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.

Author: alecthomas

.gitignore

@@ -1,4 +1,5 @@
 .henv
+.hermit
 testdata/env
 build
 pkgs

go.mod

@@ -17,7 +17,6 @@ require (
 	github.com/gobwas/glob v0.2.3
 	github.com/gofrs/flock v0.8.0
 	github.com/kballard/go-shellquote v0.0.0-20180428030007-95032a82bc51
-	github.com/kr/text v0.2.0 // indirect
 	github.com/mattn/go-isatty v0.0.12
 	github.com/mitchellh/go-ps v1.0.0
 	github.com/niemeyer/pretty v0.0.0-20200227124842-a10e7caefd8e // indirect
@@ -33,8 +32,9 @@ require (
 	github.com/xi2/xz v0.0.0-20171230120015-48954b6210f8
 	go.etcd.io/bbolt v1.3.5
 	golang.org/x/crypto v0.0.0-20201221181555-eec23a3978ad
-	golang.org/x/sys v0.0.0-20210124154548-22da62e12c0c
+	golang.org/x/sys v0.0.0-20210503080704-8803ae5d1324
 	golang.org/x/text v0.3.2 // indirect
 	gopkg.in/check.v1 v1.0.0-20200227125254-8fa46927fb4f // indirect
 	howett.net/plist v0.0.0-20200419221736-3b63eb3a43b5
+	mvdan.cc/sh/v3 v3.3.0
 )

