Use scratch as base image

The goal with this changes is to decrease the attack surface of the
published images, by relying on a scratch base image.
In order to support such approach, the following changes were made:
- Statically compile the application to use the network and os/user implementations from Pure Go.
- Embed time zone data into the application via time/tzdata.
- Embed x509 roots from Go, which was introduced on Go 1.20.
- Create a top layer dir structure that complies with FHS 3.0.
- Create files /etc/{passwd,shadow,group,nsswitch.conf}.
- Bump Go to 1.21 (given that 1.20 is half way through its life)

Signed-off-by: Paulo Gomes <paulo.gomes@suse.com>

Dockerfile.dapper

@@ -1,4 +1,4 @@
-FROM registry.suse.com/bci/golang:1.19
+FROM registry.suse.com/bci/golang:1.21
 
 # k3d and kubectl versions must be aligned with the Kubernetes versions
 # set in tests/k3s-bench-test.yaml.

go.mod

@@ -1,6 +1,6 @@
 module github.com/rancher/cis-operator
 
-go 1.19
+go 1.21
 
 require (
 	github.com/blang/semver v3.5.1+incompatible
@@ -69,6 +69,7 @@ require (
 	github.com/spf13/pflag v1.0.5 // indirect
 	github.com/spf13/viper v1.15.0 // indirect
 	github.com/subosito/gotenv v1.4.2 // indirect
+	golang.org/x/crypto/x509roots/fallback v0.0.0-20231030152948-74c2ba9521f1 // indirect
 	golang.org/x/mod v0.9.0 // indirect
 	golang.org/x/net v0.8.0 // indirect
 	golang.org/x/oauth2 v0.5.0 // indirect

go.sum

@@ -374,6 +374,8 @@ golang.org/x/crypto v0.0.0-20200220183623-bac4c82f6975/go.mod h1:LzIPMQfyMNhhGPh
 golang.org/x/crypto v0.0.0-20200622213623-75b288015ac9/go.mod h1:LzIPMQfyMNhhGPhUkYOs5KpL4U8rLKemX1yGLhDgUto=
 golang.org/x/crypto v0.0.0-20210421170649-83a5a9bb288b/go.mod h1:T9bdIzuCu7OtxOm1hfPfRQxPLYneinmdGuTeoZ9dtd4=
 golang.org/x/crypto v0.0.0-20211108221036-ceb1ce70b4fa/go.mod h1:GvvjBRRGRdwPK5ydBHafDWAxML/pGHZbMvKqRZ5+Abc=
+golang.org/x/crypto/x509roots/fallback v0.0.0-20231030152948-74c2ba9521f1 h1:wQ75dCmVn5ExryuIUzbi2MC1/10fUNIL1FP918r4jx8=
+golang.org/x/crypto/x509roots/fallback v0.0.0-20231030152948-74c2ba9521f1/go.mod h1:kNa9WdvYnzFwC79zRpLRMJbdEFlhyM5RPFBBZp/wWH8=
 golang.org/x/exp v0.0.0-20190121172915-509febef88a4/go.mod h1:CJ0aWSM057203Lf6IL+f9T1iT9GByDxfZKAQTCR3kQA=
 golang.org/x/exp v0.0.0-20190306152737-a1d7652674e8/go.mod h1:CJ0aWSM057203Lf6IL+f9T1iT9GByDxfZKAQTCR3kQA=
 golang.org/x/exp v0.0.0-20190510132918-efd6b22b2522/go.mod h1:ZjyILWgesfNpC6sMxTJOJm9Kp84zZh5NQWvqDGG3Qr8=

main.go

@@ -24,6 +24,16 @@ import (
 
 	cisoperatorapiv1 "github.com/rancher/cis-operator/pkg/apis/cis.cattle.io/v1"
 	cisoperator "github.com/rancher/cis-operator/pkg/securityscan"
+
+	// Automatically sets fallback trusted x509 roots, in case they are
+	// not available at runtime. This is required to establish trust
+	// when deployed into a scratch container.
+	_ "golang.org/x/crypto/x509roots/fallback"
+
+	// Embed a copy of the timezone database, so that it does not depend
+	// on it being available at runtime.
+	_ "time/tzdata"
+
 	corev1 "k8s.io/api/core/v1"
 )
 

package/Dockerfile

@@ -1,7 +1,44 @@
-FROM registry.suse.com/bci/bci-busybox:15.5
+FROM registry.suse.com/bci/bci-busybox:15.5 as builder
 
