Fission Container Executor Function PodSpec Injection Leading to Node Escape

Summary

Fission's Container Executor path lets a tenant supply Function.spec.podspec directly; the executor merges it into the executor-built podspec and creates a Deployment whose pods run the user's container image.

Details

Two flaws compounded:

1. pkg/apis/core/v1/validation.go::FunctionSpec.Validate only checked that spec.PodSpec != nil when executorType: container; it did not inspect the content of spec.PodSpec.
2. pkg/executor/util/merge.go::MergePodSpec unconditionally forwarded hostPID, hostNetwork, hostIPC, hostPath volumes, serviceAccountName, and container privileged into the Deployment spec via the container-executor sink (pkg/executor/executortype/container/deployment.go::getDeploymentSpec).

A tenant with only functions.fission.io/create could deploy a Function with a crafted podspec that mounted the host root filesystem and shared host namespaces. The executor — running under its high-privilege SA, which holds deployments/create on the function namespace — created that Deployment on the tenant's behalf, turning Function-create into effective deployments/create with arbitrary pod-security configuration.

This is the Function-CRD sibling of GHSA-gx55-f84r-v3r7 / GHSA-wmgg-3p4h-48x7, with a lower attack threshold: regular function developers typically hold functions/create but not environments/create.

Impact

A tenant with only functions.fission.io/create is escalated to node escape via a privileged, host-namespace pod scheduled by the executor.

Fix

Fixed in #3391 and released in v1.24.0.

• FunctionSpec.Validate now calls ValidatePodSpecSafety("Function.spec.podspec", spec.PodSpec) after the existing spec.PodSpec == nil check.
• The Function validating webhook is already registered on verbs=create;update, so it picks up the new validation with no marker change.
• The same merge-layer strip and per-container sanitize used for the Environment path applies here, since the container-executor sink calls util.MergePodSpec.

See GHSA-gx55-f84r-v3r7 for the detailed fix.

Behavioural change

Functions whose spec.podspec sets host namespaces, hostPath volumes, container privileged/allowPrivilegeEscalation, dangerous Linux capabilities, or a serviceAccountName override are now rejected at admission. Legitimate container-executor functions that set image, command, args, env, resources, nodeSelector, tolerations, affinity, non-hostPath volumes, or volumeMounts are unaffected.