Merge pull request #166 from nix-community/sd-stub-efi-variables

feat: minimal poc for exporting UEFI variables à la sd-boot

nix/tests/lanzaboote.nix

@@ -267,4 +267,81 @@ in
         f"Expected: {expected_loader_config} is not included in actual config: '{actual_loader_config}'"
     '';
   };
+
+  export-efi-variables = mkSecureBootTest {
+    name = "lanzaboote-exports-efi-variables";
+    machine.environment.systemPackages = [ pkgs.efibootmgr ];
+    testScript = ''
+      import struct
+
+      # We will choose to boot directly on the stub.
+      # To perform this trick, we will boot first with systemd-boot.
+      # Then, we will add a new boot entry in EFI with higher priority
+      # pointing to our stub.
+      # Finally, we will reboot.
+      # We will also assert that systemd-boot is not running
+      # by checking for the sd-boot's specific EFI variables.
+      machine.start()
+
+      # By construction, nixos-generation-1.efi is the stub we are interested in.
+      # TODO: this should work -- machine.succeed("efibootmgr -d /dev/vda -c -l \\EFI\\Linux\\nixos-generation-1.efi") -- efivars are not persisted
+      # across reboots atm?
+      # cheat code no 1
+      machine.succeed("cp /boot/EFI/Linux/nixos-generation-1.efi /boot/EFI/BOOT/BOOTX64.EFI")
+      machine.succeed("cp /boot/EFI/Linux/nixos-generation-1.efi /boot/EFI/systemd/systemd-bootx64.efi")
+
+      # Let's reboot.
+      machine.succeed("sync")
+      machine.crash()
+      machine.start()
+
+      # This is the sd-boot EFI variable indicator, we should not have it at this point.
+      print(machine.execute("bootctl")[1]) # Check if there's incorrect value in the output.
+      machine.succeed(
+          "test -e /sys/firmware/efi/efivars/LoaderEntrySelected-4a67b082-0a4c-41cf-b6c7-440b29bb8c4f && false || true"
+      )
+
+      SD_LOADER_GUID = "4a67b082-0a4c-41cf-b6c7-440b29bb8c4f"
+      expected_variables = ["LoaderDevicePartUUID",
+        "LoaderImageIdentifier",
+        "LoaderFirmwareInfo",
+        "LoaderFirmwareType",
+        "StubInfo",
+        "StubFeatures"
+      ]
+
+      def read_raw_variable(var: str) -> bytes:
+          attr_var = machine.succeed(f"cat /sys/firmware/efi/efivars/{var}-{SD_LOADER_GUID}").encode('raw_unicode_escape')
+          return attr_var[4:] # First 4 bytes are attributes according to https://www.kernel.org/doc/html/latest/filesystems/efivarfs.html
+      def read_string_variable(var: str, encoding='utf-16-le') -> str:
+          return read_raw_variable(var).decode(encoding).rstrip('\x00')
+      def assert_variable_string(var: str, expected: str, encoding='utf-16-le'):
+          with subtest(f"Is `{var}` correctly set"):
+              value = read_string_variable(var, encoding)
+              assert value == expected, f"Unexpected variable value in `{var}`, expected: `{expected.encode(encoding)!r}`, actual: `{value.encode(encoding)!r}`"
+      def assert_variable_string_contains(var: str, expected_substring: str):
+          with subtest(f"Do `{var}` contain expected substrings"):
+              value = read_string_variable(var).strip()
+              assert expected_substring in value, f"Did not find expected substring in `{var}`, expected substring: `{expected_substring}`, actual value: `{value}`"
+
+      # Debug all systemd loader specification GUID EFI variables loaded by the current environment.
+      print(machine.succeed(f"ls /sys/firmware/efi/efivars/*-{SD_LOADER_GUID}"))
+      with subtest("Check if supported variables are exported"):
+          for expected_var in expected_variables:
+              machine.succeed(f"test -e /sys/firmware/efi/efivars/{expected_var}-{SD_LOADER_GUID}")
+
+      with subtest("Is `StubInfo` correctly set"):
+          assert "lanzastub" in read_string_variable("StubInfo"), "Unexpected stub information, provenance is not lanzaboote project!"
+
+      assert_variable_string("LoaderImageIdentifier", "\\EFI\\BOOT\\BOOTX64.EFI")
+      # TODO: exploit QEMU test infrastructure to pass the good value all the time.
+      assert_variable_string("LoaderDevicePartUUID", "1c06f03b-704e-4657-b9cd-681a087a2fdc")
+      # OVMF tests are using EDK II tree.
+      assert_variable_string_contains("LoaderFirmwareInfo", "EDK II")
+      assert_variable_string_contains("LoaderFirmwareType", "UEFI")
+
+      with subtest("Is `StubFeatures` non-zero"):
+          assert struct.unpack('<Q', read_raw_variable("StubFeatures")) != 0
+    '';
+  };
 }

rust/stub/Cargo.lock

@@ -14,6 +14,12 @@ version = "1.3.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "bef38d45163c2f1dde094a7dfd33ccf595c92905c8f8f4fdc18d06fb1037718a"
 
