplyProtection(ptr, size, mode)) {
 			UnmapViewOfFile2(GetCurrentProcess(), ptr, MEM_PRESERVE_PLACEHOLDER);
 			SetFailure(failure_reason, FailureReason::ProtectFailed);
 			return nullptr;

tests/VirtualMemoryAllocationTests.cpp

@@ -60,6 +60,12 @@
 #endif
 #endif
 
+#if KYTY_PLATFORM == KYTY_PLATFORM_WINDOWS
+namespace Libs::LibKernel::Memory {
+bool TestWindowsBackingViewModes();
+}
+#endif
+
 namespace Libs::Fiber {
 struct FiberObject;
 struct FiberOptParam;
@@ -1355,6 +1361,15 @@ void TestPartialUnmapPreservesHostPermissions() {
 }
 #endif
 
+#if KYTY_PLATFORM == KYTY_PLATFORM_WINDOWS
+void TestWindowsBackingViewPermissions() {
+	const char* test = "WindowsBackingViewPermissions";
+	Check(test, Libs::LibKernel::Memory::TestWindowsBackingViewModes(),
+	      "backing view permissions differ from the requested mode");
+	std::printf("[host]    %-48s ok\n", test);
+}
+#endif
+
 void TestDirectMapValidationBeforeOwnerMutation() {
 	const char* test    = "DirectMapValidationBeforeOwnerMutation";
 	int64_t     invalid = -1;
@@ -4248,6 +4263,9 @@ int main(int argc, char** argv) {
 	RunTest(TestDirectPartialProtectUnmapPreservesNeighbors);
 #if defined(__linux__)
 	RunTest(TestPartialUnmapPreservesHostPermissions);
+#endif
+#if KYTY_PLATFORM == KYTY_PLATFORM_WINDOWS
+	RunTest(TestWindowsBackingViewPermissions);
 #endif
 	RunTest(TestDirectMapValidationBeforeOwnerMutation);
 	RunTest(TestDirectReleaseRollbackRestoresOwnerMapping);