Commit be51bfe9 authored by Simon McVittie's avatar Simon McVittie

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=99828Signed-off-by: default avatarSimon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Philip Withnall's avatarPhilip Withnall <withnall@endlessm.com>
parent bca5a846
......@@ -807,7 +807,7 @@ _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",
......
......@@ -2953,7 +2953,7 @@ _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;
......@@ -2976,6 +2976,35 @@ _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.
*
......
......@@ -2223,6 +2223,35 @@ _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.
......@@ -2232,7 +2261,7 @@ _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;
......
......@@ -421,6 +421,9 @@ DBUS_PRIVATE_EXPORT
dbus_bool_t _dbus_create_directory (const DBusString *filename,
DBusError *error);
DBUS_PRIVATE_EXPORT
dbus_bool_t _dbus_ensure_directory (const DBusString *filename,
DBusError *error);
DBUS_PRIVATE_EXPORT
dbus_bool_t _dbus_delete_directory (const DBusString *filename,
DBusError *error);
......
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