From f41541ab2963cb26df4c8972deef8aa0faac7b19 Mon Sep 17 00:00:00 2001
From: Remi Gacogne <remi.gacogne@powerdns.com>
Date: Wed, 28 Dec 2022 10:33:24 +0100
Subject: [PATCH] rec: Implement stack overflow protection via guard pages

---
 pdns/recursordist/lazy_allocator.hh | 71 ++++++++++++++++++++++++-----
 1 file changed, 60 insertions(+), 11 deletions(-)

diff --git a/pdns/recursordist/lazy_allocator.hh b/pdns/recursordist/lazy_allocator.hh
index 7ddd31553c..79a205d5ca 100644
--- a/pdns/recursordist/lazy_allocator.hh
+++ b/pdns/recursordist/lazy_allocator.hh
@@ -25,6 +25,7 @@
 #include <type_traits>
 #include <new>
 #include <sys/mman.h>
+#include <unistd.h>
 
 // On OpenBSD mem used as stack should be marked MAP_STACK
 #if !defined(MAP_STACK)
@@ -40,33 +41,81 @@ struct lazy_allocator
   static_assert(std::is_trivial<T>::value,
                 "lazy_allocator must only be used with trivial types");
 
+#ifndef LAZY_ALLOCATOR_USES_NEW
+  /* Pad the requested size to be a multiple of page size, so that we can
+     properly restrict read and write access to our guard pages only */
+  static size_type getAlignmentPadding(size_type requestedSize, size_type pageSize)
+  {
+    size_type remaining = requestedSize % pageSize;
+    if (remaining == 0) {
+      return 0;
+    }
+    return pageSize - remaining;
+  }
+#endif /* LAZY_ALLOCATOR_USES_NEW */
+
   pointer
   allocate(size_type const n)
   {
-#ifdef __OpenBSD__
-    void* p = mmap(nullptr, n * sizeof(value_type),
-                   PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANON | MAP_STACK, -1, 0);
-    if (p == MAP_FAILED)
-      throw std::bad_alloc();
-    return static_cast<pointer>(p);
-#else
+#ifdef LAZY_ALLOCATOR_USES_NEW
     return static_cast<pointer>(::operator new(n * sizeof(value_type)));
+#else /* LAZY_ALLOCATOR_USES_NEW */
+    /* This implements a very basic protection against stack overflow
+       by placing two guard pages around the requested memory: one
+       page right before the new stack and one right after.
+       The guard pages cannot be read or written to, any attempt to
+       do so will trigger an immediate access violation, terminating
+       the program.
+       This is much better than the default behaviour for two reasons:
+       1/ the program is stopped right before corrupting memory, which
+          prevents random corruption
+       2/ it's easy to find the point where the stack overflow occurred
+       The memory overhead is two pages (usually 4k on Linux) per stack,
+       and the runtime CPU overhead is one call to mprotect() for every
+       stack allocation.
+    */
+    static const auto pageSize = sysconf(_SC_PAGESIZE);
+
+    const size_type requestedSize = n * sizeof(value_type);
+    const auto padding = getAlignmentPadding(requestedSize, pageSize);
+    const size_type allocatedSize = requestedSize + padding + (pageSize * 2);
+
+    void* p = mmap(nullptr, allocatedSize,
+                   PROT_NONE, MAP_PRIVATE | MAP_ANON | MAP_STACK, -1, 0);
+    if (p == MAP_FAILED) {
+      throw std::bad_alloc();
+    }
+    char* basePointer = static_cast<char*>(p);
+    void* usablePointer = basePointer + pageSize;
+    int res = mprotect(usablePointer, requestedSize + padding, PROT_READ | PROT_WRITE);
+    if (res != 0) {
+      munmap(p, requestedSize + padding + (pageSize * 2));
+      throw std::bad_alloc();
+    }
+    return static_cast<pointer>(usablePointer);
 #endif
   }
 
   void
   deallocate(pointer const ptr, size_type const n) noexcept
   {
-#ifdef __OpenBSD__
-    munmap(ptr, n * sizeof(value_type));
-#else
+#ifdef LAZY_ALLOCATOR_USES_NEW
 #if defined(__cpp_sized_deallocation) && (__cpp_sized_deallocation >= 201309)
     ::operator delete(ptr, n * sizeof(value_type));
 #else
     (void)n;
     ::operator delete(ptr);
 #endif
-#endif
+#else /* LAZY_ALLOCATOR_USES_NEW */
+    static const auto pageSize = sysconf(_SC_PAGESIZE);
+
+    const size_type requestedSize = n * sizeof(value_type);
+    const auto padding = getAlignmentPadding(requestedSize, pageSize);
+    const size_type allocatedSize = requestedSize + padding + (pageSize * 2);
+
+    void* basePointer = static_cast<char*>(ptr) - pageSize;
+    munmap(basePointer, allocatedSize);
+#endif /* LAZY_ALLOCATOR_PROTECT */
   }
 
   void construct(T*) const noexcept {}
-- 
2.47.2