rootless: new function to join existing conmon processes

move the logic for joining existing namespaces down to the rootless
package.  In main_local we still retrieve the list of conmon pid files
and use it from the rootless package.

In addition, create a temporary user namespace for reading these
files, as the unprivileged user might not have enough privileges for
reading the conmon pid file, for example when running with a different
uidmap and root in the container is different than the rootless user.

Closes: https://github.com/containers/libpod/issues/3187

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

4 files changed, 207 insertions, 74 deletions

diff --git a/cmd/podman/main_local.go b/cmd/podman/main_local.go
index 5af05a11e..b4f21bd0c 100644
--- a/cmd/podman/main_local.go
+++ b/cmd/podman/main_local.go
@@ -4,11 +4,9 @@ package main
 
 import (
 	"context"
-	"io/ioutil"
 	"log/syslog"
 	"os"
 	"runtime/pprof"
-	"strconv"
 	"strings"
 	"syscall"
 
@@ -120,18 +118,10 @@ func setupRootless(cmd *cobra.Command, args []string) error {
 		return errors.Wrapf(err, "could not get pause process pid file path")
 	}
 
-	data, err := ioutil.ReadFile(pausePidPath)
-	if err != nil && !os.IsNotExist(err) {
-		return errors.Wrapf(err, "cannot read pause process pid file %s", pausePidPath)
-	}
-	if err == nil {
-		pausePid, err := strconv.Atoi(string(data))
-		if err != nil {
-			return errors.Wrapf(err, "cannot parse pause pid file %s", pausePidPath)
-		}
-		became, ret, err := rootless.JoinUserAndMountNS(uint(pausePid), "")
+	if _, err := os.Stat(pausePidPath); err == nil {
+		became, ret, err := rootless.TryJoinFromFilePaths("", false, []string{pausePidPath})
 		if err != nil {
-			logrus.Errorf("cannot join pause process pid %d.  You may need to remove %s and stop all containers", pausePid, pausePidPath)
+			logrus.Errorf("cannot join pause process.  You may need to remove %s and stop all containers", pausePidPath)
 			logrus.Errorf("you can use `system migrate` to recreate the pause process")
 			logrus.Errorf(err.Error())
 			os.Exit(1)
@@ -154,28 +144,13 @@ func setupRootless(cmd *cobra.Command, args []string) error {
 		logrus.Errorf(err.Error())
 		os.Exit(1)
 	}
-	var became bool
-	var ret int
-	if len(ctrs) == 0 {
-		became, ret, err = rootless.BecomeRootInUserNS(pausePidPath)
-	} else {
-		for _, ctr := range ctrs {
-			data, err := ioutil.ReadFile(ctr.Config().ConmonPidFile)
-			if err != nil {
-				logrus.Errorf(err.Error())
-				continue
-			}
-			conmonPid, err := strconv.Atoi(string(data))
-			if err != nil {
-				logrus.Errorf(err.Error())
-				continue
-			}
-			became, ret, err = rootless.JoinUserAndMountNS(uint(conmonPid), pausePidPath)
-			if err == nil {
-				break
-			}
-		}
+
+	paths := []string{}
+	for _, ctr := range ctrs {
+		paths = append(paths, ctr.Config().ConmonPidFile)
 	}
+
+	became, ret, err := rootless.TryJoinFromFilePaths(pausePidPath, true, paths)
 	if err != nil {
 		logrus.Errorf(err.Error())
 		os.Exit(1)
@@ -185,6 +160,7 @@ func setupRootless(cmd *cobra.Command, args []string) error {
 	}
 	return nil
 }
+
 func setRLimits() error {
 	rlimits := new(syscall.Rlimit)
 	rlimits.Cur = 1048576
diff --git a/pkg/rootless/rootless_linux.c b/pkg/rootless/rootless_linux.c
index b08a27fec..2356882e7 100644
--- a/pkg/rootless/rootless_linux.c
+++ b/pkg/rootless/rootless_linux.c
@@ -346,6 +346,26 @@ syscall_clone (unsigned long flags, void *child_stack)
 #endif
 }
 
