blog_os/blog/post/double-faults.md

+++ title = "Double Faults" date = "2016-11-08" +++

In this post we will make our kernel completely exception-proof by catching double faults on a separate kernel stack.

What is a Double Fault?

In simplified terms, a double fault is a special exception that occurs when the CPU can't invoke an exception handler. For example, it occurs when a page fault is triggered but there is no page fault handler registered in the [IDT]. So it's kind of similar to catch-all blocks in programming languages with exceptions, e.g. catch(...) in C++ or catch(Exception e) in Java or C#.

[IDT]: {{% relref "09-catching-exceptions.md#the-interrupt-descriptor-table" %}}

A double fault behaves like a normal exception. It has the vector number 8 and we can define a normal handler function for it in the IDT. It is really important to provide a double fault handler, because if a double faults is unhandled a fatal triple fault occurs. Triple faults can't be caught and most hardware reacts with a system reset.

Triggering a Double Fault

Let's provoke a double fault by triggering an exception for that we didn't define a handler function yet:

{{< highlight rust "hl_lines=10" >}} // in src/lib.rs

#[no_mangle] pub extern "C" fn rust_main(multiboot_information_address: usize) { ... // initialize our IDT interrupts::init();

// trigger a debug exception
unsafe { int!(1) };

println!("It did not crash!");
loop {}

} {{< / highlight >}}

We use the int! macro of the x86 crate to trigger the exception with vector number 1, which is the debug exception. The debug exception occurs for example when a breakpoint defined in the debug registers is hit. Like the breakpoint exception, it is mainly used for implementing debuggers.

We haven't registered a handler function for the debug exception in our [IDT], so the int!(1) line should cause a double fault in the CPU.

When we start our kernel now, we see that it enters an endless boot loop:

The reason for the boot loop is the following:

1. The CPU executes the int 1 instruction, which causes a software-invoked Debug exception.
2. The CPU looks at the corresponding entry in the IDT and sees that the present bit isn't set. Thus, it can't call the debug exception handler and a double fault occurs.
3. The CPU looks at the IDT entry of the double fault handler, but this entry is also non-present. Thus, a triple fault occurs.
4. A triple fault is fatal. QEMU reacts to it like most real hardware and issues a system reset.

So in order to prevent this triple fault, we need to either provide a handler function for Debug exceptions or a double fault handler. We will do the latter, since this post is all about the double fault.

A Double Fault Handler

A double fault is a normal exception with an error code, so we can use our handler_with_error_code macro to create a wrapper function:

{{< highlight rust "hl_lines=10 17" >}} // in src/interrupts/mod.rs

lazy_static! { static ref IDT: idt::Idt = { let mut idt = idt::Idt::new();

    idt.set_handler(0, handler!(divide_by_zero_handler));
    idt.set_handler(3, handler!(breakpoint_handler));
    idt.set_handler(6, handler!(invalid_opcode_handler));
    idt.set_handler(8, handler_with_error_code!(double_fault_handler));
    idt.set_handler(14, handler_with_error_code!(page_fault_handler));

    idt
};

}

// our new double fault handler extern "C" fn double_fault_handler(stack_frame: &ExceptionStackFrame, _error_code: u64) { println!("\nEXCEPTION: DOUBLE FAULT\n{:#?}", stack_frame); loop {} } {{< / highlight >}}

Our handler prints a short error message and dumps the exception stack frame. The error code of the double fault handler is always zero, so there's no reason to print it.

When we start our kernel now, we should see that the double fault handler is invoked:

It worked! Here is what happens this time:

1. The CPU executes the int 1 instruction macro, which causes a software-invoked Debug exception.
2. Like before, the CPU looks at the corresponding entry in the IDT and sees that the present bit isn't set. Thus, it can't call the debug exception handler and a double fault occurs.
3. The CPU jumps to the – now present – double fault handler.

The triple fault (and the boot-loop) no longer occurs, since the CPU can now call the double fault handler.

