Make changes to /etc/passwd on disk for non-read only

Bind-mounting /etc/passwd into the container is problematic
becuase of how system utilities like `useradd` work. They want
to make a copy and then rename to try to prevent breakage; this
is, unfortunately, impossible when the file they want to rename
is a bind mount. The current behavior is fine for read-only
containers, though, because we expect useradd to fail in those
cases.

Instead of bind-mounting, we can edit /etc/passwd in the
container's rootfs. This is kind of gross, because the change
will show up in `podman diff` and similar tools, and will be
included in images made by `podman commit`. However, it's a lot
better than breaking important system tools.

Fixes #6953

Signed-off-by: Matthew Heon <matthew.heon@pm.me>

libpod/container_internal_linux.go

@@ -1458,11 +1458,26 @@ func (c *Container) generateCurrentUserPasswdEntry() (string, error) {
 	if uid == 0 {
 		return "", nil
 	}
+
 	u, err := user.LookupId(strconv.Itoa(rootless.GetRootlessUID()))
 	if err != nil {
 		return "", errors.Wrapf(err, "failed to get current user")
 	}
-	return fmt.Sprintf("%s:x:%s:%s:%s:%s:/bin/sh\n", u.Username, u.Uid, u.Gid, u.Username, c.WorkingDir()), nil
+
+	// Lookup the user to see if it exists in the container image.
+	_, err = lookup.GetUser(c.state.Mountpoint, u.Username)
+	if err != User.ErrNoPasswdEntries {
+		return "", err
+	}
+
+	// If the user's actual home directory exists, or was mounted in - use
+	// that.
+	homeDir := c.WorkingDir()
+	if MountExists(c.config.Spec.Mounts, u.HomeDir) {
+		homeDir = u.HomeDir
+	}
+
+	return fmt.Sprintf("%s:x:%s:%s:%s:%s:/bin/sh\n", u.Username, u.Uid, u.Gid, u.Username, homeDir), nil
 }
 
 // generateUserPasswdEntry generates an /etc/passwd entry for the container user
@@ -1488,12 +1503,9 @@ func (c *Container) generateUserPasswdEntry() (string, error) {
 
 	// Lookup the user to see if it exists in the container image
 	_, err = lookup.GetUser(c.state.Mountpoint, userspec)
-	if err != nil && err != User.ErrNoPasswdEntries {
+	if err != User.ErrNoPasswdEntries {
 		return "", err
 	}
-	if err == nil {
-		return "", nil
-	}
 
 	if groupspec != "" {
 		ugid, err := strconv.ParseUint(groupspec, 10, 32)
@@ -1542,6 +1554,32 @@ func (c *Container) generatePasswd() (string, error) {
 	if pwd == "" {
 		return "", nil
 	}
+
+	// If we are *not* read-only - edit /etc/passwd in the container.
+	// This is *gross* (shows up in changes to the container, will be
+	// committed to images based on the container) but it actually allows us
+	// to add users to the container (a bind mount breaks useradd).
+	// We should never get here twice, because generateUserPasswdEntry will
+	// not return anything if the user already exists in /etc/passwd.
+	if !c.IsReadOnly() {
+		containerPasswd, err := securejoin.SecureJoin(c.state.Mountpoint, "/etc/passwd")
+		if err != nil {
+			return "", errors.Wrapf(err, "error looking up location of container %s /etc/passwd", c.ID())
+		}
+
+		f, err := os.OpenFile(containerPasswd, os.O_APPEND|os.O_WRONLY, 0600)
+		if err != nil {
+			return "", errors.Wrapf(err, "error opening container %s /etc/passwd", c.ID())
+		}
+		defer f.Close()
+
+		if _, err := f.WriteString(pwd); err != nil {
+			return "", errors.Wrapf(err, "unable to append to container %s /etc/passwd", c.ID())
+		}
+
+		return "", nil
+	}
+
 	originPasswdFile := filepath.Join(c.state.Mountpoint, "/etc/passwd")
 	orig, err := ioutil.ReadFile(originPasswdFile)
 	if err != nil && !os.IsNotExist(err) {

test/e2e/run_passwd_test.go

@@ -35,27 +35,27 @@ var _ = Describe("Podman run passwd", func() {
 	})
 
 	It("podman run no user specified ", func() {
-		session := podmanTest.Podman([]string{"run", BB, "mount"})
+		session := podmanTest.Podman([]string{"run", "--read-only", BB, "mount"})
 		session.WaitWithDefaultTimeout()
 		Expect(session.ExitCode()).To(Equal(0))
 		Expect(session.LineInOutputContains("passwd")).To(BeFalse())
 	})
 	It("podman run user specified in container", func() {
-		session := podmanTest.Podman([]string{"run", "-u", "bin", BB, "mount"})
+		session := podmanTest.Podman([]string{"run", "--read-only", "-u", "bin", BB, "mount"})
 		session.WaitWithDefaultTimeout()
 		Expect(session.ExitCode()).To(Equal(0))
 		Expect(session.LineInOutputContains("passwd")).To(BeFalse())
 	})
 
 	It("podman run UID specified in container", func() {
-		session := podmanTest.Podman([]string{"run", "-u", "2:1", BB, "mount"})
+		session := podmanTest.Podman([]string{"run", "--read-only", "-u", "2:1", BB, "mount"})
 		session.WaitWithDefaultTimeout()
 		Expect(session.ExitCode()).To(Equal(0))
 		Expect(session.LineInOutputContains("passwd")).To(BeFalse())
 	})
 
 	It("podman run UID not specified in container", func() {
-		session := podmanTest.Podman([]string{"run", "-u", "20001:1", BB, "mount"})
+		session := podmanTest.Podman([]string{"run", "--read-only", "-u", "20001:1", BB, "mount"})
 		session.WaitWithDefaultTimeout()
 		Expect(session.ExitCode()).To(Equal(0))
 		Expect(session.LineInOutputContains("passwd")).To(BeTrue())

test/e2e/run_userns_test.go

@@ -113,6 +113,31 @@ var _ = Describe("Podman UserNS support", func() {
 		Expect(session.OutputToString()).To(Equal("0"))
 	})
 
+	It("podman run --userns=keep-id can add users", func() {
+		if os.Geteuid() == 0 {
+			Skip("Test only runs without root")
+		}
+
+		userName := os.Getenv("USER")
+		if userName == "" {
+			Skip("Can't complete test if no username available")
+		}
+
+		ctrName := "ctr-name"
+		session := podmanTest.Podman([]string{"run", "--userns=keep-id", "--user", "root:root", "-d", "--stop-signal", "9", "--name", ctrName, fedoraMinimal, "sleep", "600"})
+		session.WaitWithDefaultTimeout()
+		Expect(session.ExitCode()).To(Equal(0))
+
+		exec1 := podmanTest.Podman([]string{"exec", "-t", "-i", ctrName, "cat", "/etc/passwd"})
+		exec1.WaitWithDefaultTimeout()
+		Expect(exec1.ExitCode()).To(Equal(0))
+		Expect(exec1.OutputToString()).To(ContainSubstring(userName))
+
+		exec2 := podmanTest.Podman([]string{"exec", "-t", "-i", ctrName, "useradd", "testuser"})
+		exec2.WaitWithDefaultTimeout()
+		Expect(exec2.ExitCode()).To(Equal(0))
+	})
+
 	It("podman --userns=auto", func() {
 		u, err := user.Current()
 		Expect(err).To(BeNil())