Change _dbus_create_directory to fail for existing directories

If we don't trap EEXIST and its Windows equivalent, we are unable to
detect the situation where we create an ostensibly unique
subdirectory in a shared /tmp, but an attacker has already created it.
This affects dbus-nonce (the nonce-tcp transport) and the activation
reload test.

Add a new _dbus_ensure_directory() for the one case where we want it to
succeed even on EEXIST: the DBUS_COOKIE_SHA1 keyring, which we know
we are creating in our own trusted "official" $HOME. In the new
transient service support on Bug #99825, ensure_owned_directory()
would need the same treatment.

We are not treating this as a serious security problem, because the
nonce-tcp transport is rarely enabled on Unix and there are multiple
mitigations.

The nonce-tcp transport creates a new unique file with O_EXCL and 0600
(private to user) permissions, then overwrites the requested filename
via atomic-overwrite, so the worst that could happen there is that an
attacker could place a symbolic link matching the name of a directory
we are going to create, causing a dbus-daemon configured for nonce-tcp
to traverse the symlink and atomically overwrite a file named "nonce"
in a directory of the attacker's choice, with new random contents that
are not known to the attacker. This seems unlikely to be exploitable
for anything worse than denial of service in practice. In mainline
Linux since 3.6, this attack is also defeated by the
fs.protected_symlinks sysctl, which many distributions enable by default.

The activation reload test suffers from a classic symlink attack
due to time-of-check/time-of-use errors in its implementation, but as
part of the developer-only "embedded tests" that are only intended
to be run on a trusted machine, it is not treated as security-sensitive.
That code path will be fixed in a subsequent commit.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=99828
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Philip Withnall <withnall@endlessm.com>

diff --git a/dbus/dbus-keyring.c b/dbus/dbus-keyring.c
index f0c64de3ebc50d20ff71b27c29f942b4deddc171..490c72d4ed67abc7165879f6a6ce657885586e35 100644 (file)
--- a/dbus/dbus-keyring.c
+++ b/dbus/dbus-keyring.c
@@ -811,7 +811,7 @@ _dbus_keyring_new_for_credentials (DBusCredentials  *credentials,
    * unless someone else manages to create it
    */
   dbus_error_init (&tmp_error);
-  if (!_dbus_create_directory (&keyring->directory,
+  if (!_dbus_ensure_directory (&keyring->directory,
                                &tmp_error))
     {
       _dbus_verbose ("Creating keyring directory: %s\n",

diff --git a/dbus/dbus-sysdeps-unix.c b/dbus/dbus-sysdeps-unix.c
index fe891ab747d914b1a54613c9afdc9c54d250d4f2..39d0f0a47a41c760de572d9c7794b5a210a4eabe 100644 (file)
--- a/dbus/dbus-sysdeps-unix.c
+++ b/dbus/dbus-sysdeps-unix.c
@@ -2721,7 +2721,7 @@ _dbus_get_real_time (long *tv_sec,
  * @returns #TRUE on success
  */
 dbus_bool_t
-_dbus_create_directory (const DBusString *filename,
+_dbus_ensure_directory (const DBusString *filename,
                         DBusError        *error)
 {
   const char *filename_c;
@@ -2744,6 +2744,35 @@ _dbus_create_directory (const DBusString *filename,
     return TRUE;
 }
 
+/**
+ * Creates a directory. Unlike _dbus_ensure_directory(), this only succeeds
+ * if the directory is genuinely newly-created.
+ *
+ * @param filename directory filename
+ * @param error initialized error object
+ * @returns #TRUE on success
+ */
+dbus_bool_t
+_dbus_create_directory (const DBusString *filename,
+                        DBusError        *error)
+{
+  const char *filename_c;
+
+  _DBUS_ASSERT_ERROR_IS_CLEAR (error);
+
+  filename_c = _dbus_string_get_const_data (filename);
+
+  if (mkdir (filename_c, 0700) < 0)
+    {
+      dbus_set_error (error, DBUS_ERROR_FAILED,
+                      "Failed to create directory %s: %s\n",
+                      filename_c, _dbus_strerror (errno));
+      return FALSE;
+    }
+  else
+    return TRUE;
+}
+
 /**
  * Appends the given filename to the given directory.
  *

diff --git a/dbus/dbus-sysdeps-win.c b/dbus/dbus-sysdeps-win.c
index 75f07e9c4da0f9feea95c25d06ba54691c1c62fc..8d3540a3ae51f1685e14d26969d13e311499adbc 100644 (file)
--- a/dbus/dbus-sysdeps-win.c
+++ b/dbus/dbus-sysdeps-win.c
@@ -2211,6 +2211,35 @@ _dbus_disable_sigpipe (void)
 {
 }
 
+/**
+ * Creates a directory. Unlike _dbus_ensure_directory(), this only succeeds
+ * if the directory is genuinely newly-created.
+ *
+ * @param filename directory filename
+ * @param error initialized error object
+ * @returns #TRUE on success
+ */
+dbus_bool_t
+_dbus_create_directory (const DBusString *filename,
+                        DBusError        *error)
+{
+  const char *filename_c;
+
+  _DBUS_ASSERT_ERROR_IS_CLEAR (error);
+
+  filename_c = _dbus_string_get_const_data (filename);
+
+  if (!CreateDirectoryA (filename_c, NULL))
+    {
+      dbus_set_error (error, DBUS_ERROR_FAILED,
+                      "Failed to create directory %s: %s\n",
+                      filename_c, _dbus_strerror_from_errno ());
+      return FALSE;
+    }
+  else
+    return TRUE;
+}
+
 /**
  * Creates a directory; succeeds if the directory
  * is created or already existed.
@@ -2220,7 +2249,7 @@ _dbus_disable_sigpipe (void)
  * @returns #TRUE on success
  */
 dbus_bool_t
-_dbus_create_directory (const DBusString *filename,
+_dbus_ensure_directory (const DBusString *filename,
                         DBusError        *error)
 {
   const char *filename_c;

diff --git a/dbus/dbus-sysdeps.h b/dbus/dbus-sysdeps.h
index 5465128e236febbe6b29a784eda8738393fb535a..a320d0adc3ee97e6abc8dbe5c4bc6960accf05f2 100644 (file)
--- a/dbus/dbus-sysdeps.h
+++ b/dbus/dbus-sysdeps.h
@@ -331,6 +331,8 @@ void _dbus_get_real_time (long *tv_sec,
  */
 dbus_bool_t    _dbus_create_directory        (const DBusString *filename,
                                               DBusError        *error);
+dbus_bool_t    _dbus_ensure_directory        (const DBusString *filename,
+                                              DBusError        *error);
 dbus_bool_t    _dbus_delete_directory        (const DBusString *filename,
                                              DBusError        *error);