+int
+reexec_in_user_namespace_wait (int pid, int options)
+{
+  pid_t p;
+  int status;
+
+  do
+    p = waitpid (pid, &status, 0);
+  while (p < 0 && errno == EINTR);
+
+  if (p < 0)
+    return -1;
+
+  if (WIFEXITED (status))
+    return WEXITSTATUS (status);
+  if (WIFSIGNALED (status))
+    return 128 + WTERMSIG (status);
+  return -1;
+}
+
 static int
 create_pause_process (const char *pause_pid_file_path, char **argv)
 {
@@ -369,6 +389,8 @@ create_pause_process (const char *pause_pid_file_path, char **argv)
       while (r < 0 && errno == EINTR);
       close (p[0]);
 
+      reexec_in_user_namespace_wait(r, 0);
+
       return r == 1 && b == '0' ? 0 : -1;
     }
   else
@@ -573,8 +595,51 @@ check_proc_sys_userns_file (const char *path)
     }
 }
 
+static int
+copy_file_to_fd (const char *file_to_read, int outfd)
+{
+  char buf[512];
+  int fd;
+
+  fd = open (file_to_read, O_RDONLY);
+  if (fd < 0)
+    return fd;
+
+  for (;;)
+    {
+      ssize_t r, w, t = 0;
+
+      do
+        r = read (fd, buf, sizeof buf);
+      while (r < 0 && errno == EINTR);
+      if (r < 0)
+        {
+          close (fd);
+          return r;
+        }
+
+      if (r == 0)
+        break;
+
+      while (t < r)
+        {
+          do
+            w = write (outfd, &buf[t], r - t);
+          while (w < 0 && errno == EINTR);
+          if (w < 0)
+            {
+              close (fd);
+              return w;
+            }
+          t += w;
+        }
+    }
+  close (fd);
+  return 0;
+}
+
 int
-reexec_in_user_namespace (int ready, char *pause_pid_file_path)
+reexec_in_user_namespace (int ready, char *pause_pid_file_path, char *file_to_read, int outputfd)
 {
   int ret;
   pid_t pid;
@@ -598,11 +663,11 @@ reexec_in_user_namespace (int ready, char *pause_pid_file_path)
   listen_pid = getenv("LISTEN_PID");
   listen_fds = getenv("LISTEN_FDS");
 
-  if (listen_pid != NULL && listen_fds != NULL) {
-    if (strtol(listen_pid, NULL, 10) == getpid()) {
-      do_socket_activation = true;
+  if (listen_pid != NULL && listen_fds != NULL)
+    {
+      if (strtol(listen_pid, NULL, 10) == getpid())
+        do_socket_activation = true;
     }
-  }
 
   sprintf (uid, "%d", geteuid ());
   sprintf (gid, "%d", getegid ());
@@ -658,11 +723,12 @@ reexec_in_user_namespace (int ready, char *pause_pid_file_path)
       _exit (EXIT_FAILURE);
     }
 
-  if (do_socket_activation) {
-    char s[32];
-    sprintf (s, "%d", getpid());
-    setenv ("LISTEN_PID", s, true);
-  }
+  if (do_socket_activation)
+    {
+      char s[32];
+      sprintf (s, "%d", getpid());
+      setenv ("LISTEN_PID", s, true);
+    }
 
   setenv ("_CONTAINERS_USERNS_CONFIGURED", "init", 1);
   setenv ("_CONTAINERS_ROOTLESS_UID", uid, 1);
@@ -721,27 +787,14 @@ reexec_in_user_namespace (int ready, char *pause_pid_file_path)
       _exit (EXIT_FAILURE);
     }
 
+  if (file_to_read && file_to_read[0])
+    {
+      ret = copy_file_to_fd (file_to_read, outputfd);
+      close (outputfd);
+      _exit (ret == 0 ? EXIT_SUCCESS : EXIT_FAILURE);
+    }
+
   execvp (argv[0], argv);
 
