Turn page of old P4 into guard page

README.md

@@ -1,7 +1,7 @@
-# Blog OS (Page Tables)
-[![Build Status](https://travis-ci.org/phil-opp/blog_os.svg?branch=post_6)](https://travis-ci.org/phil-opp/blog_os/branches)
+# Blog OS (Remap the Kernel)
+[![Build Status](https://travis-ci.org/phil-opp/blog_os.svg?branch=post_7)](https://travis-ci.org/phil-opp/blog_os/branches)
 
-This repository contains the source code for the [Page Tables](http://os.phil-opp.com/modifying-page-tables.html) post of the [Writing an OS in Rust](http://os.phil-opp.com) series.
+This repository contains the source code for the [Remap the Kernel](http://os.phil-opp.com/remap-the-kernel.html) post of the [Writing an OS in Rust](http://os.phil-opp.com) series.
 
 **Check out the [master branch](https://github.com/phil-opp/blog_os) for more information.**
 

src/arch/x86_64/linker.ld

@@ -1,24 +1,53 @@
 ENTRY(start)
 
 SECTIONS {
-    . = 1M;
+  . = 1M;
 
-    .boot :
-    {
-        /* ensure that the multiboot header is at the beginning */
-        KEEP(*(.multiboot_header))
-    }
+  .rodata :
+  {
+    /* ensure that the multiboot header is at the beginning */
+    KEEP(*(.multiboot_header))
+    *(.rodata .rodata.*)
+    . = ALIGN(4K);
+  }
 
-    .text :
-    {
-        *(.text .text.*)
-    }
+  .text :
+  {
+    *(.text .text.*)
+    . = ALIGN(4K);
+  }
 
-    .rodata : {
-        *(.rodata .rodata.*)
-    }
+  .data :
+  {
+    *(.data .data.*)
+    . = ALIGN(4K);
+  }
 
-    .data.rel.ro : {
-        *(.data.rel.ro.local*) *(.data.rel.ro .data.rel.ro.*)
-    }
+  .bss :
+  {
+    *(.bss .bss.*)
+    . = ALIGN(4K);
+  }
+
+  .got :
+  {
+    *(.got)
+    . = ALIGN(4K);
+  }
+
+  .got.plt :
+  {
+    *(.got.plt)
+    . = ALIGN(4K);
+  }
+
+  .data.rel.ro : ALIGN(4K) {
+    *(.data.rel.ro.local*) *(.data.rel.ro .data.rel.ro.*)
+    . = ALIGN(4K);
+  }
+
+  .gcc_except_table : ALIGN(4K) {
+    *(.gcc_except_table)
+    . = ALIGN(4K);
+  }
 }

src/lib.rs

@@ -26,21 +26,8 @@ pub extern fn rust_main(multiboot_information_address: usize) {
     let boot_info = unsafe{ multiboot2::load(multiboot_information_address) };
     let memory_map_tag = boot_info.memory_map_tag()
         .expect("Memory map tag required");
-
-    println!("memory areas:");
-    for area in memory_map_tag.memory_areas() {
-        println!("    start: 0x{:x}, length: 0x{:x}",
-            area.base_addr, area.length);
-    }
-
     let elf_sections_tag = boot_info.elf_sections_tag()
-        .expect("Elf-sections tag required");
-
-    println!("kernel sections:");
-    for section in elf_sections_tag.sections() {
-        println!("    addr: 0x{:x}, size: 0x{:x}, flags: 0x{:x}",
-            section.addr, section.size, section.flags);
-    }
+        .expect("Elf sections tag required");
 
     let kernel_start = elf_sections_tag.sections().map(|s| s.addr)
         .min().unwrap();
@@ -49,11 +36,37 @@ pub extern fn rust_main(multiboot_information_address: usize) {
     let multiboot_start = multiboot_information_address;
     let multiboot_end = multiboot_start + (boot_info.total_size as usize);
 
+    println!("kernel start: 0x{:x}, kernel end: 0x{:x}",
+        kernel_start, kernel_end);
+    println!("multiboot start: 0x{:x}, multiboot end: 0x{:x}",
+        multiboot_start, multiboot_end);
+
     let mut frame_allocator = memory::AreaFrameAllocator::new(
         kernel_start as usize, kernel_end as usize, multiboot_start,
         multiboot_end, memory_map_tag.memory_areas());
 
