libboot/src/x86.rs

// Copyright 2023, The Android Open Source Project
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

//! Boot protocol implementation for x86 platforms.
//!
//! For linux, the library currently only supports bzimage and protocol version 2.06+.
//! Specifically, modern memory layout is used, protected kernel is loaded to high address at
//! 0x100000 and command line size can be greater than 255 characters.
//!
//!                     ~                        ~
//!                     |  Protected-mode kernel |
//!             100000  +------------------------+
//!                     |  I/O memory hole       |
//!             0A0000  +------------------------+
//!                     |  Reserved for BIOS     |      Leave as much as possible unused
//!                     ~                        ~
//!                     |  Command line          |      (Can also be below the X+10000 mark)
//!                     +------------------------+
//!                     |  Stack/heap            |      For use by the kernel real-mode code.
//!  low_mem_addr+08000 +------------------------+
//!                     |  Kernel setup          |      The kernel real-mode code.
//!                     |  Kernel boot sector    |      The kernel legacy boot sector.
//!        low_mem_addr +------------------------+
//!                     |  Boot loader           |      <- Boot sector entry point 0000:7C00
//!             001000  +------------------------+
//!                     |  Reserved for MBR/BIOS |
//!             000800  +------------------------+
//!                     |  Typically used by MBR |
//!             000600  +------------------------+
//!                     |  BIOS use only         |
//!             000000  +------------------------+
//!
//! See https://www.kernel.org/doc/html/v5.11/x86/boot.html#the-linux-x86-boot-protocol for more
//! detail.

use crate::*;

use core::arch::asm;
use core::slice::from_raw_parts_mut;

pub use x86_bootparam_defs::{boot_params, e820entry, setup_header};
use zerocopy::{AsBytes, FromBytes, FromZeroes, Ref};

// Sector size is fixed to 512
const SECTOR_SIZE: usize = 512;
/// Boot sector and setup code section is 32K at most.
const BOOT_SETUP_LOAD_SIZE: usize = 0x8000;
/// Address for loading the protected mode kernel
const LOAD_ADDR_HIGH: usize = 0x10_0000;
// Flag value to use high address for protected mode kernel.
const LOAD_FLAG_LOADED_HIGH: u8 = 0x1;

/// Constant for E820 address range type.
pub const E820_ADDRESS_TYPE_RAM: u32 = 1;
pub const E820_ADDRESS_TYPE_RESERVED: u32 = 2;
pub const E820_ADDRESS_TYPE_ACPI: u32 = 3;
pub const E820_ADDRESS_TYPE_NVS: u32 = 4;
pub const E820_ADDRESS_TYPE_UNUSABLE: u32 = 5;
pub const E820_ADDRESS_TYPE_PMEM: u32 = 7;

/// Wrapper for `struct boot_params {}` C structure
#[repr(transparent)]
#[derive(Copy, Clone, AsBytes, FromBytes, FromZeroes)]
pub struct BootParams(boot_params);

impl BootParams {
    /// Cast a bytes into a reference of BootParams header
    pub fn from_bytes_ref(buffer: &[u8]) -> Result<&BootParams> {
        Ok(Ref::<_, BootParams>::new_from_prefix(buffer)
            .ok_or_else(|| BootError::InvalidInput)?
            .0
            .into_ref())
    }

    /// Cast a bytes into a mutable reference of BootParams header.
    pub fn from_bytes_mut(buffer: &mut [u8]) -> Result<&mut BootParams> {
        Ok(Ref::<_, BootParams>::new_from_prefix(buffer)
            .ok_or_else(|| BootError::InvalidInput)?
            .0
            .into_mut())
    }

    /// Return a mutable reference of the `setup_header` struct field in `boot_params`
    pub fn setup_header_mut(&mut self) -> &mut setup_header {
        &mut self.0.hdr
    }

    /// Return a const reference of the `setup_header` struct field in `boot_params`
    pub fn setup_header_ref(&self) -> &setup_header {
        &self.0.hdr
    }

    /// Checks whether image is valid and version is supported.
    pub fn check(&self) -> Result<()> {
        // Check magic.
        if !(self.setup_header_ref().boot_flag == 0xAA55
            && self.setup_header_ref().header.to_le_bytes() == *b"HdrS")
        {
            return Err(BootError::InvalidZImage);
        }

        // Check if it is bzimage and version is supported.
        if !(self.0.hdr.version >= 0x0206
            && ((self.setup_header_ref().loadflags & LOAD_FLAG_LOADED_HIGH) != 0))
        {
            return Err(BootError::UnsupportedZImage);
        }

        Ok(())
    }

    /// Gets the number of sectors in the setup code section.
    pub fn setup_sects(&self) -> usize {
        match self.setup_header_ref().setup_sects {
            0 => 4,
            v => v as usize,
        }
    }

    /// Gets the offset to the protected mode kernel in the image.
    ///
    /// The value is also the same as the sum of legacy boot sector plus setup code size.
    pub fn kernel_off(&self) -> usize {
        // one boot sector + setup sectors
        (1 + self.setup_sects()) * SECTOR_SIZE
    }