+[[package]]
+name = "bitflags"
+version = "2.2.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "24a6904aef64d73cf10ab17ebace7befb918b82164785cb89907993be7f83813"
+
 [[package]]
 name = "block-buffer"
 version = "0.10.4"
@@ -31,9 +37,9 @@ checksum = "baf1de4339761588bc0619e3cbc0120ee582ebb74b53b4efbf79117bd2da40fd"
 
 [[package]]
 name = "cpufeatures"
-version = "0.2.6"
+version = "0.2.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "280a9f2d8b3a38871a3c8a46fb80db65e5e5ed97da80c4d08bf27fb63e35e181"
+checksum = "3e4c1eaa2012c47becbbad2ab175484c2a84d1185b566fb2cc5b8707343dfe58"
 dependencies = [
  "libc",
 ]
@@ -83,6 +89,7 @@ dependencies = [
 name = "lanzaboote_stub"
 version = "0.1.0"
 dependencies = [
+ "bitflags 2.2.1",
  "goblin",
  "log",
  "sha2",
@@ -92,9 +99,9 @@ dependencies = [
 
 [[package]]
 name = "libc"
-version = "0.2.141"
+version = "0.2.142"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3304a64d199bb964be99741b7a14d26972741915b3649639149b2479bb46f4b5"
+checksum = "6a987beff54b60ffa6d51982e1aa1146bc42f19bd26be28b0586f252fccf5317"
 
 [[package]]
 name = "log"
@@ -212,7 +219,7 @@ version = "0.20.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ab39d5e7740f21ed4c46d6659f31038bbe3fe7a8be1f702d8a984348837c43b1"
 dependencies = [
- "bitflags",
+ "bitflags 1.3.2",
  "log",
  "ptr_meta",
  "ucs2",

rust/stub/Cargo.toml

@@ -4,12 +4,13 @@ version = "0.1.0"
 edition = "2021"
 publish = false
 # For UEFI target
-rust_version = "1.68"
+rust-version = "1.68"
 
 [dependencies]
 uefi = { version = "0.20.0", default-features = false, features = [ "alloc", "global_allocator" ] }
 uefi-services = { version = "0.17.0", default-features = false, features = [ "panic_handler", "logger" ] }
 goblin = { version = "0.6.1", default-features = false, features = [ "pe64", "alloc" ]}
+bitflags = "2.2.1"
 
 # Even in debug builds, we don't enable the debug logs, because they generate a lot of spam from goblin.
 log = { version = "0.4.17", default-features = false, features = [ "max_level_info", "release_max_level_warn" ]}

