feat(shim): initial shim support

src/integrations/shim.rs

@@ -0,0 +1,193 @@
+use crate::utils;
+use crate::utils::ResolvedPath;
+use anyhow::{Context, Result, anyhow, bail};
+use std::ffi::c_void;
+use uefi::Handle;
+use uefi::boot::LoadImageSource;
+use uefi::proto::device_path::DevicePath;
+use uefi::proto::unsafe_protocol;
+use uefi_raw::{Guid, Status, guid};
+
+/// Support for the shim loader application for Secure Boot.
+pub struct ShimSupport;
+
+/// Input to the shim mechanisms.
+pub enum ShimInput<'a> {
+    /// Data loaded into a buffer and ready to be verified, owned.
+    OwnedDataBuffer(Option<&'a ResolvedPath>, Vec<u8>),
+    /// Data loaded into a buffer and ready to be verified.
+    DataBuffer(Option<&'a ResolvedPath>, &'a [u8]),
+    /// Data is provided as a resolved path. We will need to load the data to verify it.
+    /// The output will them return the loaded data.
+    ResolvedPath(&'a ResolvedPath),
+}
+
+impl<'a> ShimInput<'a> {
+    /// Accesses the buffer behind the shim input, if available.
+    pub fn buffer(&self) -> Option<&[u8]> {
+        match self {
+            ShimInput::OwnedDataBuffer(_, data) => Some(data),
+            ShimInput::DataBuffer(_, data) => Some(data),
+            ShimInput::ResolvedPath(_) => None,
+        }
+    }
+
+    /// Accesses the full device path to the input.
+    pub fn file_path(&self) -> Option<&DevicePath> {
+        match self {
+            ShimInput::OwnedDataBuffer(path, _) => path.as_ref().map(|it| it.full_path.as_ref()),
+            ShimInput::DataBuffer(path, _) => path.as_ref().map(|it| it.full_path.as_ref()),
+            ShimInput::ResolvedPath(path) => Some(path.full_path.as_ref()),
+        }
+    }
+
+    /// Converts this input into an owned data buffer, where the data is loaded.
+    /// For ResolvedPath, this will read the file.
+    pub fn into_owned_data_buffer(self) -> Result<ShimInput<'a>> {
+        match self {
+            ShimInput::OwnedDataBuffer(root, data) => Ok(ShimInput::OwnedDataBuffer(root, data)),
+
+            ShimInput::DataBuffer(root, data) => {
+                Ok(ShimInput::OwnedDataBuffer(root, data.to_vec()))
+            }
+
+            ShimInput::ResolvedPath(path) => {
+                Ok(ShimInput::OwnedDataBuffer(Some(path), path.read_file()?))
+            }
+        }
+    }
+}
+
+/// Output of the shim verification function.
+/// Since the shim needs to load the data from disk, we will optimize by using that as the data
+/// to actually boot.
+pub enum ShimVerificationOutput {
+    /// The verification failed.
+    VerificationFailed,
+    /// The data provided to the verifier was already a buffer.
+    VerifiedDataNotLoaded,
+    /// Verifying the data resulted in loading the data from the source.
+    /// This contains the data that was loaded, so it won't need to be loaded again.
+    VerifiedDataBuffer(Vec<u8>),
+}
+
+/// The shim lock protocol as defined by the shim loader application.
+#[unsafe_protocol(ShimSupport::SHIM_LOCK_GUID)]
+struct ShimLockProtocol {
+    /// Verify the data in `buffer` with the size `buffer_size` to determine if it is valid.
+    pub shim_verify: unsafe extern "efiapi" fn(buffer: *mut c_void, buffer_size: u32) -> Status,
+    /// Unused function that is defined by the shim.
+    _generate_header: *mut c_void,
+    /// Unused function that is defined by the shim.
+    _read_header: *mut c_void,
+}
+
+impl ShimSupport {
+    /// GUID for the shim lock protocol.
+    const SHIM_LOCK_GUID: Guid = guid!("605dab50-e046-4300-abb6-3dd810dd8b23");
+    /// GUID for the shim image loader protocol.
+    const SHIM_IMAGE_LOADER_GUID: Guid = guid!("1f492041-fadb-4e59-9e57-7cafe73a55ab");
+
+    /// Determines whether the shim is loaded.
