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The Nomad client renders templates in the same privileged process used for most other client operations. During internal testing, we discovered that a malicious task can create a symlink that can cause template rendering to read and write to arbitrary files outside the allocation sandbox. Because the Nomad agent can be restarted without restarting tasks, we can't simply check that the path is safe at the time we write without encountering a time-of-check/time-of-use race. To protect Nomad client hosts from this attack, we'll now read and write templates in a subprocess: * On Linux/Unix, this subprocess is sandboxed via chroot to the allocation directory. This requires that Nomad is running as a privileged process. A non-root Nomad agent will warn that it cannot sandbox the template renderer. * On Windows, this process is sandboxed via a Windows AppContainer which has been granted access to only to the allocation directory. This does not require special privileges on Windows. (Creating symlinks in the first place can be prevented by running workloads as non-Administrator or non-ContainerAdministrator users.) Both sandboxes cause encountered symlinks to be evaluated in the context of the sandbox, which will result in a "file not found" or "access denied" error, depending on the platform. This change will also require an update to Consul-Template to allow callers to inject a custom `ReaderFunc` and `RenderFunc`. This design is intended as a workaround to allow us to fix this bug without creating backwards compatibility issues for running tasks. A future version of Nomad may introduce a read-only mount specifically for templates and artifacts so that tasks cannot write into the same location that the Nomad agent is. Fixes: https://github.com/hashicorp/nomad/issues/19888 Fixes: CVE-2024-1329
40 lines
1.0 KiB
Go
40 lines
1.0 KiB
Go
// Copyright (c) HashiCorp, Inc.
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// SPDX-License-Identifier: BUSL-1.1
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//go:build !windows
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package renderer
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import (
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"fmt"
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"os"
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"path/filepath"
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"syscall"
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)
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// sandbox is the non-Windows sandbox implementation, which relies on chroot.
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// Although chroot is not an appropriate boundary for tasks (implicitly
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// untrusted), here the only code that's executing is Nomad itself. Returns the
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// new destPath inside the chroot.
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func sandbox(sandboxPath, destPath string) (string, error) {
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err := syscall.Chroot(sandboxPath)
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if err != nil {
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// if the user is running in unsupported non-root configuration, we
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// can't build the sandbox, but need to handle this gracefully
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fmt.Fprintf(os.Stderr, "template-render sandbox %q not available: %v",
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sandboxPath, err)
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return destPath, nil
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}
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destPath, err = filepath.Rel(sandboxPath, destPath)
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if err != nil {
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return "", fmt.Errorf("could not find destination path relative to chroot: %w", err)
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}
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if !filepath.IsAbs(destPath) {
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destPath = "/" + destPath
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}
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return destPath, nil
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}
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