   _exit (EXIT_FAILURE);
 }
-
-int
-reexec_in_user_namespace_wait (int pid)
-{
-  pid_t p;
-  int status;
-
-  do
-    p = waitpid (pid, &status, 0);
-  while (p < 0 && errno == EINTR);
-
-  if (p < 0)
-    return -1;
-
-  if (WIFEXITED (status))
-    return WEXITSTATUS (status);
-  if (WIFSIGNALED (status))
-    return 128 + WTERMSIG (status);
-  return -1;
-}
diff --git a/pkg/rootless/rootless_linux.go b/pkg/rootless/rootless_linux.go
index 9132c0fe5..27d64d1fc 100644
--- a/pkg/rootless/rootless_linux.go
+++ b/pkg/rootless/rootless_linux.go
@@ -26,8 +26,8 @@ import (
 #include <stdlib.h>
 extern uid_t rootless_uid();
 extern uid_t rootless_gid();
-extern int reexec_in_user_namespace(int ready, char *pause_pid_file_path);
-extern int reexec_in_user_namespace_wait(int pid);
+extern int reexec_in_user_namespace(int ready, char *pause_pid_file_path, char *file_to_read, int fd);
+extern int reexec_in_user_namespace_wait(int pid, int options);
 extern int reexec_userns_join(int userns, int mountns, char *pause_pid_file_path);
 */
 import "C"
@@ -226,7 +226,7 @@ func JoinUserAndMountNS(pid uint, pausePid string) (bool, int, error) {
 		return false, -1, errors.Errorf("cannot re-exec process")
 	}
 
-	ret := C.reexec_in_user_namespace_wait(pidC)
+	ret := C.reexec_in_user_namespace_wait(pidC, 0)
 	if ret < 0 {
 		return false, -1, errors.New("error waiting for the re-exec process")
 	}
@@ -234,11 +234,7 @@ func JoinUserAndMountNS(pid uint, pausePid string) (bool, int, error) {
 	return true, int(ret), nil
 }
 
-// BecomeRootInUserNS re-exec podman in a new userNS.  It returns whether podman was re-executed
-// into a new user namespace and the return code from the re-executed podman process.
-// If podman was re-executed the caller needs to propagate the error code returned by the child
-// process.
-func BecomeRootInUserNS(pausePid string) (bool, int, error) {
+func becomeRootInUserNS(pausePid, fileToRead string, fileOutput *os.File) (bool, int, error) {
 	if os.Geteuid() == 0 || os.Getenv("_CONTAINERS_USERNS_CONFIGURED") != "" {
 		if os.Getenv("_CONTAINERS_USERNS_CONFIGURED") == "init" {
 			return false, 0, runInUser()
@@ -249,6 +245,13 @@ func BecomeRootInUserNS(pausePid string) (bool, int, error) {
 	cPausePid := C.CString(pausePid)
 	defer C.free(unsafe.Pointer(cPausePid))
 
+	cFileToRead := C.CString(fileToRead)
+	defer C.free(unsafe.Pointer(cFileToRead))
+	var fileOutputFD C.int
+	if fileOutput != nil {
+		fileOutputFD = C.int(fileOutput.Fd())
+	}
+
 	runtime.LockOSThread()
 	defer runtime.UnlockOSThread()
 
@@ -262,7 +265,7 @@ func BecomeRootInUserNS(pausePid string) (bool, int, error) {
 	defer w.Close()
 	defer w.Write([]byte("0"))
 
-	pidC := C.reexec_in_user_namespace(C.int(r.Fd()), cPausePid)
+	pidC := C.reexec_in_user_namespace(C.int(r.Fd()), cPausePid, cFileToRead, fileOutputFD)
 	pid := int(pidC)
 	if pid < 0 {
 		return false, -1, errors.Errorf("cannot re-exec process")
@@ -328,6 +331,10 @@ func BecomeRootInUserNS(pausePid string) (bool, int, error) {
 		return false, -1, errors.Wrapf(err, "read from sync pipe")
 	}
 
+	if fileOutput != nil {
+		return true, 0, nil
+	}
+
 	if b[0] == '2' {
 		// We have lost the race for writing the PID file, as probably another
 		// process created a namespace and wrote the PID.
@@ -368,10 +375,95 @@ func BecomeRootInUserNS(pausePid string) (bool, int, error) {
 		}
 	}()
 
