kernel: use per-thread signal stacks for guest stack faults

src/common/hostException.cpp

@@ -9,7 +9,9 @@
 #include <csignal>
 #include <sys/ucontext.h>
 #else
+#include <algorithm>
 #include <csignal>
+#include <cstdlib>
 #include <initializer_list>
 #include <ucontext.h> // IWYU pragma: keep
 #include <unistd.h>
@@ -164,6 +166,46 @@ static void SignalHandler(int sig, siginfo_t* si, void* uctx) {
 
 #else
 
+class ThreadSignalStack {
+public:
+	ThreadSignalStack() {
+		const auto page_size = static_cast<size_t>(::getpagesize());
+		const auto stack_size =
+		    (std::max<size_t>(64 * 1024, MINSIGSTKSZ) + page_size - 1) & ~(page_size - 1);
+		if (::posix_memalign(&m_memory, page_size, stack_size) != 0) {
+			return;
+		}
+
+		stack_t stack {};
+		stack.ss_sp   = m_memory;
+		stack.ss_size = stack_size;
+		if (::sigaltstack(&stack, &m_previous) != 0) {
+			std::free(m_memory);
+			m_memory = nullptr;
+		}
+	}
+
+	~ThreadSignalStack() {
+		if (m_memory != nullptr && ::sigaltstack(&m_previous, nullptr) == 0) {
+			std::free(m_memory);
+		}
+	}
+
+	[[nodiscard]] bool IsInitialized() const { return m_memory != nullptr; }
+
+	KYTY_CLASS_NO_COPY(ThreadSignalStack)
+
+private:
+	void*   m_memory = nullptr;
+	stack_t m_previous {};
+};
+
+bool InitializeThreadSignalStack() {
+	// Keep fault handling off guest stacks, which GPU tracking can make read-only.
+	thread_local ThreadSignalStack signal_stack;
+	return signal_stack.IsInitialized();
+}
+
 // x86-64 page-fault error bits.
 constexpr uint64_t PAGE_FAULT_ERROR_WRITE       = 0x02;
 constexpr uint64_t PAGE_FAULT_ERROR_INSTRUCTION = 0x10;
@@ -274,8 +316,7 @@ bool InstallHandler(Handler handler) {
 	struct sigaction action {};
 	action.sa_sigaction = SignalHandler;
 	sigemptyset(&action.sa_mask);
-	// Fault resolution needs the normal thread stack.
+	action.sa_flags = SA_SIGINFO | SA_RESTART | SA_ONSTACK;
-	action.sa_flags = SA_SIGINFO | SA_RESTART;
 
 	for (const int signal_number: {SIGSEGV, SIGBUS, SIGILL}) {
 		if (::sigaction(signal_number, &action, nullptr) != 0) {

src/common/hostException.h

@@ -39,6 +39,10 @@ using Handler = bool (*)(const ExceptionInfo&);
 
 bool InstallHandler(Handler handler);
 
+#if KYTY_PLATFORM == KYTY_PLATFORM_LINUX
+bool InitializeThreadSignalStack();
+#endif
+
 } // namespace Common::HostException
 
 #endif /* KYTY_COMMON_HOST_EXCEPTION_H_ */

src/kernel/pthread.cpp

@@ -4,6 +4,7 @@
 #include "common/common.h"
 #include "common/dateTime.h"
 #include "common/emulatorConfig.h"
+#include "common/hostException.h"
 #include "common/logging/log.h"
 #include "common/singleton.h"
 #include "common/stringUtils.h"
@@ -1061,6 +1062,12 @@ void PthreadDeleteStaticObjects(Loader::Program* program) {
 void PthreadInitSelfForMainThread() {
 	EXIT_IF(g_pthread_self != nullptr);
 