+    pub fn loaded() -> Result<bool> {
+        Ok(utils::find_handle(&Self::SHIM_LOCK_GUID)
+            .context("unable to find shim lock protocol")?
+            .is_some())
+    }
+
+    /// Determines whether the shim loader is available.
+    pub fn loader_available() -> Result<bool> {
+        Ok(utils::find_handle(&Self::SHIM_IMAGE_LOADER_GUID)
+            .context("unable to find shim image loader protocol")?
+            .is_some())
+    }
+
+    /// Use the shim to validate the `input`, returning [ShimVerificationOutput] when complete.
+    pub fn validate(input: ShimInput) -> Result<ShimVerificationOutput> {
+        // Acquire the handle to the shim lock protocol.
+        let handle = utils::find_handle(&Self::SHIM_LOCK_GUID)
+            .context("unable to find shim lock protocol")?
+            .ok_or_else(|| anyhow!("unable to find shim lock protocol"))?;
+        // Acquire the protocol exclusively to the shim lock.
+        let protocol = uefi::boot::open_protocol_exclusive::<ShimLockProtocol>(handle)
+            .context("unable to open shim lock protocol")?;
+
+        // If the input type is a device path, we need to load the data.
+        let maybe_loaded_data = match input {
+            ShimInput::OwnedDataBuffer(_, _data) => {
+                bail!("owned data buffer is not supported in the verification function");
+            }
+            ShimInput::DataBuffer(_, _) => None,
+            ShimInput::ResolvedPath(path) => Some(path.read_file()?),
+        };
+
+        // Convert the input to a buffer.
+        // If the input provides the data buffer, we will use that.
+        // Otherwise, we will use the data loaded by this function.
+        let buffer = match input {
+            ShimInput::OwnedDataBuffer(_root, _data) => {
+                bail!("owned data buffer is not supported in the verification function");
+            }
+            ShimInput::DataBuffer(_root, data) => data,
+            ShimInput::ResolvedPath(_path) => maybe_loaded_data
+                .as_deref()
+                .context("expected data buffer to be loaded already")?,
+        };
+
+        // Check if the buffer is too large to verify.
+        if buffer.len() > u32::MAX as usize {
+            bail!("buffer is too large to verify with shim lock protocol");
+        }
+
+        // Call the shim verify function.
+        // SAFETY: The shim verify function is specified by the shim lock protocol.
+        // Calling this function is considered safe because
+        let status =
+            unsafe { (protocol.shim_verify)(buffer.as_ptr() as *mut c_void, buffer.len() as u32) };
+
+        // If the verification failed, return the verification failure output.
+        if !status.is_success() {
+            return Ok(ShimVerificationOutput::VerificationFailed);
+        }
+
+        // If verification succeeded, return the validation output,
+        // which might include the loaded data.
+        Ok(maybe_loaded_data
+            .map(ShimVerificationOutput::VerifiedDataBuffer)
+            .unwrap_or(ShimVerificationOutput::VerifiedDataNotLoaded))
+    }
+
+    /// Load the image specified by the `input` and returns an image handle.
+    pub fn load(current_image: Handle, input: ShimInput) -> Result<Handle> {
+        // Determine whether the shim is loaded.
+        let shim_loaded = Self::loaded().context("unable to determine if shim is loaded")?;
+
+        // Determine whether the shim loader is available.
+        let shim_loader_available =
+            Self::loader_available().context("unable to determine if shim loader is available")?;
+
+        // Determines whether LoadImage in Boot Services must be patched.
+        // Version 16 of the shim doesn't require extra effort to load Secure Boot binaries.
+        // If the image loader is installed, we can skip over the security override.
+        let requires_security_override = shim_loaded && !shim_loader_available;
+
+        // If the security override is required, we will bail for now.
+        if requires_security_override {
+            bail!("shim image loader protocol is not available, please upgrade to shim version 16");
+        }
+
+        // Converts the shim input to an owned data buffer.
+        let input = input
+            .into_owned_data_buffer()
+            .context("unable to convert input to loaded data buffer")?;
+
+        // Constructs a LoadImageSource from the input.
+        let source = LoadImageSource::FromBuffer {
+            buffer: input.buffer().context("unable to get buffer from input")?,
+            file_path: input.file_path(),
+        };
+
+        // Loads the image using Boot Services LoadImage function.
+        uefi::boot::load_image(current_image, source).context("unable to load image")
+    }
+}