-    loop{}
+    enable_nxe_bit();
+    enable_write_protect_bit();
+    memory::remap_the_kernel(&mut frame_allocator, boot_info);
+    println!("It did not crash!");
+
+    loop {}
+}
+
+fn enable_nxe_bit() {
+    use x86_64::registers::msr::{IA32_EFER, rdmsr, wrmsr};
+
+    let nxe_bit = 1 << 11;
+    unsafe {
+        let efer = rdmsr(IA32_EFER);
+        wrmsr(IA32_EFER, efer | nxe_bit);
+    }
+}
+
+fn enable_write_protect_bit() {
+    use x86_64::registers::control_regs::{cr0, cr0_write, Cr0};
+
+    unsafe { cr0_write(cr0() | Cr0::WRITE_PROTECT) };
 }
 
 #[lang = "eh_personality"] extern fn eh_personality() {}

src/memory/mod.rs

@@ -1,4 +1,5 @@
 pub use self::area_frame_allocator::AreaFrameAllocator;
+pub use self::paging::remap_the_kernel;
 use self::paging::PhysicalAddress;
 
 mod area_frame_allocator;
@@ -19,8 +20,38 @@ impl Frame {
     fn start_address(&self) -> PhysicalAddress {
         self.number * PAGE_SIZE
     }
+
+    fn clone(&self) -> Frame {
+        Frame { number: self.number }
+    }
+
+    fn range_inclusive(start: Frame, end: Frame) -> FrameIter {
+        FrameIter {
+            start: start,
+            end: end,
+        }
+    }
 }
 
+struct FrameIter {
+    start: Frame,
+    end: Frame,
+}
+
+impl Iterator for FrameIter {
+    type Item = Frame;
+
+    fn next(&mut self) -> Option<Frame> {
+        if self.start <= self.end {
+            let frame = self.start.clone();
+            self.start.number += 1;
+            Some(frame)
+        } else {
+            None
+        }
+    }
+ }
+
 pub trait FrameAllocator {
     fn allocate_frame(&mut self) -> Option<Frame>;
     fn deallocate_frame(&mut self, frame: Frame);

src/memory/paging/entry.rs

@@ -1,4 +1,5 @@
 use memory::Frame;
+use multiboot2::ElfSection;
 
 pub struct Entry(u64);
 
@@ -45,3 +46,26 @@ bitflags! {
         const NO_EXECUTE =      1 << 63,
     }
 }
+
+
+impl EntryFlags {
+    pub fn from_elf_section_flags(section: &ElfSection) -> EntryFlags {
+        use multiboot2::{ELF_SECTION_ALLOCATED, ELF_SECTION_WRITABLE,
+            ELF_SECTION_EXECUTABLE};
+
+        let mut flags = EntryFlags::empty();
+
+        if section.flags().contains(ELF_SECTION_ALLOCATED) {
+            // section is loaded to memory
+            flags = flags | PRESENT;
+        }
+        if section.flags().contains(ELF_SECTION_WRITABLE) {
+            flags = flags | WRITABLE;
+        }
+        if !section.flags().contains(ELF_SECTION_EXECUTABLE) {
+            flags = flags | NO_EXECUTE;
+        }
+
+        flags
+    }
+}