+#if KYTY_PLATFORM == KYTY_PLATFORM_LINUX
+	if (!Common::HostException::InitializeThreadSignalStack()) {
+		EXIT("Failed to initialize the thread signal stack\n");
+	}
+#endif
+
 	g_pthread_self = new PthreadPrivate {};
 	PthreadAttrInit(&g_pthread_self->attr);
 	UpdateCurrentThreadStackAttr(&g_pthread_self->attr);
@@ -3382,6 +3389,13 @@ static void CleanupThread(void* arg) {
 }
 
 static void* RunThread(void* arg) {
+
+#if KYTY_PLATFORM == KYTY_PLATFORM_LINUX
+	if (!Common::HostException::InitializeThreadSignalStack()) {
+		EXIT("Failed to initialize the thread signal stack\n");
+	}
+#endif
+
 	auto* thread = static_cast<Pthread>(arg);
 	void* ret    = nullptr;
 

tests/MemoryTrackerTests.cpp

@@ -1,3 +1,4 @@
+#include "common/hostException.h"
 #include "common/virtualMemory.h"
 #include "graphics/host_gpu/memoryTracker.h"
 #include "graphics/host_gpu/rangeSet.h"
@@ -21,6 +22,7 @@
 #undef min
 #undef max
 #else
+#include <csignal>
 #include <map>
 #include <sys/mman.h>
 #include <sys/wait.h>
@@ -832,6 +834,68 @@ void TestFatalPaths() {
   }
 }
 
+#if KYTY_PLATFORM == KYTY_PLATFORM_LINUX
+void *g_fault_stack = nullptr;
+constexpr size_t FAULT_STACK_SIZE = 64 * 1024;
+volatile sig_atomic_t g_stack_faults = 0;
+
+bool HandleStackFault(const Common::HostException::ExceptionInfo &info) {
+  using namespace Common::HostException;
+  stack_t active_stack{};
+  const auto fault_address = reinterpret_cast<uintptr_t>(g_fault_stack) +
+                             FAULT_STACK_SIZE - sizeof(uintptr_t);
+  if (info.type != ExceptionType::AccessViolation ||
+      info.access_violation_type != AccessViolationType::Write ||
+      info.access_violation_vaddr != fault_address ||
+      ::sigaltstack(nullptr, &active_stack) != 0 ||
+      (active_stack.ss_flags & SS_ONSTACK) == 0) {
+    std::_Exit(1);
+  }
+  g_stack_faults = 1;
+  return ::mprotect(g_fault_stack, FAULT_STACK_SIZE,
+                    PROT_READ | PROT_WRITE) == 0;
+}
+
+// A stack write must fault before any signal frame can use the protected stack.
+__attribute__((naked)) void WriteProtectedStack(void *) {
+  asm volatile("mov %rsp, %rax\n"
+               "mov %rdi, %rsp\n"
+               "push %rax\n"
+               "pop %rsp\n"
+               "ret\n");
+}
+
+void TestFaultOnProtectedStack() {
+  const pid_t pid = ::fork();
+  Check(pid >= 0, "stack fault fork failed");
+  if (pid == 0) {
+    std::thread worker([] {
+      Check(Common::HostException::InitializeThreadSignalStack(),
+            "initialize thread signal stack failed");
+      Check(Common::HostException::InstallHandler(HandleStackFault),
+            "install stack fault handler failed");
+      g_fault_stack = ::mmap(nullptr, FAULT_STACK_SIZE, PROT_READ,
+                            MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+      Check(g_fault_stack != MAP_FAILED, "allocate protected stack failed");
+      WriteProtectedStack(static_cast<char *>(g_fault_stack) + FAULT_STACK_SIZE);
+      Check(g_stack_faults == 1, "protected stack write did not resume");
+      struct sigaction action{};
+      Check(::sigaction(SIGSEGV, nullptr, &action) == 0 &&
+                action.sa_handler != SIG_DFL,
+            "stack fault reset the process handler");
+      Check(::munmap(g_fault_stack, FAULT_STACK_SIZE) == 0,
+            "release protected stack failed");
+    });
+    worker.join();
+    std::_Exit(0);
+  }
+  int status = 0;
+  Check(::waitpid(pid, &status, 0) == pid && WIFEXITED(status) &&
+            WEXITSTATUS(status) == 0,
+        "fault on a protected stack did not recover");
+}
+#endif
+
 } // namespace
 
 namespace Libs::LibKernel::Memory {
@@ -863,6 +927,9 @@ int main(int argc, char **argv) {
   TestGpuUnmarkUsesRegionMask();
   TestFullRegionGpuUnmarkBatching();
   TestFatalPaths();
+#if KYTY_PLATFORM == KYTY_PLATFORM_LINUX
+  TestFaultOnProtectedStack();
+#endif
   std::puts("MemoryTrackerTests: all cases passed");
   return 0;
 }