runsc: Run sandbox process inside minimal chroot.

We construct a dir with the executable bind-mounted at /exe, and proc mounted
at /proc.  Runsc now executes the sandbox process inside this chroot, thus
limiting access to the host filesystem.  The mounts and chroot dir are removed
when the sandbox is destroyed.

Because this requires bind-mounts, we can only do the chroot if we have
CAP_SYS_ADMIN.

PiperOrigin-RevId: 211994001
Change-Id: Ia71c515e26085e0b69b833e71691830148bc70d1

1 files changed, 29 insertions, 4 deletions

diff --git a/runsc/sandbox/sandbox.go b/runsc/sandbox/sandbox.go
index dd5a0aa56..f6264d5b2 100644
--- a/runsc/sandbox/sandbox.go
+++ b/runsc/sandbox/sandbox.go
@@ -51,6 +51,10 @@ type Sandbox struct {
 	// Pid is the pid of the running sandbox (immutable). May be 0 is the sandbox
 	// is not running.
 	Pid int `json:"pid"`
+
+	// Chroot is the path to the chroot directory that the sandbox process
+	// is running in.
+	Chroot string `json:"chroot"`
 }
 
 // Create creates the sandbox process.
@@ -392,12 +396,11 @@ func (s *Sandbox) createSandboxProcess(spec *specs.Spec, conf *boot.Config, bund
 		log.Infof("Sandbox will be started in new user namespace")
 		nss = append(nss, specs.LinuxNamespace{Type: specs.UserNamespace})
 
-		if conf.TestOnlyAllowRunAsCurrentUser {
+		// If we have CAP_SETUID and CAP_SETGID, then we can also run
+		// as user nobody.
+		if conf.TestOnlyAllowRunAsCurrentUserWithoutChroot {
 			log.Warningf("Running sandbox in test mode as current user (uid=%d gid=%d). This is only safe in tests!", os.Getuid(), os.Getgid())
 		} else if specutils.CanSetUIDGID() {
-			// If we have CAP_SETUID and CAP_SETGID, then we can also run
-			// as user nobody.
-
 			// Map nobody in the new namespace to nobody in the parent namespace.
 			const nobody = 65534
 			cmd.SysProcAttr.UidMappings = []syscall.SysProcIDMap{{
@@ -419,6 +422,23 @@ func (s *Sandbox) createSandboxProcess(spec *specs.Spec, conf *boot.Config, bund
 		} else {
 			return fmt.Errorf("can't run sandbox process as user nobody since we don't have CAP_SETUID or CAP_SETGID")
 		}
+
+		// If we have CAP_SYS_ADMIN, we can create an empty chroot and
+		// bind-mount the executable inside it.
+		if conf.TestOnlyAllowRunAsCurrentUserWithoutChroot {
+			log.Warningf("Running sandbox in test mode without chroot. This is only safe in tests!")
+		} else if specutils.HasCapSysAdmin() {
+			log.Infof("Sandbox will be started in minimal chroot")
+			chroot, err := setUpChroot(conf.Platform)
+			if err != nil {
+				return fmt.Errorf("error setting up chroot: %v", err)
+			}
+			cmd.SysProcAttr.Chroot = chroot
+			cmd.Args[0] = "/runsc"
+			cmd.Path = "/runsc"
+		} else {
+			return fmt.Errorf("can't run sandbox process in minimal chroot since we don't have CAP_SYS_ADMIN")
+		}
 	}
 
 	// Log the fds we are donating to the sandbox process.
@@ -525,6 +545,11 @@ func (s *Sandbox) Destroy() error {
 		log.Debugf("Killing sandbox %q", s.ID)
 		signalProcess(s.Pid, unix.SIGKILL)
 	}
+
+	if s.Chroot != "" {
+		return tearDownChroot(s.Chroot)
+	}
+
 	return nil
 }