Add --mount-devapi option

By default, we had supported `--mount-bind /dev /dev` to get
access to devices.  But in many cases, build systems and the
like will want to avoid exposing host physical devices.

For example, if I'm building something locally, I don't want the
makefile etc. to be able to access `/dev/dri`.

Makefile-user-chroot.am

@@ -19,6 +19,7 @@ bin_PROGRAMS += linux-user-chroot
 
 linux_user_chroot_SOURCES = \
 	src/setup-seccomp.c \
+	src/setup-dev.c \
 	src/linux-user-chroot.c \
 	$(NULL)
 

src/cleanup.h

+/* -*- mode: c; tab-width: 2; indent-tabs-mode: nil -*-
+ *
+ * linux-user-chroot: A setuid program that allows non-root users to safely chroot(2)
+ *
+ * Copyright 2015 Colin Walters <walters@verbum.org>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it would be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ */
+
+#pragma once
+
+#include <assert.h>
+
+static inline void
+cleanup_close_fdp (int *fdp)
+{
+  int fd;
+
+  assert (fdp);
+  
+  fd = *fdp;
+  if (fd != -1)
+    (void) close (fd);
+}
+
+#define _cleanup_fd_close_ __attribute__((cleanup(cleanup_close_fdp)))

src/linux-user-chroot.c

@@ -48,6 +48,7 @@
 #include <sched.h>
 
 #include "setup-seccomp.h"
+#include "setup-dev.h"
 
 #ifndef PR_SET_NO_NEW_PRIVS
 #define PR_SET_NO_NEW_PRIVS	38
@@ -91,7 +92,8 @@ fatal_errno (const char *message)
 typedef enum {
   MOUNT_SPEC_BIND,
   MOUNT_SPEC_READONLY,
-  MOUNT_SPEC_PROCFS
+  MOUNT_SPEC_PROCFS,
+  MOUNT_SPEC_DEVAPI
 } MountSpecType;
 
 typedef struct _MountSpec MountSpec;
@@ -251,6 +253,22 @@ main (int      argc,
           mount->dest = argv[after_mount_arg_index+1];
           mount->next = bind_mounts;
           
+          bind_mounts = mount;
+          after_mount_arg_index += 2;
+        }
+      else if (strcmp (arg, "--mount-devapi") == 0)
+        {
+          MountSpec *mount;
+
+          if ((argc - after_mount_arg_index) < 2)
+            fatal ("--mount-devapi takes one argument");
+
+          mount = malloc (sizeof (MountSpec));
+          mount->type = MOUNT_SPEC_DEVAPI;
+          mount->source = NULL;
+          mount->dest = argv[after_mount_arg_index+1];
+          mount->next = bind_mounts;
+          
           bind_mounts = mount;
           after_mount_arg_index += 2;
         }
@@ -394,6 +412,11 @@ main (int      argc,
                          "proc", MS_MGC_VAL | MS_PRIVATE, NULL) < 0)
                 fatal_errno ("mount (\"proc\")");
             }
+          else if (bind_mount_iter->type == MOUNT_SPEC_DEVAPI)
+            {
+              if (setup_dev (dest) < 0)
+                fatal_errno ("setting up devapi");
+            }
           else
             assert (0);
           free (dest);

src/setup-dev.c

+/* 
+ * Copyright (C) 2015 Colin Walters <walters@verbum.org>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.	 See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library. If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+#include "config.h"
+/* Core libc/linux-headers stuff */
+#define _GNU_SOURCE
+#include <unistd.h>
+#include <stdio.h>
+#include <fcntl.h>
+#include <errno.h>
+#include <stdarg.h>
+#include <string.h>
+#include <assert.h>
+#include <stdlib.h>
+#include <stdarg.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/prctl.h>
+#include <sys/fsuid.h>
+#include <sys/mount.h>
+#include <sys/syscall.h>
+#include <sys/wait.h>
+#include <sys/socket.h>
+#include <sys/utsname.h>
+#include <sched.h>
+
+#include "setup-dev.h"
+#include "cleanup.h"
+
+#define N_ELEMENTS(arr)		(sizeof (arr) / sizeof ((arr)[0]))
+
+int
+setup_dev (const char  *dest_devdir)
+{
+  _cleanup_fd_close_ int src_fd = -1;
+  _cleanup_fd_close_ int dest_fd = -1;
+  struct stat stbuf;
+  unsigned int i;
+  static const char *const devnodes[] = { "null", "zero", "full", "random", "urandom", "tty" };
+
+  src_fd = openat (AT_FDCWD, "/dev", O_RDONLY | O_NONBLOCK | O_DIRECTORY | O_CLOEXEC | O_NOCTTY);
+  if (src_fd == -1)
+    return -1;
+
+  if (mount ("tmpfs", dest_devdir,
+	     "tmpfs", MS_MGC_VAL | MS_PRIVATE | MS_NOSUID, "mode=0755") < 0)
+    return -1;
+
+  dest_fd = openat (AT_FDCWD, dest_devdir, O_RDONLY | O_NONBLOCK | O_DIRECTORY | O_CLOEXEC | O_NOCTTY);
+  if (dest_fd == -1)
+    return -1;
+
+  for (i = 0; i < N_ELEMENTS (devnodes); i++)
+    {
+      const char *nodename = devnodes[i];
+      
+      if (fstatat (src_fd, nodename, &stbuf, 0) == -1)
+	return -1;
+      if (mknodat (dest_fd, nodename, stbuf.st_mode, stbuf.st_rdev) != 0)
+        return -1;
+      if (fchmodat (dest_fd, nodename, stbuf.st_mode, 0) != 0)
+        return -1;
+    }
+
+  if (symlinkat ("/proc/self/fd/0", dest_fd, "stdin") < 0)
+    return -1;
+  if (symlinkat ("/proc/self/fd/1", dest_fd, "stdout") < 0)
+    return -1;
+  if (symlinkat ("/proc/self/fd/2", dest_fd, "stderr") < 0)
+    return -1;
+
+  return 0;
+}
+

src/setup-dev.h

+/* -*- mode: c; tab-width: 2; indent-tabs-mode: nil -*-
+ *
+ * linux-user-chroot: A setuid program that allows non-root users to safely chroot(2)
+ *
+ * Copyright 2015 Colin Walters <walters@verbum.org>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it would be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ */
+
+#pragma once
+
+int setup_dev (const char *dest);