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PoC for a shell interpreter sandbox.
This uses mvdan/sh/interp to provide a builtin POSIX shell interpreter whose file operations are (mostly) restricted to a sandbox root. I say mostly because the builtin shell globbing (eg. `echo /etc/*`) still allows listing directories outside the sandbox, though accessing them is prevented. The basics seem to work though there would be quite a bit more work fleshing out the supported utilities. Security wise I'm not sure this gives us much, as one of the goals of this is to allow executables _within_ the sandbox to be executed (eg. Java's keytool). If this is allowed then the package can basically execute arbitrary code without restriction, so I don't think there are many/any security benefits whatsoever. However there are other benefits: consistent shell support across any OS, including Windows. There is no need to rely on particular versions of bash being present. Some safety guarantees around accidentally violating the sandbox - eg. a script that accidentally rm's some files. Are these benefits large enough to warrant fleshing this out? I am not certain.
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| @@ -1,4 +1,5 @@ | ||
| .henv | ||
| .hermit | ||
| testdata/env | ||
| build | ||
| pkgs | ||
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@@ -44,6 +44,7 @@ linters: | |
| - paralleltest | ||
| - ifshort # so annoying | ||
| - prealloc | ||
| - nolintlint | ||
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| linters-settings: | ||
| govet: | ||
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,76 @@ | ||
| package sandbox | ||
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| import ( | ||
| "fmt" | ||
| "path/filepath" | ||
| "strings" | ||
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| "github.com/alecthomas/kong" | ||
| "github.com/pkg/errors" | ||
| "mvdan.cc/sh/v3/interp" | ||
| ) | ||
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| var builtins = map[string]func() builtinCmd{ | ||
| "ls": func() builtinCmd { return &lsCmd{} }, | ||
| "mkdir": func() builtinCmd { return &mkdirCmd{} }, | ||
| "rm": func() builtinCmd { return &rmCmd{} }, | ||
| "cat": func() builtinCmd { return &catCmd{} }, | ||
| "grep": func() builtinCmd { return &grepCmd{} }, | ||
| } | ||
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| type cmdCtx struct { | ||
| *Sandbox | ||
| interp.HandlerContext | ||
| runner *interp.Runner | ||
| } | ||
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| // Sanitise a path within the sandbox. | ||
| func (c *cmdCtx) Sanitise(path string) (string, error) { | ||
| if !filepath.IsAbs(path) { | ||
| path = filepath.Join(c.Dir, path) | ||
| } | ||
| if !strings.HasPrefix(path, c.root) { | ||
| return "", errors.Wrap(ErrSandboxViolation, path) | ||
| } | ||
| return path, nil | ||
| } | ||
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| type builtinCmd interface { | ||
| Run(bctx cmdCtx) error | ||
| } | ||
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| func runBuiltinCmd(bctx cmdCtx, args []string) (present bool, err error) { | ||
| defer func() { | ||
| if status, ok := recover().(int); ok { | ||
| present = true | ||
| if status != 0 { | ||
| err = interp.NewExitStatus(uint8(status)) | ||
| } | ||
| } | ||
| }() | ||
| // fmt.Fprintf(bctx.Stderr, "+ %s\n", shellquote.Join(args...)) | ||
| factory, ok := builtins[args[0]] | ||
| if !ok { | ||
| return false, nil | ||
| } | ||
| cmd := factory() | ||
| exitStatus := 0 | ||
| _, err = kong.Must(cmd, | ||
| kong.Exit(func(i int) { panic(i) }), | ||
| kong.ShortUsageOnError(), | ||
| kong.Name(args[0]), | ||
| ).Parse(args[1:]) | ||
| if err != nil { | ||
| fmt.Fprintf(bctx.Stderr, "%s: %s\n", args[0], err) | ||
| return true, interp.NewExitStatus(1) | ||
| } | ||
| if exitStatus != 0 { | ||
| return true, nil | ||
| } | ||
| err = cmd.Run(bctx) | ||
| if err != nil { | ||
| fmt.Fprintf(bctx.Stderr, "%s: %s\n", args[0], err) | ||
| return true, interp.NewExitStatus(1) | ||
| } | ||
| return true, nil | ||
| } |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,36 @@ | ||
| package sandbox | ||
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| import ( | ||
| "io" | ||
| "os" | ||
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| "github.com/pkg/errors" | ||
| ) | ||
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| type catCmd struct { | ||
| Paths []string `arg:"" optional:"" help:"Files to cat, if any."` | ||
| } | ||
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| func (c *catCmd) Run(ctx cmdCtx) error { | ||
| if len(c.Paths) == 0 { | ||
| _, err := io.Copy(ctx.Stdout, ctx.Stdin) | ||
| return errors.WithStack(err) | ||
| } | ||
| for _, path := range c.Paths { | ||
| var err error | ||
| path, err = ctx.Sanitise(path) | ||
| if err != nil { | ||
| return errors.WithStack(err) | ||
| } | ||
| r, err := os.Open(path) | ||
| if err != nil { | ||
| return errors.WithStack(err) | ||
| } | ||
| _, err = io.Copy(ctx.Stdout, r) | ||
| _ = r.Close() | ||
| if err != nil { | ||
| return errors.WithStack(err) | ||
| } | ||
| } | ||
| return nil | ||
| } |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,61 @@ | ||
| package sandbox | ||
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| import ( | ||
| "bufio" | ||
| "fmt" | ||
| "io" | ||
| "os" | ||
| "regexp" | ||
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| "github.com/pkg/errors" | ||
| ) | ||
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| type grepCmd struct { | ||
| Invert bool `short:"v" help:"Invert match."` | ||
| List bool `short:"l" help:"List matching filenames."` | ||
| Pattern string `arg:"" help:"Pattern to match."` | ||
| Files []string `arg:"" optional:"" help:"Files to search."` | ||
| } | ||
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| func (g *grepCmd) Run(ctx cmdCtx) error { | ||
| re, err := regexp.CompilePOSIX(g.Pattern) | ||
| if err != nil { | ||
| return errors.WithStack(err) | ||
| } | ||
| if len(g.Files) == 0 { | ||
| return errors.WithStack(g.grep(ctx, re, "-", ctx.Stdin)) | ||
| } | ||
| for _, file := range g.Files { | ||
| file, err = ctx.Sanitise(file) | ||
| if err != nil { | ||
| return errors.Wrap(err, "grep") | ||
| } | ||
| r, err := os.Open(file) | ||
| if err != nil { | ||
| return errors.WithStack(err) | ||
| } | ||
| err = g.grep(ctx, re, file, r) | ||
| _ = r.Close() | ||
| if err != nil { | ||
| return errors.WithStack(err) | ||
| } | ||
| } | ||
| return nil | ||
| } | ||
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| func (g *grepCmd) grep(ctx cmdCtx, re *regexp.Regexp, filename string, r io.Reader) error { | ||
| s := bufio.NewScanner(r) | ||
| for s.Scan() { | ||
| line := s.Bytes() | ||
| if re.Find(line) == nil { | ||
| continue | ||
| } | ||
| if g.List { | ||
| fmt.Fprintln(ctx.Stdout, filename) | ||
| return nil | ||
| } | ||
| fmt.Fprintln(ctx.Stdout, string(line)) | ||
| } | ||
| fmt.Fprint(ctx.Stdout) | ||
| return errors.WithStack(s.Err()) | ||
| } |
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