Kernel: Add bounds checking to recognized_symbols in dump_backtrace_i…

…mpl (SerenityOS#372)

This adds a bounds check to the loop that writes to the buffer
'recognized_symbols'. This prevents buffer overflows in the
case when a programs backtrace is particularly large.

Fixes SerenityOS#371.

Kernel/KSyms.cpp

@@ -94,7 +94,7 @@ static void load_ksyms_from_data(const ByteBuffer& buffer)
  RecognizedSymbol recognized_symbols[max_recognized_symbol_count];
  int recognized_symbol_count = 0;
  if (use_ksyms) {
- for (u32* stack_ptr = (u32*)ebp; current->process().validate_read_from_kernel(VirtualAddress((u32)stack_ptr)); stack_ptr = (u32*)*stack_ptr) {
+ for (u32* stack_ptr = (u32*)ebp; current->process().validate_read_from_kernel(VirtualAddress((u32)stack_ptr)) && recognized_symbol_count < max_recognized_symbol_count; stack_ptr = (u32*)*stack_ptr) {
  u32 retaddr = stack_ptr[1];
  recognized_symbols[recognized_symbol_count++] = { retaddr, ksymbolicate(retaddr) };
  }
@@ -105,7 +105,7 @@ static void load_ksyms_from_data(const ByteBuffer& buffer)
  }
  return;
  }
- ASSERT(recognized_symbol_count < max_recognized_symbol_count);
+ ASSERT(recognized_symbol_count <= max_recognized_symbol_count);
  size_t bytes_needed = 0;
  for (int i = 0; i < recognized_symbol_count; ++i) {
  auto& symbol = recognized_symbols[i];