runsc: Run sandbox as user nobody.

When starting a sandbox without direct file or network access, we create an
empty user namespace and run the sandbox in there.  However, the root user in
that namespace is still mapped to the root user in the parent namespace.

This CL maps the "nobody" user from the parent namespace into the child
namespace, and runs the sandbox process as user "nobody" inside the new
namespace.

PiperOrigin-RevId: 211572223
Change-Id: I1b1f9b1a86c0b4e7e5ca7bc93be7d4887678bab6

1 files changed, 31 insertions, 2 deletions

diff --git a/runsc/sandbox/sandbox.go b/runsc/sandbox/sandbox.go
index e0fadefcd..dd5a0aa56 100644
--- a/runsc/sandbox/sandbox.go
+++ b/runsc/sandbox/sandbox.go
@@ -373,8 +373,8 @@ func (s *Sandbox) createSandboxProcess(spec *specs.Spec, conf *boot.Config, bund
 	// User namespace depends on the following options:
 	//   - Host network/filesystem: requires to run inside the user namespace
 	//       specified in the spec or the current namespace if none is configured.
-	//   - Gofer: when using a Gofer, the sandbox process can run isolated in an
-	//       empty namespace.
+	//   - Gofer: when using a Gofer, the sandbox process can run isolated in a
+	//       new user namespace with only the "nobody" user and group.
 	if conf.Network == boot.NetworkHost || conf.FileAccess == boot.FileAccessDirect {
 		if userns, ok := specutils.GetNS(specs.UserNamespace, spec); ok {
 			log.Infof("Sandbox will be started in container's user namespace: %+v", userns)
@@ -391,6 +391,34 @@ func (s *Sandbox) createSandboxProcess(spec *specs.Spec, conf *boot.Config, bund
 	} else {
 		log.Infof("Sandbox will be started in new user namespace")
 		nss = append(nss, specs.LinuxNamespace{Type: specs.UserNamespace})
+
+		if conf.TestOnlyAllowRunAsCurrentUser {
+			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{{
+				ContainerID: int(nobody),
+				HostID:      int(nobody),
+				Size:        int(1),
+			}}
+			cmd.SysProcAttr.GidMappings = []syscall.SysProcIDMap{{
+				ContainerID: int(nobody),
+				HostID:      int(nobody),
+				Size:        int(1),
+			}}
+
+			// Set credentials to run as user and group nobody.
+			cmd.SysProcAttr.Credential = &syscall.Credential{
+				Uid: nobody,
+				Gid: nobody,
+			}
+		} else {
+			return fmt.Errorf("can't run sandbox process as user nobody since we don't have CAP_SETUID or CAP_SETGID")
+		}
 	}
 
 	// Log the fds we are donating to the sandbox process.
@@ -399,6 +427,7 @@ func (s *Sandbox) createSandboxProcess(spec *specs.Spec, conf *boot.Config, bund
 	}
 
 	log.Debugf("Starting sandbox: %s %v", binPath, cmd.Args)
+	log.Debugf("SysProcAttr: %+v", cmd.SysProcAttr)
 	if err := specutils.StartInNS(cmd, nss); err != nil {
 		return err
 	}