    /// Gets e820 map entries.
    pub fn e820_map(&mut self) -> &mut [e820entry] {
        &mut self.0.e820_map[..]
    }
}

/// Boots a Linux bzimage.
///
/// # Args
///
/// * `kernel`: Buffer holding the loaded bzimage.
///
/// * `ramdisk`: Buffer holding the loaded ramdisk.
///
/// * `cmdline`: Command line argument blob.
///
/// * `mmap_cb`: A caller provided callback for setting the e820 memory map. The callback takes in
///     a mutable reference of e820 map entries (&mut [e820entry]). On success, it should return
///     the number of used entries. On error, it can return a
///     `BootError::E820MemoryMapCallbackError(<code>)` to propagate a custom error code.
///
/// * `low_mem_addr`: The lowest memory touched by the bootloader section. This is where boot param
///      starts.
///
/// * The API is not expected to return on success.
///
/// # Safety
///
/// * Caller must ensure that `kernel` contains a valid Linux kernel and `low_mem_addr` is valid
///
/// * Caller must ensure that there is enough memory at address 0x10_0000 for relocating `kernel`.
pub unsafe fn boot_linux_bzimage<F>(
    kernel: &[u8],
    ramdisk: &[u8],
    cmdline: &[u8],
    mmap_cb: F,
    low_mem_addr: usize,
) -> Result<()>
where
    F: FnOnce(&mut [e820entry]) -> Result<u8>,
{
    let bootparam = BootParams::from_bytes_ref(&kernel[..])?;
    bootparam.check()?;

    // low memory address greater than 0x9_0000 is bogus.
    assert!(low_mem_addr <= 0x9_0000);
    // SAFETY: By safety requirement of this function, `low_mem_addr` points to sufficiently large
    // memory.
    let boot_param_buffer =
        unsafe { from_raw_parts_mut(low_mem_addr as *mut _, BOOT_SETUP_LOAD_SIZE) };
    // Note: We currently boot directly from protected mode kernel and bypass real-mode kernel.
    // Thus we omit the heap section. Revisit this if we encounter platforms that have to boot from
    // real-mode kernel.
    let cmdline_start = low_mem_addr + BOOT_SETUP_LOAD_SIZE;
    // Should not reach into I/O memory hole section.
    assert!(cmdline_start + cmdline.len() <= 0x0A0000);
    // SAFETY: By safety requirement of this function, `low_mem_addr` points to sufficiently large
    // memory.
    let cmdline_buffer = unsafe { from_raw_parts_mut(cmdline_start as *mut _, cmdline.len()) };

    let boot_sector_size = bootparam.kernel_off();
    // Copy protected mode kernel to load address
    // SAFETY: By safety requirement of this function, `LOAD_ADDR_HIGH` points to sufficiently
    // large memory.
    unsafe {
        from_raw_parts_mut(LOAD_ADDR_HIGH as *mut u8, kernel[boot_sector_size..].len())
            .clone_from_slice(&kernel[boot_sector_size..]);
    }

    // Copy over boot params to boot sector to prepare for fix-up.
    boot_param_buffer.fill(0);
    boot_param_buffer[..boot_sector_size].clone_from_slice(&kernel[..boot_sector_size]);

    let bootparam_fixup = BootParams::from_bytes_mut(boot_param_buffer)?;

    // Sets commandline.
    cmdline_buffer.clone_from_slice(cmdline);
    bootparam_fixup.setup_header_mut().cmd_line_ptr = cmdline_start.try_into().unwrap();
    bootparam_fixup.setup_header_mut().cmdline_size = cmdline.len().try_into().unwrap();

    // Sets ramdisk address.
    bootparam_fixup.setup_header_mut().ramdisk_image =
        (ramdisk.as_ptr() as usize).try_into().map_err(|_| BootError::IntegerOverflow)?;
    bootparam_fixup.setup_header_mut().ramdisk_size =
        ramdisk.len().try_into().map_err(|_| BootError::IntegerOverflow)?;

    // Sets to loader type "special loader". (Anything other than 0, otherwise linux kernel ignores
    // ramdisk.)
    bootparam_fixup.setup_header_mut().type_of_loader = 0xff;

    // Fix up e820 memory map.
    let num_entries = mmap_cb(bootparam_fixup.e820_map())?;
    bootparam_fixup.0.e820_entries = num_entries;

    // Clears stack pointers, interrupt and jumps to protected mode kernel.
    // SAFETY: By safety requirement of this function, input contains a valid linux kernel.
    #[cfg(target_arch = "x86_64")]
    unsafe {
        asm!(
            "xor ebp, ebp",
            "xor esp, esp",
            "cld",
            "cli",
            "jmp {ep}",
            ep = in(reg) LOAD_ADDR_HIGH,
            in("rsi") low_mem_addr,
        );
    }
    // SAFETY: By safety requirement of this function, input contains a valid linux kernel.
    #[cfg(target_arch = "x86")]
    unsafe {
        asm!(
            "xor ebp, ebp",
            "xor esp, esp",
            "mov esi, eax",
            "cld",
            "cli",
            "jmp {ep}",
            ep = in(reg) LOAD_ADDR_HIGH,
            in("eax") low_mem_addr,
        );
    }

    Ok(())
}