go.sum

@@ -22,6 +22,8 @@ github.com/avvmoto/buf-readerat v0.0.0-20171115124131-a17c8cb89270/go.mod h1:2Xt
 github.com/blakesmith/ar v0.0.0-20190502131153-809d4375e1fb h1:m935MPodAbYS46DG4pJSv7WO+VECIWUQ7OJYSoTrMh4=
 github.com/blakesmith/ar v0.0.0-20190502131153-809d4375e1fb/go.mod h1:PkYb9DJNAwrSvRx5DYA+gUcOIgTGVMNkfSCbZM8cWpI=
 github.com/creack/pty v1.1.9/go.mod h1:oKZEueFk5CKHvIhNR5MUki03XCEU+Q6VDXinZuGJ33E=
+github.com/creack/pty v1.1.11 h1:07n33Z8lZxZ2qwegKbObQohDhXDQxiMMz1NOUGYlesw=
+github.com/creack/pty v1.1.11/go.mod h1:oKZEueFk5CKHvIhNR5MUki03XCEU+Q6VDXinZuGJ33E=
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
@@ -31,6 +33,7 @@ github.com/gobwas/glob v0.2.3 h1:A4xDbljILXROh+kObIiy5kIaPYD8e96x1tgBhUI5J+Y=
 github.com/gobwas/glob v0.2.3/go.mod h1:d3Ez4x06l9bZtSvzIay5+Yzi0fmZzPgnTbPcKjJAkT8=
 github.com/gofrs/flock v0.8.0 h1:MSdYClljsF3PbENUUEx85nkWfJSGfzYI9yEBZOJz6CY=
 github.com/gofrs/flock v0.8.0/go.mod h1:F1TvTiK9OcQqauNUHlbJvyl9Qa1QvF/gOUDKA14jxHU=
+github.com/google/renameio v1.0.1-0.20210406141108-81588dbe0453/go.mod h1:t/HQoYBZSsWSNK35C6CO/TpPLDVWvxOHboWUAweKUpk=
 github.com/hashicorp/hcl v1.0.0 h1:0Anlzjpi4vEasTeNFn2mLJgTSwt0+6sfsiTG8qcWGx4=
 github.com/hashicorp/hcl v1.0.0/go.mod h1:E5yfLk+7swimpb2L/Alb/PJmXilQ/rhwaUYs4T20WEQ=
 github.com/jessevdk/go-flags v1.4.0/go.mod h1:4FA24M0QyGHXBuZZK/XkWh8h0e1EYbRYJSGM75WSRxI=
@@ -39,6 +42,8 @@ github.com/kballard/go-shellquote v0.0.0-20180428030007-95032a82bc51/go.mod h1:C
 github.com/klauspost/compress v1.11.7 h1:0hzRabrMN4tSTvMfnL3SCv1ZGeAP23ynzodBgaHeMeg=
 github.com/klauspost/compress v1.11.7/go.mod h1:aoV0uJVorq1K+umq18yTdKaF57EivdYsUV+/s2qKfXs=
 github.com/kr/pretty v0.1.0/go.mod h1:dAy3ld7l9f0ibDNOQOHHMYYIIbhfbHSm3C4ZsoJORNo=
+github.com/kr/pretty v0.2.1 h1:Fmg33tUaq4/8ym9TJN1x7sLJnHVwhP33CNkpYV/7rwI=
+github.com/kr/pretty v0.2.1/go.mod h1:ipq/a2n7PKx3OHsz4KJII5eveXtPO4qwEXGdVfWzfnI=
 github.com/kr/pty v1.1.1/go.mod h1:pFQYn66WHrOpPYNljwOMqo10TkYh1fy3cYio2l3bCsQ=
 github.com/kr/text v0.1.0/go.mod h1:4Jbv+DJW3UT/LiOwJeYQe1efqtUx/iVham/4vfdArNI=
 github.com/kr/text v0.2.0 h1:5Nx0Ya0ZqY2ygV366QzturHI13Jq95ApcVaJBhpS+AY=
@@ -49,13 +54,15 @@ github.com/mitchellh/go-ps v1.0.0 h1:i6ampVEEF4wQFF+bkYfwYgY+F/uYJDktmvLPf7qIgjc
 github.com/mitchellh/go-ps v1.0.0/go.mod h1:J4lOc8z8yJs6vUwklHw2XEIiT4z4C40KtWVN3nvg8Pg=
 github.com/niemeyer/pretty v0.0.0-20200227124842-a10e7caefd8e h1:fD57ERR4JtEqsWbfPhv4DMiApHyliiK5xCTNVSPiaAs=
 github.com/niemeyer/pretty v0.0.0-20200227124842-a10e7caefd8e/go.mod h1:zD1mROLANZcx1PVRCS0qkT7pwLkGfwJo4zjcN/Tysno=
+github.com/pkg/diff v0.0.0-20210226163009-20ebb0f2a09e/go.mod h1:pJLUxLENpZxwdsKMEsNbx1VGcRFpLqf3715MtcvvzbA=
 github.com/pkg/errors v0.8.1/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
 github.com/pkg/errors v0.9.1 h1:FEBLx1zS214owpjy7qsBeixbURkuhQAwrK5UwLGTwt4=
 github.com/pkg/errors v0.9.1/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/qdm12/reprint v0.0.0-20200326205758-722754a53494 h1:wSmWgpuccqS2IOfmYrbRiUgv+g37W5suLLLxwwniTSc=
 github.com/qdm12/reprint v0.0.0-20200326205758-722754a53494/go.mod h1:yipyliwI08eQ6XwDm1fEwKPdF/xdbkiHtrU+1Hg+vc4=
+github.com/rogpeppe/go-internal v1.7.0/go.mod h1:xXDCJY+GAPziupqXw64V24skbSoqbTEfhy4qGm1nDQc=
 github.com/saracen/go7z v0.0.0-20191010121135-9c09b6bd7fda h1:h+YpzUB/bGVJcLqW+d5GghcCmE/A25KbzjXvWJQi/+o=
 github.com/saracen/go7z v0.0.0-20191010121135-9c09b6bd7fda/go.mod h1:MSotTrCv1PwoR8QgU1JurEx+lNNbtr25I+m0zbLyAGw=
 github.com/saracen/go7z-fixtures v0.0.0-20190623165746-aa6b8fba1d2f h1:PF9WV5j/x6MT+x/sauUHd4objCvJbZb0wdxZkHSdd5A=
@@ -90,14 +97,18 @@ golang.org/x/net v0.0.0-20190404232315-eb5bcb51f2a3/go.mod h1:t9HGtf8HONx5eT2rtn
 golang.org/x/net v0.0.0-20190620200207-3b0461eec859 h1:R/3boaszxrf1GEUWTVDzSKVwLmSJpwZ1yqXm8j0v2QI=
 golang.org/x/net v0.0.0-20190620200207-3b0461eec859/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=
 golang.org/x/sync v0.0.0-20190423024810-112230192c58/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
+golang.org/x/sync v0.0.0-20210220032951-036812b2e83c h1:5KslGYwFpkhGh+Q16bwMP3cOontH8FOep7tGV86Y7SQ=
+golang.org/x/sync v0.0.0-20210220032951-036812b2e83c/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20191026070338-33540a1f6037/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20200116001909-b77594299b42/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20200202164722-d101bd2416d5/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
-golang.org/x/sys v0.0.0-20210124154548-22da62e12c0c h1:VwygUrnw9jn88c4u8GD3rZQbqrP/tgas88tPUbBxQrk=
-golang.org/x/sys v0.0.0-20210124154548-22da62e12c0c/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
-golang.org/x/term v0.0.0-20201117132131-f5c789dd3221 h1:/ZHdbVpdR/jk3g30/d4yUL0JU9kksj8+F/bnQUVLGDM=
+golang.org/x/sys v0.0.0-20201119102817-f84b799fce68/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/sys v0.0.0-20210503080704-8803ae5d1324 h1:pAwJxDByZctfPwzlNGrDN2BQLsdPb9NkhoTJtUkAO28=
+golang.org/x/sys v0.0.0-20210503080704-8803ae5d1324/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/term v0.0.0-20201117132131-f5c789dd3221/go.mod h1:Nr5EML6q2oocZ2LXRh80K7BxOlk5/8JxuGnuhpl+muw=
+golang.org/x/term v0.0.0-20210503060354-a79de5458b56 h1:b8jxX3zqjpqb2LklXPzKSGJhzyxCOZSz8ncv8Nv+y7w=
+golang.org/x/term v0.0.0-20210503060354-a79de5458b56/go.mod h1:tfny5GFUkzUvx4ps4ajbZsCe5lw1metzhBm9T3x7oIY=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
 golang.org/x/text v0.3.2 h1:tW2bmiBqwgJj/UpqtC8EpXEZVYOwU0yG4iWbprSVAcs=
 golang.org/x/text v0.3.2/go.mod h1:bEr9sfX3Q8Zfm5fL9x+3itogRgK3+ptLWKqgva+5dAk=
@@ -110,9 +121,13 @@ gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8
 gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/check.v1 v1.0.0-20200227125254-8fa46927fb4f h1:BLraFXnmrev5lT+xlilqcH8XK9/i0At2xKjWk4p6zsU=
 gopkg.in/check.v1 v1.0.0-20200227125254-8fa46927fb4f/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/errgo.v2 v2.1.0/go.mod h1:hNsd1EY+bozCKY1Ytp96fpM3vjJbqLJn88ws8XvfDNI=
 gopkg.in/yaml.v2 v2.2.1/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
 gopkg.in/yaml.v2 v2.2.2/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
 gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c h1:dUUwHk2QECo/6vqA44rthZ8ie2QXMNeKRTHCNY2nXvo=
 gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
 howett.net/plist v0.0.0-20200419221736-3b63eb3a43b5 h1:AQkaJpH+/FmqRjmXZPELom5zIERYZfwTjnHpfoVMQEc=
 howett.net/plist v0.0.0-20200419221736-3b63eb3a43b5/go.mod h1:vMygbs4qMhSZSc4lCUl2OEE+rDiIIJAIdR4m7MiMcm0=
+mvdan.cc/editorconfig v0.2.0/go.mod h1:lvnnD3BNdBYkhq+B4uBuFFKatfp02eB6HixDvEz91C0=
+mvdan.cc/sh/v3 v3.3.0 h1:ujzElMnry63f4I5sjPFxzo6xia+gwsHZM0yyauuyZ6k=
+mvdan.cc/sh/v3 v3.3.0/go.mod h1:dh3avhLDhJJ/MJKzbak6FYn+DJKUWk7Fb6Dh5mGdv6Y=