src/memory/paging/mapper.rs

@@ -0,0 +1,118 @@
+use super::{VirtualAddress, PhysicalAddress, Page, ENTRY_COUNT};
+use super::entry::*;
+use super::table::{self, Table, Level4, Level1};
+use memory::{PAGE_SIZE, Frame, FrameAllocator};
+use core::ptr::Unique;
+
+pub struct Mapper {
+    p4: Unique<Table<Level4>>,
+}
+
+impl Mapper {
+    pub unsafe fn new() -> Mapper {
+        Mapper {
+            p4: Unique::new_unchecked(table::P4),
+        }
+    }
+
+    pub fn p4(&self) -> &Table<Level4> {
+        unsafe { self.p4.as_ref() }
+    }
+
+    pub fn p4_mut(&mut self) -> &mut Table<Level4> {
+        unsafe { self.p4.as_mut() }
+    }
+
+    pub fn translate(&self, virtual_address: VirtualAddress) -> Option<PhysicalAddress> {
+        let offset = virtual_address % PAGE_SIZE;
+        self.translate_page(Page::containing_address(virtual_address))
+            .map(|frame| frame.number * PAGE_SIZE + offset)
+    }
+
+    pub fn translate_page(&self, page: Page) -> Option<Frame> {
+        let p3 = self.p4().next_table(page.p4_index());
+
+        let huge_page = || {
+            p3.and_then(|p3| {
+                let p3_entry = &p3[page.p3_index()];
+                // 1GiB page?
+                if let Some(start_frame) = p3_entry.pointed_frame() {
+                    if p3_entry.flags().contains(HUGE_PAGE) {
+                        // address must be 1GiB aligned
+                        assert!(start_frame.number % (ENTRY_COUNT * ENTRY_COUNT) == 0);
+                        return Some(Frame {
+                            number: start_frame.number + page.p2_index() *
+                                    ENTRY_COUNT + page.p1_index(),
+                        });
+                    }
+                }
+                if let Some(p2) = p3.next_table(page.p3_index()) {
+                    let p2_entry = &p2[page.p2_index()];
+                    // 2MiB page?
+                    if let Some(start_frame) = p2_entry.pointed_frame() {
+                        if p2_entry.flags().contains(HUGE_PAGE) {
+                            // address must be 2MiB aligned
+                            assert!(start_frame.number % ENTRY_COUNT == 0);
+                            return Some(Frame {
+                                number: start_frame.number + page.p1_index()
+                            });
+                        }
+                    }
+                }
+                None
+            })
+        };
+
+        p3.and_then(|p3| p3.next_table(page.p3_index()))
+        .and_then(|p2| p2.next_table(page.p2_index()))
+        .and_then(|p1| p1[page.p1_index()].pointed_frame())
+        .or_else(huge_page)
+    }
+
+    pub fn map_to<A>(&mut self, page: Page, frame: Frame, flags: EntryFlags,
+                    allocator: &mut A)
+        where A: FrameAllocator
+    {
+        let p4 = self.p4_mut();
+        let mut p3 = p4.next_table_create(page.p4_index(), allocator);
+        let mut p2 = p3.next_table_create(page.p3_index(), allocator);
+        let mut p1 = p2.next_table_create(page.p2_index(), allocator);
+
+        assert!(p1[page.p1_index()].is_unused());
+        p1[page.p1_index()].set(frame, flags | PRESENT);
+    }
+
+    pub fn map<A>(&mut self, page: Page, flags: EntryFlags, allocator: &mut A)
+        where A: FrameAllocator
+    {
+        let frame = allocator.allocate_frame().expect("out of memory");
+        self.map_to(page, frame, flags, allocator)
+    }
+
+    pub fn identity_map<A>(&mut self, frame: Frame, flags: EntryFlags, allocator: &mut A)
+        where A: FrameAllocator
+    {
+        let page = Page::containing_address(frame.start_address());
+        self.map_to(page, frame, flags, allocator)
+    }
+
+    pub fn unmap<A>(&mut self, page: Page, allocator: &mut A)
+        where A: FrameAllocator
+    {
+        use x86_64::instructions::tlb;
+        use x86_64::VirtualAddress;
+
+        assert!(self.translate(page.start_address()).is_some());
+
+        let p1 = self.p4_mut()
+                    .next_table_mut(page.p4_index())
+                    .and_then(|p3| p3.next_table_mut(page.p3_index()))
+                    .and_then(|p2| p2.next_table_mut(page.p2_index()))
+                    .expect("mapping code does not support huge pages");
+        let frame = p1[page.p1_index()].pointed_frame().unwrap();
+        p1[page.p1_index()].set_unused();
+        tlb::flush(VirtualAddress(page.start_address()));
+        // TODO free p(1,2,3) table if empty
+        //allocator.deallocate_frame(frame);
+    }
+}

src/memory/paging/mod.rs

@@ -1,16 +1,23 @@
 pub use self::entry::*;
+pub use self::mapper::Mapper;
+use core::ops::{Deref, DerefMut};
 use core::ptr::Unique;
 use memory::{PAGE_SIZE, Frame, FrameAllocator};
+use multiboot2::BootInformation;
 use self::table::{Table, Level4};
+use self::temporary_page::TemporaryPage;
 
 mod entry;
 mod table;
+mod temporary_page;
+mod mapper;
 
 const ENTRY_COUNT: usize = 512;
 
 pub type PhysicalAddress = usize;
 pub type VirtualAddress = usize;
 
