libct/start: use execabs for newuidmap lookup

Since we are looking up the path to newuidmap/newgidmap in one context,
and executing those in another (libct/nsenter), it might make sense to
use a stricter rules for looking up path to those binaries.

Practically it means that if someone wants to use custom newuidmap and
newgidmap binaries from $PATH, it would be impossible to use these from
the current directory by means of PATH=.:$PATH; instead one would have
to do something like PATH=$(pwd):$PATH.

See https://go.dev/blog/path-security for background.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/container_linux.go

@@ -23,6 +23,7 @@ import (
 	"github.com/opencontainers/runtime-spec/specs-go"
 	"github.com/sirupsen/logrus"
 	"github.com/vishvananda/netlink/nl"
+	"golang.org/x/sys/execabs"
 	"golang.org/x/sys/unix"
 	"google.golang.org/protobuf/proto"
 
@@ -2150,7 +2151,7 @@ func (c *linuxContainer) bootstrapData(cloneFlags uintptr, nsMaps map[configs.Na
 				// We resolve the paths for new{u,g}idmap from
 				// the context of runc to avoid doing a path
 				// lookup in the nsexec context.
-				if path, err := exec.LookPath("newuidmap"); err == nil {
+				if path, err := execabs.LookPath("newuidmap"); err == nil {
 					r.AddData(&Bytemsg{
 						Type:  UidmapPathAttr,
 						Value: []byte(path),
@@ -2178,7 +2179,7 @@ func (c *linuxContainer) bootstrapData(cloneFlags uintptr, nsMaps map[configs.Na
 				Value: b,
 			})
 			if c.config.RootlessEUID {
-				if path, err := exec.LookPath("newgidmap"); err == nil {
+				if path, err := execabs.LookPath("newgidmap"); err == nil {
 					r.AddData(&Bytemsg{
 						Type:  GidmapPathAttr,
 						Value: []byte(path),

vendor/golang.org/x/sys/execabs/execabs.go

@@ -0,0 +1,102 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package execabs is a drop-in replacement for os/exec
+// that requires PATH lookups to find absolute paths.
+// That is, execabs.Command("cmd") runs the same PATH lookup
+// as exec.Command("cmd"), but if the result is a path
+// which is relative, the Run and Start methods will report
+// an error instead of running the executable.
+//
+// See https://blog.golang.org/path-security for more information
+// about when it may be necessary or appropriate to use this package.
+package execabs
+
+import (
+	"context"
+	"fmt"
+	"os/exec"
+	"path/filepath"
+	"reflect"
+	"unsafe"
+)
+
+// ErrNotFound is the error resulting if a path search failed to find an executable file.
+// It is an alias for exec.ErrNotFound.
+var ErrNotFound = exec.ErrNotFound
+
+// Cmd represents an external command being prepared or run.
+// It is an alias for exec.Cmd.
+type Cmd = exec.Cmd
+
+// Error is returned by LookPath when it fails to classify a file as an executable.
+// It is an alias for exec.Error.
+type Error = exec.Error
+
+// An ExitError reports an unsuccessful exit by a command.
+// It is an alias for exec.ExitError.
+type ExitError = exec.ExitError
+
+func relError(file, path string) error {
+	return fmt.Errorf("%s resolves to executable in current directory (.%c%s)", file, filepath.Separator, path)
+}
+
+// LookPath searches for an executable named file in the directories
+// named by the PATH environment variable. If file contains a slash,
+// it is tried directly and the PATH is not consulted. The result will be
+// an absolute path.
+//
+// LookPath differs from exec.LookPath in its handling of PATH lookups,
+// which are used for file names without slashes. If exec.LookPath's
+// PATH lookup would have returned an executable from the current directory,
+// LookPath instead returns an error.
+func LookPath(file string) (string, error) {
+	path, err := exec.LookPath(file)
+	if err != nil {
+		return "", err
+	}
+	if filepath.Base(file) == file && !filepath.IsAbs(path) {
+		return "", relError(file, path)
+	}
+	return path, nil
+}
+
+func fixCmd(name string, cmd *exec.Cmd) {
+	if filepath.Base(name) == name && !filepath.IsAbs(cmd.Path) {
+		// exec.Command was called with a bare binary name and
+		// exec.LookPath returned a path which is not absolute.
+		// Set cmd.lookPathErr and clear cmd.Path so that it
+		// cannot be run.
+		lookPathErr := (*error)(unsafe.Pointer(reflect.ValueOf(cmd).Elem().FieldByName("lookPathErr").Addr().Pointer()))
+		if *lookPathErr == nil {
+			*lookPathErr = relError(name, cmd.Path)
+		}
+		cmd.Path = ""
+	}
+}
+
+// CommandContext is like Command but includes a context.
+//
+// The provided context is used to kill the process (by calling os.Process.Kill)
+// if the context becomes done before the command completes on its own.
+func CommandContext(ctx context.Context, name string, arg ...string) *exec.Cmd {
+	cmd := exec.CommandContext(ctx, name, arg...)
+	fixCmd(name, cmd)
+	return cmd
+
+}
+
+// Command returns the Cmd struct to execute the named program with the given arguments.
+// See exec.Command for most details.
+//
+// Command differs from exec.Command in its handling of PATH lookups,
+// which are used when the program name contains no slashes.
+// If exec.Command would have returned an exec.Cmd configured to run an
+// executable from the current directory, Command instead
+// returns an exec.Cmd that will return an error from Start or Run.
+func Command(name string, arg ...string) *exec.Cmd {
+	cmd := exec.Command(name, arg...)
+	fixCmd(name, cmd)
+	return cmd
+}

vendor/modules.txt

@@ -72,6 +72,7 @@ github.com/vishvananda/netns
 golang.org/x/net/bpf
 # golang.org/x/sys v0.0.0-20211116061358-0a5406a5449c
 ## explicit
+golang.org/x/sys/execabs
 golang.org/x/sys/internal/unsafeheader
 golang.org/x/sys/unix
 golang.org/x/sys/windows