install: tpm2-luks: Do not bind to any PCRs with cryptenroll

The default binding to PCR7 just causes problems without adding
much value in reality.  With a generic OS/distribution being installed
(i.e. no custom Secure Boot chain) a malicious actor who managers to
get access to a disk outside of a machine can usually arrange to
boot using the same PCR7 state.

The problem it causes is it creates a hard version locking
requirement between the host system running `bootc install`
and the target OS.

A related, but opposite problem in a way is that today
we don't update shim by default, except when opted-in via
`bootupctl update`; to do so while doing PCR 7 locking will
require e.g. bootupd to learn how to re-enroll with
new shim's PCR7 state.

Signed-off-by: Colin Walters <[email protected]>

.github/workflows/ci.yml

@@ -187,3 +187,27 @@ jobs:
           truncate -s 20G ${tmpdisk}
           sudo podman run --rm -ti --privileged --env RUST_LOG=debug -v /dev:/dev -v /:/target -v /var/lib/containers:/var/lib/containers -v ./usr/bin/bootc:/usr/bin/bootc --pid=host --security-opt label=disable \
             -v ${tmpdisk}:/disk ${image} bootc install to-disk --via-loopback /disk
+  install-tpm2-luks:
+    name: "Test install to-disk --via-loopback tpm2-luks"
+    needs: [build-c9s]
+    runs-on: ubuntu-latest
+    steps:
+      - name: Download
+        uses: actions/download-artifact@v4
+        with:
+          name: bootc-c9s.tar.zst
+      - name: Install
+        run: tar -xvf bootc.tar.zst
+      - name: Integration tests
+        run: |
+          set -xeuo pipefail
+          cat > Containerfile << 'EOF'
+          FROM quay.io/centos-bootc/centos-bootc-dev:stream9
+          COPY tests/50-enable-tpm2-luks.toml /usr/lib/bootc/install
+          EOF
+          image=localhost/test
+          podman build -t "${image}" .
+          tmpdisk=$(mktemp -p /var/tmp)
+          truncate -s 20G ${tmpdisk}
+          sudo podman run --rm -ti --privileged --env RUST_LOG=debug -v /dev:/dev -v /:/target -v /var/lib/containers:/var/lib/containers -v ./usr/bin/bootc:/usr/bin/bootc --pid=host --security-opt label=disable \
+            -v ${tmpdisk}:/disk ${image} bootc install to-disk --block-setup tpm2-luks --via-loopback /disk

docs/src/man-md/bootc-install-config.md

@@ -26,6 +26,22 @@ The `install`` section supports two subfields:
 - `filesystem`: See below.
 - `kargs`: An array of strings; this will be appended to the set of kernel arguments.
 
+## block: direct
+
+This configuration will use a "plain" layout with the default filesystem configured
+in the image for `/`, alongside a separate `/boot` partition and other platform
+state such as an EFI System Partition (if applicable), etc.
+
+## block: tpm2-luks
+
+Bind unlock of filesystem to presence of the default TPM2 device.
+This is implemented using a hardcoded invocation of `systemd-cryptenroll`
+that does not do any PCR binding, and does not enable a password or recovery
+key.  This is suitable as a basic default for for e.g. headless cloud environments.
+More sophisticated use cases will currently most likely want to use `bootc install to-filesystem`
+and explicitly invoke `systemd-cryptenroll` or other tools to set up
+the desired encryption state.
+
 # filesystem
 
 There is one valid field:

lib/src/install/baseline.rs

@@ -77,8 +77,17 @@ pub(crate) struct InstallBlockDeviceOpts {
 
     /// Target root block device setup.
     ///
-    /// direct: Filesystem written directly to block device
-    /// tpm2-luks: Bind unlock of filesystem to presence of the default tpm2 device.
+    /// - direct: Filesystem written directly to block device
+    /// - tpm2-luks: Bind unlock of filesystem to presence of the default TPM2 device.
+    ///   This is implemented using a hardcoded invocation of `systemd-cryptenroll`
+    ///   that does not do any PCR binding, and does not enable a password or recovery
+    ///   key.  Effectively this just guards the encryption state to the disk being "physically"
+    ///   connected to the same machine.  This is a very simple model
+    ///   that can serve as a basic default for for e.g. headless cloud environments.
+    ///   More sophisticated use cases will most likely want to use `bootc install to-filesystem`
+    ///   and explicitly invoke `systemd-cryptenroll` or other tools to set up
+    ///   the desired encryption state.
+    ///
     #[clap(long, value_enum)]
     pub(crate) block_setup: Option<BlockSetup>,
 
@@ -336,7 +345,12 @@ pub(crate) fn install_create_rootfs(
                 .run()?;
             // The --wipe-slot=all removes our temporary passphrase, and binds to the local TPM device.
             Task::new("Enrolling root device with TPM", "systemd-cryptenroll")
-                .args(["--wipe-slot=all", "--tpm2-device=auto", "--unlock-key-file"])
+                .args([
+                    "--wipe-slot=all",
+                    "--tpm2-pcrs=",
+                    "--tpm2-device=auto",
+                    "--unlock-key-file",
+                ])
                 .args([tmp_keyfile])
                 .args([base_rootdev.as_str()])
                 .run_with_stdin_buf(dummy_passphrase_input)?;

tests/50-enable-tpm2-luks.toml

@@ -0,0 +1,2 @@
+[install]
+block = ["direct", "tpm2-luks"]