+#[derive(Debug, Clone, Copy)]
 pub struct Page {
    number: usize,
 }
@@ -42,116 +49,158 @@ impl Page {
 }
 
 pub struct ActivePageTable {
-    p4: Unique<Table<Level4>>,
+    mapper: Mapper,
+}
+
+impl Deref for ActivePageTable {
+    type Target = Mapper;
+
+    fn deref(&self) -> &Mapper {
+        &self.mapper
+    }
+}
+
+impl DerefMut for ActivePageTable {
+    fn deref_mut(&mut self) -> &mut Mapper {
+        &mut self.mapper
+    }
 }
 
 impl ActivePageTable {
-    pub unsafe fn new() -> ActivePageTable {
+    unsafe fn new() -> ActivePageTable {
         ActivePageTable {
-            p4: Unique::new_unchecked(table::P4),
+            mapper: Mapper::new(),
         }
     }
 
-    fn p4(&self) -> &Table<Level4> {
-        unsafe { self.p4.as_ref() }
-    }
-
-    fn p4_mut(&mut self) -> &mut Table<Level4> {
-        unsafe { self.p4.as_mut() }
-    }
-
-    pub fn translate(&self, virtual_address: VirtualAddress) -> Option<PhysicalAddress> {
-        let offset = virtual_address % PAGE_SIZE;
-        self.translate_page(Page::containing_address(virtual_address))
-            .map(|frame| frame.number * PAGE_SIZE + offset)
-    }
-
-    fn translate_page(&self, page: Page) -> Option<Frame> {
-        use self::entry::HUGE_PAGE;
-
-        let p3 = self.p4().next_table(page.p4_index());
-
-        let huge_page = || {
-            p3.and_then(|p3| {
-                let p3_entry = &p3[page.p3_index()];
-                // 1GiB page?
-                if let Some(start_frame) = p3_entry.pointed_frame() {
-                    if p3_entry.flags().contains(HUGE_PAGE) {
-                        // address must be 1GiB aligned
-                        assert!(start_frame.number % (ENTRY_COUNT * ENTRY_COUNT) == 0);
-                        return Some(Frame {
-                            number: start_frame.number + page.p2_index() *
-                                    ENTRY_COUNT + page.p1_index(),
-                        });
-                    }
-                }
-                if let Some(p2) = p3.next_table(page.p3_index()) {
-                    let p2_entry = &p2[page.p2_index()];
-                    // 2MiB page?
-                    if let Some(start_frame) = p2_entry.pointed_frame() {
-                        if p2_entry.flags().contains(HUGE_PAGE) {
-                            // address must be 2MiB aligned
-                            assert!(start_frame.number % ENTRY_COUNT == 0);
-                            return Some(Frame {
-                                number: start_frame.number + page.p1_index()
-                            });
-                        }
-                    }
-                }
-                None
-            })
-        };
-
-        p3.and_then(|p3| p3.next_table(page.p3_index()))
-        .and_then(|p2| p2.next_table(page.p2_index()))
-        .and_then(|p1| p1[page.p1_index()].pointed_frame())
-        .or_else(huge_page)
-    }
-
-    pub fn map_to<A>(&mut self, page: Page, frame: Frame, flags: EntryFlags,
-                    allocator: &mut A)
-        where A: FrameAllocator
-    {
-        let p4 = self.p4_mut();
-        let mut p3 = p4.next_table_create(page.p4_index(), allocator);
-        let mut p2 = p3.next_table_create(page.p3_index(), allocator);
-        let mut p1 = p2.next_table_create(page.p2_index(), allocator);
-
-        assert!(p1[page.p1_index()].is_unused());
-        p1[page.p1_index()].set(frame, flags | PRESENT);
-    }
-
-    pub fn map<A>(&mut self, page: Page, flags: EntryFlags, allocator: &mut A)
-        where A: FrameAllocator
-    {
-        let frame = allocator.allocate_frame().expect("out of memory");
-        self.map_to(page, frame, flags, allocator)
-    }
-
-    pub fn identity_map<A>(&mut self, frame: Frame, flags: EntryFlags, allocator: &mut A)
-        where A: FrameAllocator
-    {
-        let page = Page::containing_address(frame.start_address());
-        self.map_to(page, frame, flags, allocator)
-    }
-
-    fn unmap<A>(&mut self, page: Page, allocator: &mut A)
-        where A: FrameAllocator
+    pub fn with<F>(&mut self,
+                   table: &mut InactivePageTable,
+                   temporary_page: &mut temporary_page::TemporaryPage, // new
+                   f: F)
+        where F: FnOnce(&mut Mapper)
     {
         use x86_64::instructions::tlb;
-        use x86_64::VirtualAddress;
+        use x86_64::registers::control_regs;
 