rust/stub/src/efivars.rs

@@ -0,0 +1,221 @@
+use alloc::{vec::Vec, vec, string::ToString, format};
+use uefi::{Result, Handle, prelude::{BootServices, RuntimeServices}, proto::{device_path::{DevicePath, media::{HardDrive, PartitionSignature}, DeviceType, DeviceSubType, text::DevicePathToText}, loaded_image::LoadedImage}, Guid, guid, table::{runtime::{VariableVendor, VariableAttributes}, SystemTable, Boot}, cstr16, CStr16};
+
+use bitflags::bitflags;
+
+fn disk_get_part_uuid(boot_services: &BootServices, disk_handle: Handle) -> Result<Guid> {
+    let dp = boot_services.open_protocol_exclusive::<DevicePath>(disk_handle)?;
+
+    for node in dp.node_iter() {
+        if node.device_type() != DeviceType::MEDIA || node.sub_type() != DeviceSubType::MEDIA_HARD_DRIVE {
+            continue;
+        }
+
+        if let Ok(hd_path) = <&HardDrive>::try_from(node) {
+            if let PartitionSignature::Guid(guid) = hd_path.partition_signature() {
+                return Ok(guid);
+            }
+        }
+    }
+
+    Err(uefi::Status::UNSUPPORTED.into())
+}
+
+
+/// systemd loader's GUID
+/// != systemd's GUID
+/// https://github.com/systemd/systemd/blob/main/src/boot/efi/util.h#L114-L121
+/// https://systemd.io/BOOT_LOADER_INTERFACE/
+pub const BOOT_LOADER_VENDOR_UUID: VariableVendor = VariableVendor(guid!("4a67b082-0a4c-41cf-b6c7-440b29bb8c4f"));
+
+/// Lanzaboote stub name
+pub static STUB_INFO_STRING: &str = concat!("lanzastub ", env!("CARGO_PKG_VERSION"));
+
+bitflags! {
+
+    #[repr(transparent)]
+    #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, Default)]
+    /// Feature flags as described in https://systemd.io/BOOT_LOADER_INTERFACE/
+    pub struct EfiLoaderFeatures: u64 {
+       const ConfigTimeout = 1 << 0;
+       const ConfigTimeoutOneShot = 1 << 1;
+       const EntryDefault = 1 << 2;
+       const EntryOneshot = 1 << 3;
+       const BootCounting = 1 << 4;
+       const XBOOTLDR = 1 << 5;
+       const RandomSeed = 1 << 6;
+       const LoadDriver = 1 << 7;
+       const SortKey = 1 << 8;
+       const SavedEntry = 1 << 9;
+       const DeviceTree = 1 << 10;
+    }
+}
+
+/// Get the currently supported EFI features from the loader if they do exist
+/// https://systemd.io/BOOT_LOADER_INTERFACE/
+pub fn get_loader_features(runtime_services: &RuntimeServices) -> Result<EfiLoaderFeatures> {
+    if let Ok(size) = runtime_services.get_variable_size(cstr16!("LoaderFeatures"), &BOOT_LOADER_VENDOR_UUID) {
+        let mut buffer = vec![0; size].into_boxed_slice();
+        runtime_services.get_variable(
+            cstr16!("LoaderFeatures"),
+            &BOOT_LOADER_VENDOR_UUID,
+            &mut buffer)?;
+
+        return EfiLoaderFeatures::from_bits(
+            u64::from_le_bytes(
+                (*buffer).try_into()
+                .map_err(|_err| uefi::Status::BAD_BUFFER_SIZE)?
+            ))
+            .ok_or_else(|| uefi::Status::INCOMPATIBLE_VERSION.into());
+    }
+
+    Ok(Default::default())
+}
+
+bitflags! {
+    #[repr(transparent)]
+    #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+    /// Feature flags as described in https://www.freedesktop.org/software/systemd/man/systemd-stub.html
+    pub struct EfiStubFeatures: u64 {
+       /// Is `LoaderDevicePartUUID` loaded in UEFI variables?
+       const ReportBootPartition = 1 << 0;
+       /// Are credentials picked up from the boot partition?
+       const PickUpCredentials = 1 << 1;
+       /// Are system extensions picked up from the boot partition?
+       const PickUpSysExts = 1 << 2;
+       /// Are we able to measure kernel image, parameters and sysexts?
+       const ThreePcrs = 1 << 3;
+       /// Can we pass a random seed to the kernel?
+       const RandomSeed = 1 << 4;
+    }
+}
+
+// This won't work on a big endian system.
+// But okay, we do not really care, do we?
+#[cfg(target_endian = "little")]
+pub fn from_u16(from: &[u16]) -> &[u8] {
+    unsafe {
+        core::slice::from_raw_parts(from.as_ptr() as *mut u8, from.len().checked_mul(2).unwrap())
+    }
+}
+
+// Remove me when https://github.com/rust-osdev/uefi-rs/pull/788 lands
+pub fn cstr16_to_bytes(s: &CStr16) -> &[u8] {
+    from_u16(s.to_u16_slice_with_nul())
+}
+
+/// Ensures that an UEFI variable is set or set it with a fallback value
+/// computed in a lazy way.
+pub fn ensure_efi_variable<F>(runtime_services: &RuntimeServices,
+    name: &CStr16,
+    vendor: &VariableVendor,
+    attributes: VariableAttributes,
+    get_fallback_value: F) -> uefi::Result
+    where F: FnOnce() -> uefi::Result<Vec<u8>>
+{
+    // If we get a variable size, a variable already exist.
+    if runtime_services.get_variable_size(name, vendor).is_err() {
+        runtime_services.set_variable(
+            name,
+            vendor,
+            attributes,
+            &get_fallback_value()?
+        )?;
+    }
+
+    uefi::Status::SUCCESS.into()
+}
+
+/// Exports systemd-stub style EFI variables
+pub fn export_efi_variables(system_table: &SystemTable<Boot>) -> Result<()> {
+    let boot_services = system_table.boot_services();
+    let runtime_services = system_table.runtime_services();
+
+    let stub_features: EfiStubFeatures =
+        EfiStubFeatures::ReportBootPartition;
+
+    let loaded_image =
+        boot_services.open_protocol_exclusive::<LoadedImage>(boot_services.image_handle())?;
+
+    let default_attributes = VariableAttributes::BOOTSERVICE_ACCESS | VariableAttributes::RUNTIME_ACCESS;
+
+    #[allow(unused_must_use)]
+    // LoaderDevicePartUUID
+    ensure_efi_variable(runtime_services,
+        cstr16!("LoaderDevicePartUUID"),
+        &BOOT_LOADER_VENDOR_UUID,
+        default_attributes,
+        || disk_get_part_uuid(boot_services, loaded_image.device()).map(|guid| guid.to_string()
+            .encode_utf16()
+            .flat_map(|c| c.to_le_bytes())
+            .collect::<Vec<u8>>()
+        )
+    ).ok();
+    // LoaderImageIdentifier
+    ensure_efi_variable(runtime_services,
+        cstr16!("LoaderImageIdentifier"),
+        &BOOT_LOADER_VENDOR_UUID,
+        default_attributes,
+        || {
+            if let Some(dp) = loaded_image.file_path() {
+                let dp_protocol = boot_services.open_protocol_exclusive::<DevicePathToText>(
+                    boot_services.get_handle_for_protocol::<DevicePathToText>()?
+                )?;
+                dp_protocol.convert_device_path_to_text(
+                    boot_services,
+                    dp,
+                    uefi::proto::device_path::text::DisplayOnly(false),
+                    uefi::proto::device_path::text::AllowShortcuts(false)
+                ).map(|ps| cstr16_to_bytes(&ps).to_vec())
+            } else {
+                // If we cannot retrieve the filepath of the loaded image
+                // Then, we cannot set `LoaderImageIdentifier`.
+                Err(uefi::Status::UNSUPPORTED.into())
+            }
+    }).ok();
+    // LoaderFirmwareInfo
+    ensure_efi_variable(runtime_services,
+        cstr16!("LoaderFirmwareInfo"),
+        &BOOT_LOADER_VENDOR_UUID,
+        default_attributes,
+        || Ok(
+            format!("{} {}.{:02}", system_table.firmware_vendor(), system_table.firmware_revision() >> 16, system_table.firmware_revision() & 0xFFFFF)
+            .encode_utf16()
+            .flat_map(|c| c.to_le_bytes())
+            .collect::<Vec<u8>>()
+        )
+    ).ok();
+    // LoaderFirmwareType
+    ensure_efi_variable(runtime_services,
+        cstr16!("LoaderFirmwareType"),
+        &BOOT_LOADER_VENDOR_UUID,
+        default_attributes,
+        || Ok(
+            format!("UEFI {:02}", system_table.uefi_revision())
+            .encode_utf16()
+            .flat_map(|c| c.to_le_bytes())
+            .collect::<Vec<u8>>()
+        )
+    ).ok();
+    // StubInfo
+    // FIXME: ideally, no one should be able to overwrite `StubInfo`, but that would require
+    // constructing an EFI authenticated variable payload. This seems overcomplicated for now.
+    runtime_services.set_variable(
+        cstr16!("StubInfo"),
+        &BOOT_LOADER_VENDOR_UUID,
+        default_attributes,
+        &STUB_INFO_STRING.encode_utf16().flat_map(|c| c.to_le_bytes()).collect::<Vec<u8>>()
+    ).ok();
+
+    // StubFeatures
+    runtime_services.set_variable(
+        cstr16!("StubFeatures"),
+        &BOOT_LOADER_VENDOR_UUID,
+        default_attributes,
+        &stub_features.bits().to_le_bytes()
+    ).ok();
+
+    Ok(())
+}
+
+

