net/netutil: confine ipForwardingEnabledLinux read with os.OpenInRoot

Signed-off-by: basavaraj-sm05 <basavaraj@digiscrypt.com>

net/netutil/ip_forward.go

@@ -6,10 +6,10 @@ package netutil
 import (
 	"bytes"
 	"fmt"
+	"io"
 	"net/netip"
 	"os"
 	"os/exec"
-	"path/filepath"
 	"runtime"
 	"strconv"
 	"strings"
@@ -198,7 +198,10 @@ const (
 // sysctl (which on Linux just reads from /proc/sys anyway).
 func ipForwardingEnabledLinux(p protocol, iface string) (bool, error) {
 	k := ipForwardSysctlKey(slashFormat, p, iface)
-	bs, err := os.ReadFile(filepath.Join("/proc/sys", k))
+	// Open k under /proc/sys with os.OpenInRoot so a ".." or symlinked
+	// component in the interface name can't read outside /proc/sys. It's
+	// openat-based, so it's also TOCTOU-resistant. See https://go.dev/blog/osroot.
+	f, err := os.OpenInRoot("/proc/sys", k)
 	if err != nil {
 		if os.IsNotExist(err) {
 			// If IPv6 is disabled, sysctl keys like "net.ipv6.conf.all.forwarding" just don't
@@ -213,6 +216,11 @@ func ipForwardingEnabledLinux(p protocol, iface string) (bool, error) {
 		}
 		return false, err
 	}
+	defer f.Close()
+	bs, err := io.ReadAll(f)
+	if err != nil {
+		return false, err
+	}
 
 	val, err := strconv.ParseInt(string(bytes.TrimSpace(bs)), 10, 32)
 	if err != nil {