-        assert!(self.translate(page.start_address()).is_some());
+        {
+            let backup = Frame::containing_address(
+                control_regs::cr3().0 as usize);
 
-        let p1 = self.p4_mut()
-                    .next_table_mut(page.p4_index())
-                    .and_then(|p3| p3.next_table_mut(page.p3_index()))
-                    .and_then(|p2| p2.next_table_mut(page.p2_index()))
-                    .expect("mapping code does not support huge pages");
-        let frame = p1[page.p1_index()].pointed_frame().unwrap();
-        p1[page.p1_index()].set_unused();
-        tlb::flush(VirtualAddress(page.start_address()));
-        // TODO free p(1,2,3) table if empty
-        //allocator.deallocate_frame(frame);
+            // map temporary_page to current p4 table
+            let p4_table = temporary_page.map_table_frame(backup.clone(), self);
+
+            // overwrite recursive mapping
+            self.p4_mut()[511].set(table.p4_frame.clone(), PRESENT | WRITABLE);
+            tlb::flush_all();
+
+            // execute f in the new context
+            f(self);
+
+            // restore recursive mapping to original p4 table
+            p4_table[511].set(backup, PRESENT | WRITABLE);
+            tlb::flush_all();
+        }
+
+        temporary_page.unmap(self);
+    }
+
+    pub fn switch(&mut self, new_table: InactivePageTable) -> InactivePageTable {
+        use x86_64::PhysicalAddress;
+        use x86_64::registers::control_regs;
+
+        let old_table = InactivePageTable {
+            p4_frame: Frame::containing_address(
+                control_regs::cr3().0 as usize
+            ),
+        };
+        unsafe {
+            control_regs::cr3_write(PhysicalAddress(
+                new_table.p4_frame.start_address() as u64));
+        }
+        old_table
     }
 }
+
+pub struct InactivePageTable {
+    p4_frame: Frame,
+}
+
+impl InactivePageTable {
+    pub fn new(frame: Frame,
+               active_table: &mut ActivePageTable,
+               temporary_page: &mut TemporaryPage)
+               -> InactivePageTable {
+        {
+            let table = temporary_page.map_table_frame(frame.clone(),
+                active_table);
+            // now we are able to zero the table
+            table.zero();
+            // set up recursive mapping for the table
+            table[511].set(frame.clone(), PRESENT | WRITABLE);
+        }
+        temporary_page.unmap(active_table);
+
+        InactivePageTable { p4_frame: frame }
+    }
+}
+
+pub fn remap_the_kernel<A>(allocator: &mut A, boot_info: &BootInformation)
+    where A: FrameAllocator
+{
+    let mut temporary_page = TemporaryPage::new(Page { number: 0xcafebabe },
+        allocator);
+
+    let mut active_table = unsafe { ActivePageTable::new() };
+    let mut new_table = {
+        let frame = allocator.allocate_frame().expect("no more frames");
+        InactivePageTable::new(frame, &mut active_table, &mut temporary_page)
+    };
+
+    active_table.with(&mut new_table, &mut temporary_page, |mapper| {
+        let elf_sections_tag = boot_info.elf_sections_tag()
+            .expect("Memory map tag required");
+
+        for section in elf_sections_tag.sections() {
+            use self::entry::WRITABLE;
+
+            if !section.is_allocated() {
+                // section is not loaded to memory
+                continue;
+            }
+            assert!(section.start_address() % PAGE_SIZE == 0,
+                    "sections need to be page aligned");
+
+            println!("mapping section at addr: {:#x}, size: {:#x}",
+                section.addr, section.size);
+
+            let flags = EntryFlags::from_elf_section_flags(section);
+
+            let start_frame = Frame::containing_address(section.start_address());
+            let end_frame = Frame::containing_address(section.end_address() - 1);
+            for frame in Frame::range_inclusive(start_frame, end_frame) {
+                mapper.identity_map(frame, flags, allocator);
+            }
+        }
+
+        // identity map the VGA text buffer
+        let vga_buffer_frame = Frame::containing_address(0xb8000);
+        mapper.identity_map(vga_buffer_frame, WRITABLE, allocator);
+
+        // identity map the multiboot info structure
+        let multiboot_start = Frame::containing_address(boot_info.start_address());
+        let multiboot_end = Frame::containing_address(boot_info.end_address() - 1);
+        for frame in Frame::range_inclusive(multiboot_start, multiboot_end) {
+            mapper.identity_map(frame, PRESENT, allocator);
+        }
+    });
+
+    let old_table = active_table.switch(new_table);
+    println!("NEW TABLE!!!");
+
+    // turn the old p4 page into a guard page
+    let old_p4_page = Page::containing_address(
+      old_table.p4_frame.start_address()
+    );
+    active_table.unmap(old_p4_page, allocator);
+    println!("guard page at {:#x}", old_p4_page.start_address());
+}