shell/sandbox/builtins.go

@@ -0,0 +1,54 @@
+package interp
+
+import (
+	"path/filepath"
+	"strings"
+
+	"github.com/alecthomas/kong"
+	"github.com/pkg/errors"
+	"mvdan.cc/sh/v3/interp"
+)
+
+var builtins = map[string]func() builtinCmd{
+	"ls":    func() builtinCmd { return &lsCmd{} },
+	"mkdir": func() builtinCmd { return &mkdirCmd{} },
+	"rm":    func() builtinCmd { return &rmCmd{} },
+}
+
+type cmdCtx struct {
+	*Sandbox
+	interp.HandlerContext
+	runner *interp.Runner
+}
+
+// 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
+}
+
+type builtinCmd interface {
+	Run(bctx cmdCtx) error
+}
+
+func runBuiltinCmd(bctx cmdCtx, args []string) (error, bool) {
+	factory, ok := builtins[args[0]]
+	if !ok {
+		return nil, false
+	}
+	cmd := factory()
+	_, err := kong.Must(
+		cmd,
+		kong.Exit(func(i int) {}),
+		kong.Description("List files."),
+	).Parse(args[1:])
+	if err != nil {
+		return errors.Wrap(err, args[0]), true
+	}
+	return errors.Wrap(cmd.Run(bctx), args[0]), true
+}