That was pretty straightforward! So why do we need a whole post for this topic? Well, we're now able to catch most double faults, but there are some cases where our current approach doesn't suffice.

Causes of Double Faults

Before we look at the special cases, we need to know the exact causes of double faults. Above, we used a pretty vague definition:

A double fault is a special exception that occurs when the CPU can't invoke an exception handler.

What does “can't invoke” mean exactly? The handler is not present? The handler is swapped out? And what happens if a handler causes exceptions itself?

For example, what happens if… :

1. a divide-by-zero exception occurs, but the corresponding handler function is swapped out?
2. a page fault occurs, but the page fault handler is swapped out?
3. a divide-by-zero handler invokes a breakpoint exception, but the breakpoint handler is swapped out?
4. our kernel overflows its stack and the [guard page] is hit?

[guard page]: {{% relref "07-remap-the-kernel.md#creating-a-guard-page" %}}

Fortunately, the AMD64 manual (PDF) has an exact definition (in Section 8.2.9). According to it, a “double fault exception can occur when a second exception occurs during the handling of a prior (first) exception handler”. The “can” is important: Only very specific combinations of exceptions lead to a double fault. These combinations are:

First Exception	Second Exception
Divide-by-zero, Invalid TSS, Segment Not Present, Stack-Segment Fault, General Protection Fault	Invalid TSS, Segment Not Present, Stack-Segment Fault, General Protection Fault
Page Fault	Page Fault, Invalid TSS, Segment Not Present, Stack-Segment Fault, General Protection Fault

So for example a divide-by-zero fault followed by a page fault is fine (the page fault handler is invoked), but a divide-by-zero fault followed by a general-protection fault leads to a double fault.

With the help of this table, we can answer the first three of the above questions:

1. When a divide-by-zero exception occurs and the corresponding handler function is swapped out, a page fault occurs and the page fault handler is invoked.
2. When a page fault occurs and the page fault handler is swapped out, a double fault occurs and the double fault handler is invoked.
3. When a divide-by-zero handler invokes a breakpoint exception and the breakpoint handler is swapped out, a breakpoint exception occurs first. However, the corresponding handler is swapped out, so a page fault occurs and the page fault handler is invoked.

In fact, even the case of a non-present handler follows this scheme: A non-present handler causes a segment-not-present exception. We didn't define a segment-not-present handler, so another segment-not-present exception occurs. According to the table, this leads to a double fault.

Kernel Stack Overflow

Let's look at the fourth question:

What happens if our kernel overflows its stack and the [guard page] is hit?

When our kernel overflows its stack and hits the guard page, a page fault occurs and the CPU invokes the page fault handler. However, the CPU also tries to push the exception stack frame onto the stack. This fails of course, since our current stack pointer still points to the guard page. Thus, a second page fault occurs, which causes a double fault (according to the above table).

So the CPU tries to call our double fault handler now. However, on a double fault the CPU tries to push the exception stack frame, too. Our stack pointer still points to the guard page, so a third page fault occurs, which causes a triple fault and a system reboot. So our current double fault handler can't avoid a triple fault in this case.

Let's try it ourselves! We can easily provoke a kernel stack overflow by calling a function that recurses endlessly:

{{< highlight rust "hl_lines=9 10 11 14" >}} // in src/lib.rs

#[no_mangle] pub extern "C" fn rust_main(multiboot_information_address: usize) { ... // initialize our IDT interrupts::init();

fn stack_overflow() {
    stack_overflow(); // for each recursion, the return address is pushed
}

// trigger a stack overflow
stack_overflow();

println!("It did not crash!");
loop {}

} {{< / highlight >}}

When we try this code in QEMU, we see that the system enters a boot-loop again.

So how can we avoid this problem? We can't omit the pushing of the exception stack frame, since the CPU itself does it. So we need to ensure somehow that the stack is always valid when a double fault exception occurs. Fortunately, the x86_64 architecture has a solution to this problem.