rust/stub/src/main.rs

@@ -8,9 +8,10 @@ mod linux_loader;
 mod pe_loader;
 mod pe_section;
 mod uefi_helpers;
+mod efivars;
 
 use alloc::vec::Vec;
-use log::{info, warn};
+use log::{info, warn, debug};
 use pe_loader::Image;
 use pe_section::{pe_section, pe_section_as_string};
 use sha2::{Digest, Sha256};
@@ -22,6 +23,7 @@ use uefi::{
     },
     CStr16, CString16, Result,
 };
+use efivars::{EfiLoaderFeatures, export_efi_variables, get_loader_features};
 
 use crate::{
     linux_loader::InitrdLoader,
@@ -236,6 +238,15 @@ fn main(handle: Handle, mut system_table: SystemTable<Boot>) -> Status {
         warn!("Hash mismatch for initrd!");
     }
 
+    if let Ok(features) = get_loader_features(system_table.runtime_services()) {
+        if !features.contains(EfiLoaderFeatures::RandomSeed) {
+            // FIXME: process random seed then on the disk.
+            debug!("Random seed is available, but lanzaboote does not support it yet.");
+        }
+    }
+    export_efi_variables(&system_table)
+        .expect("Failed to export stub EFI variables");
+
     if is_kernel_hash_correct && is_initrd_hash_correct {
         boot_linux_unchecked(
             handle,