src/memory/paging/temporary_page.rs

@@ -0,0 +1,79 @@
+use super::{Page, ActivePageTable, VirtualAddress};
+use super::table::{Table, Level1};
+use memory::{Frame, FrameAllocator};
+
+pub struct TemporaryPage {
+    page: Page,
+    allocator: TinyAllocator,
+}
+
+impl TemporaryPage {
+    pub fn new<A>(page: Page, allocator: &mut A) -> TemporaryPage
+        where A: FrameAllocator
+    {
+        TemporaryPage {
+            page: page,
+            allocator: TinyAllocator::new(allocator),
+        }
+    }
+
+    /// Maps the temporary page to the given frame in the active table.
+    /// Returns the start address of the temporary page.
+    pub fn map(&mut self, frame: Frame, active_table: &mut ActivePageTable)
+        -> VirtualAddress
+    {
+        use super::entry::WRITABLE;
+
+        assert!(active_table.translate_page(self.page).is_none(),
+                "temporary page is already mapped");
+        active_table.map_to(self.page, frame, WRITABLE, &mut self.allocator);
+        self.page.start_address()
+    }
+
+    /// Unmaps the temporary page in the active table.
+    pub fn unmap(&mut self, active_table: &mut ActivePageTable) {
+        active_table.unmap(self.page, &mut self.allocator)
+    }
+
+    /// Maps the temporary page to the given page table frame in the active
+    /// table. Returns a reference to the now mapped table.
+    pub fn map_table_frame(&mut self,
+                        frame: Frame,
+                        active_table: &mut ActivePageTable)
+                        -> &mut Table<Level1> {
+        unsafe { &mut *(self.map(frame, active_table) as *mut Table<Level1>) }
+    }
+}
+
+struct TinyAllocator([Option<Frame>; 3]);
+
+impl TinyAllocator {
+    fn new<A>(allocator: &mut A) -> TinyAllocator
+        where A: FrameAllocator
+    {
+        let mut f = || allocator.allocate_frame();
+        let frames = [f(), f(), f()];
+        TinyAllocator(frames)
+    }
+}
+
+impl FrameAllocator for TinyAllocator {
+    fn allocate_frame(&mut self) -> Option<Frame> {
+        for frame_option in &mut self.0 {
+            if frame_option.is_some() {
+                return frame_option.take();
+            }
+        }
+        None
+    }
+
+    fn deallocate_frame(&mut self, frame: Frame) {
+        for frame_option in &mut self.0 {
+            if frame_option.is_none() {
+                *frame_option = Some(frame);
+                return;
+            }
+        }
+        panic!("Tiny allocator can hold only 3 frames.");
+    }
+}