Switching Stacks

The x86_64 architecture is able to switch to a predefined stack for some exceptions through an Interrupt Stack Table (IST). The IST is a table of 7 pointers to known-good stacks. In Rust-like pseudo code:

struct InterruptStackTable {
    stack_pointers: [Option<StackPointer>; 7],
}

For each exception handler, we can choose an IST stack through the options field in the [IDT entry]. For example, we could use the first stack in the IST for our double fault handler. Then the CPU would automatically switch to this stack before it pushes anything. Thus, we are able to avoid the triple fault.

[IDT entry]: {{% relref "09-catching-exceptions.md#the-interrupt-descriptor-table" %}}

The Task State Segment

The Interrupt Stack Table (IST) is part of an old legacy structure called Task State Segment (TSS). The TSS used to hold various information (e.g. processor register state) about a task in 32-bit x86 and was for example used for hardware context switching. However, hardware context switching is no longer supported in 64-bit mode and the format of the TSS changed completely.

On x86_64, the TSS no longer holds any task specific information at all. Instead, it holds two stack tables (the IST is one of them). The only common field between the 32-bit and 64-bit TSS is the pointer to the I/O port permissions bitmap.

The 64-bit TSS has the following format:

Field	Type
(reserved)	u32
Privilege Stack Table	[u64; 3]
(reserved)	u64
Interrupt Stack Table	[u64; 7]
(reserved)	u64
(reserved)	u16
I/O Map Base Address	u16

The Privilege Stack Table is used by the CPU when the privilege level changes. For example, if an exception occurs while the CPU is in user mode (privilege level 3), the CPU normally switches to kernel mode (privilege level 0) before invoking the exception handler. In that case, the CPU would switch to the 0th stack in the Privilege Stack Table (since 0 is the target privilege level). We don't have any user mode programs yet, so we can safely ignore this table for now.

Let's create a TaskStateSegment struct in new tss submodule:

// in src/interrupts/mod.rs

mod tss;

// in src/interrupts/tss.rs

use core::nonzero::NonZero;

#[derive(Debug)]
#[repr(C, packed)]
pub struct TaskStateSegment {
    reserved_0: u32,
    pub privilege_stacks: PrivilegeStackTable,
    reserved_1: u64,
    pub interrupt_stacks: InterruptStackTable,
    reserved_2: u64,
    reserved_3: u16,
    iomap_base: u16,
}

#[derive(Debug)]
pub struct PrivilegeStackTable([Option<StackPointer>; 3]);

#[derive(Debug)]
pub struct InterruptStackTable([Option<StackPointer>; 7]);

#[derive(Debug)]
pub struct StackPointer(NonZero<u64>);

TODO lang item

However, it is a bit cumbersome to setup this mechanism.

The mechanism consists of two main components: An Interrupt Stack Table and a Task State Segment.

Switching stacks The Interrupt Stack Table The Task State Segment The Global Descriptor Table (again) Putting it together What’s next?

In the previous post, we learned how to return from exceptions correctly. In this post, we will explore a special type of exception: the double fault. The double fault occurs whenever the invokation of an exception handler fails. For example, if we didn't declare any exception hanlder in the IDT.

Let's start by creating a handler function for double faults:

Next, we need to register the double fault handler in our IDT:

Double faults also occur when an exception occurs while the CPU is trying to invoke an exception handler. For example, let's assume a divide-by-zero exception occurs but the OS accidentally swapped out the corresponding handler function. Now the CPU tries to call the divide-by-zero handler, which

A double fault occurs whenever the CPU fails to call an exception handler. On a high level it's like a catch-all handler, similar to catch(...) in C++ or catch(Exception e) in Java or C#.

The most common case is that there isn't a handler defined in the IDT. However, a double fault also occurs if the exception handler lies on a unaccessible page of if the CPU fails to push the exception stack frame.