-COPY bin/cis-operator /usr/bin/
+# There is no real need for containers to fully comply with the
+# Filesystem Hierarchy Standard (FHS). However, some applications
+# could malfunction if some specific basic dirs are not available.
+# Therefore, create top level structure.
+#
+# https://refspecs.linuxfoundation.org/FHS_3.0/fhs/index.html
+RUN mkdir -p /final/boot && \
+    mkdir -p /final/etc && \
+    mkdir -p /final/home && \
+    mkdir -p /final/lib && \
+    mkdir -p /final/lib64 && \
+    mkdir -p /final/media && \
+    mkdir -p /final/mnt && \
+    mkdir -p /final/opt && \
+    mkdir -p /final/run && \
+    mkdir -p /final/usr/sbin && \
+    mkdir -p /final/var/lib/nobody
 
-USER 65535:65535
+# Some dirs require very specific permissions.
+RUN install -dv -m 0750 /final/root && \
+    install -dv -m 1777 /final/tmp /final/var/tmp
+
+# Keep name search configured in line with BCI.
+RUN cp /etc/nsswitch.conf /final/etc
+
+# Differs from BCI, by removing /bin/sh from root:
+RUN echo "root:x:0:0:root:/root:/usr/bin/false\nnobody:x:65534:65534:nobody:/var/lib/nobody:/usr/bin/false" > /final/etc/passwd
+
+RUN cp /etc/shadow /final/etc
+RUN cp /etc/group /final/etc
+
+COPY bin/cis-operator /final/usr/bin/
+
+FROM scratch as final
+
+COPY --from=builder /final/ /
+
+# Aligns nobody user ID with BCI.
+USER 65534:65534
+ENV PATH=/usr/bin
 
 CMD ["cis-operator"]

scripts/build

@@ -6,13 +6,20 @@ source $(dirname $0)/version
 cd $(dirname $0)/..
 
 mkdir -p bin
+
+# Leans on Pure Go for the network stack and os/user. For more information:
+# - https://github.com/golang/go/blob/4cd201b14b6216e72ffa175747c20d1191e5eb57/src/net/net.go#L39-L81
+# - https://github.com/golang/go/blob/4cd201b14b6216e72ffa175747c20d1191e5eb57/src/os/user/user.go#L6-L17
+GO_TAGS="netgo osusergo"
+LINKFLAGS="-X github.com/rancher/cis-operator.Version=${VERSION}"
+LINKFLAGS="-X github.com/rancher/cis-operator.GitCommit=${COMMIT} ${LINKFLAGS}"
+
 if [ "$(uname)" = "Linux" ]; then
-    OTHER_LINKFLAGS="-extldflags -static -s"
+    LINKFLAGS="${LINKFLAGS} -extldflags -static -s -w"
 fi
-LINKFLAGS="-X github.com/rancher/cis-operator.Version=$VERSION"
-LINKFLAGS="-X github.com/rancher/cis-operator.GitCommit=$COMMIT $LINKFLAGS"
-CGO_ENABLED=0 go build -ldflags "$LINKFLAGS $OTHER_LINKFLAGS" -o bin/cis-operator
-if [ "$CROSS" = "true" ] && [ "$ARCH" = "amd64" ]; then
-    GOOS=darwin go build -ldflags "$LINKFLAGS" -o bin/cis-operator-darwin
-    GOOS=windows go build -ldflags "$LINKFLAGS" -o bin/cis-operator-windows
+
+CGO_ENABLED=0 go build -trimpath -tags "${GO_TAGS}" -ldflags "${LINKFLAGS}" -o bin/cis-operator
+if [ "${CROSS}" = "true" ] && [ "${ARCH}" = "amd64" ]; then
+    GOOS=darwin go build -trimpath -ldflags "${LINKFLAGS}" -o bin/cis-operator-darwin
+    GOOS=windows go build -trimpath -ldflags "${LINKFLAGS}" -o bin/cis-operator-windows
 fi