shell/sandbox/find.go

@@ -0,0 +1,4 @@
+package interp
+
+type findCmd struct {
+}

shell/sandbox/ls.go

@@ -0,0 +1,102 @@
+package interp
+
+import (
+	"fmt"
+	"io/fs"
+	"io/ioutil"
+	"os"
+	"os/user"
+	"path/filepath"
+	"strings"
+	"syscall"
+	"text/tabwriter"
+
+	"github.com/pkg/errors"
+)
+
+type lsCmd struct {
+	All  bool `short:"a" help:"List all files."`
+	Long bool `short:"l" help:"Long listing."`
+
+	Paths []string `arg:"" optional:"" help:"Paths to list."`
+}
+
+func (l *lsCmd) Run(ctx cmdCtx) error {
+	paths := l.Paths
+	if len(paths) == 0 {
+		paths = append(paths, ctx.Dir)
+	}
+	w := tabwriter.NewWriter(ctx.Stdout, 4, 4, 1, ' ', 0)
+	for _, path := range paths {
+		path, err := ctx.Sanitise(path)
+		if err != nil {
+			return err
+		}
+		info, err := os.Stat(path)
+		if err != nil {
+			return errors.WithStack(err)
+		}
+		var entries []fs.FileInfo
+		if info.IsDir() {
+			entries, err = ioutil.ReadDir(path)
+			if err != nil {
+				return errors.WithStack(err)
+			}
+			// Synthesise . and ..
+			if l.All {
+				dotdot, err := os.Stat(filepath.Join(path, ".."))
+				if err != nil {
+					return errors.WithStack(err)
+				}
+				entries = append([]fs.FileInfo{
+					&renamedFileInfo{info, "."},
+					&renamedFileInfo{dotdot, ".."},
+				}, entries...)
+			}
+		} else {
+			entries = append(entries, info)
+		}
+		for _, entry := range entries {
+			if !l.All && strings.HasPrefix(entry.Name(), ".") {
+				continue
+			}
+			if !l.Long {
+				fmt.Fprintln(w, entry.Name())
+				continue
+			}
+			var (
+				size      uint16
+				username  = "user"
+				groupname = "group"
+			)
+			if stat, ok := entry.Sys().(*syscall.Stat_t); ok {
+				usr, err := user.LookupId(fmt.Sprintf("%d", stat.Uid))
+				if err != nil {
+					return errors.WithStack(err)
+				}
+				group, err := user.LookupGroupId(fmt.Sprintf("%d", stat.Gid))
+				if err != nil {
+					return errors.WithStack(err)
+				}
+				username = usr.Username
+				groupname = group.Name
+				size = stat.Nlink
+
+			}
+			fmt.Fprintf(w, "%s\t%d\t%s\t%s\t%d\t%s\t%s\n",
+				entry.Mode(), size, username, groupname, entry.Size(),
+				entry.ModTime().Format("2 Jan 15:04"),
+				entry.Name())
+		}
+	}
+	return w.Flush()
+}
+
+type renamedFileInfo struct {
+	fs.FileInfo
+	name string
+}
+
+func (r *renamedFileInfo) Name() string {
+	return r.name
+}

shell/sandbox/mkdir.go

@@ -0,0 +1,36 @@
+package interp
+
+import (
+	"os"
+
+	"github.com/pkg/errors"
+)
+
+type mkdirCmd struct {
+	Mode         os.FileMode `short:"m" help:"Set the file permission bits of the final created directory to the specified mode."`
+	Intermediate bool        `short:"p" help:"Create intermediate directories as needed."`
+	Dirs         []string    `arg:"" help:"Directories to create"`
+}
+
+func (m *mkdirCmd) Run(ctx cmdCtx) error {
+	mode := m.Mode
+	if mode == 0 {
+		mode = 0777
+	}
+	for _, dir := range m.Dirs {
+		dir, err := ctx.Sanitise(dir)
+		if err != nil {
+			return err
+		}
+		if m.Intermediate {
+			if err := os.MkdirAll(dir, mode); err != nil {
+				return errors.Wrap(err, dir)
+			}
+		} else {
+			if err := os.Mkdir(dir, mode); err != nil {
+				return errors.Wrap(err, dir)
+			}
+		}
+	}
+	return nil
+}

shell/sandbox/rm.go

@@ -0,0 +1,33 @@
+package interp
+
+import (
+	"os"
+
+	"github.com/pkg/errors"
+)
+
+type rmCmd struct {
+	Recursive bool `short:"r" help:"Recursively delete."`
+	Force     bool `short:"f" help:"Force deletion of read-only files."`
+
+	Paths []string `arg:"" help:"Paths to delete."`
+}
+
+func (r *rmCmd) Run(bctx cmdCtx) error {
+	for _, path := range r.Paths {
+		path, err := bctx.Sanitise(path)
+		if err != nil {
+			return err
+		}
+		if r.Recursive {
+			if err = os.RemoveAll(path); err != nil {
+				return errors.WithStack(err)
+			}
+		} else {
+			if err = os.Remove(path); err != nil {
+				return errors.WithStack(err)
+			}
+		}
+	}
+	return nil
+}