-	ret := C.reexec_in_user_namespace_wait(pidC)
+	ret := C.reexec_in_user_namespace_wait(pidC, 0)
 	if ret < 0 {
 		return false, -1, errors.New("error waiting for the re-exec process")
 	}
 
 	return true, int(ret), nil
 }
+
+// BecomeRootInUserNS re-exec podman in a new userNS.  It returns whether podman was re-executed
+// into a new user namespace and the return code from the re-executed podman process.
+// If podman was re-executed the caller needs to propagate the error code returned by the child
+// process.
+func BecomeRootInUserNS(pausePid string) (bool, int, error) {
+	return becomeRootInUserNS(pausePid, "", nil)
+}
+
+// TryJoinFromFilePaths attempts to join the namespaces of the pid files in paths.
+// This is useful when there are already running containers and we
+// don't have a pause process yet.  We can use the paths to the conmon
+// processes to attempt joining their namespaces.
+// If needNewNamespace is set, the file is read from a temporary user
+// namespace, this is useful for containers that are running with a
+// different uidmap and the unprivileged user has no way to read the
+// file owned by the root in the container.
+func TryJoinFromFilePaths(pausePidPath string, needNewNamespace bool, paths []string) (bool, int, error) {
+	if len(paths) == 0 {
+		return BecomeRootInUserNS(pausePidPath)
+	}
+
+	var lastErr error
+	var pausePid int
+
+	for _, path := range paths {
+		if !needNewNamespace {
+			data, err := ioutil.ReadFile(path)
+			if err != nil {
+				lastErr = err
+				continue
+			}
+
+			pausePid, err = strconv.Atoi(string(data))
+			if err != nil {
+				lastErr = errors.Wrapf(err, "cannot parse file %s", path)
+				continue
+			}
+
+			lastErr = nil
+			break
+		} else {
+			fds, err := syscall.Socketpair(syscall.AF_UNIX, syscall.SOCK_DGRAM, 0)
+			if err != nil {
+				lastErr = err
+				continue
+			}
+
+			r, w := os.NewFile(uintptr(fds[0]), "read file"), os.NewFile(uintptr(fds[1]), "write file")
+
+			defer w.Close()
+			defer r.Close()
+
+			if _, _, err := becomeRootInUserNS("", path, w); err != nil {
+				lastErr = err
+				continue
+			}
+
+			w.Close()
+			defer func() {
+				r.Close()
+				C.reexec_in_user_namespace_wait(-1, 0)
+			}()
+
+			b := make([]byte, 32)
+
+			n, err := r.Read(b)
+			if err != nil {
+				lastErr = errors.Wrapf(err, "cannot read %s\n", path)
+				continue
+			}
+
+			pausePid, err = strconv.Atoi(string(b[:n]))
+			if err == nil {
+				lastErr = nil
+				break
+			}
+		}
+	}
+	if lastErr != nil {
+		return false, 0, lastErr
+	}
+
+	return JoinUserAndMountNS(uint(pausePid), pausePidPath)
+}
diff --git a/pkg/rootless/rootless_unsupported.go b/pkg/rootless/rootless_unsupported.go
index 221baff97..06781e6ce 100644
--- a/pkg/rootless/rootless_unsupported.go
+++ b/pkg/rootless/rootless_unsupported.go
@@ -36,3 +36,15 @@ func GetRootlessGID() int {
 func JoinUserAndMountNS(pid uint, pausePid string) (bool, int, error) {
 	return false, -1, errors.New("this function is not supported on this os")
 }
+
+// TryJoinFromFilePaths attempts to join the namespaces of the pid files in paths.
+// This is useful when there are already running containers and we
+// don't have a pause process yet.  We can use the paths to the conmon
+// processes to attempt joining their namespaces.
+// If needNewNamespace is set, the file is read from a temporary user
+// namespace, this is useful for containers that are running with a
+// different uidmap and the unprivileged user has no way to read the
+// file owned by the root in the container.
+func TryJoinFromFilePaths(pausePidPath string, needNewNamespace bool, paths []string) (bool, int, error) {
+	return false, -1, errors.New("this function is not supported on this os")
+}