QSB-068: Disconnecting a video output can cause XScreenSaver to crash

[Andrew David Wong]

Dear Qubes Community,

We have just published Qubes Security Bulletin (QSB) 068: Disconnecting
a video output can cause XScreenSaver to crash. The text of this QSB is
reproduced below. This QSB and its accompanying signatures will always
be available in the Qubes Security Pack (qubes-secpack).

View QSB-068 in the qubes-secpack:

https://github.com/QubesOS/qubes-secpack/blob/master/QSBs/qsb-068-2021.txt

Learn about the qubes-secpack, including how to obtain, verify, and read it:

https://www.qubes-os.org/security/pack/

View all past QSBs:

https://www.qubes-os.org/security/bulletins/

```


---===[ Qubes Security Bulletin 068 ]===---

2021-06-04


Disconnecting a video output can cause XScreenSaver to crash


User action required
=====================

Users must install the following specific packages in order to address
the issues discussed in this bulletin:

For Qubes 4.0, in dom0:
- xscreensaver 5.45-5

For Qubes 4.1, in dom0:
- xscreensaver 5.45-5

These packages will migrate from the security-testing repository to the
current (stable) repository over the next two weeks after being tested
by the community. [1] Once available, the packages are to be installed
via the Qubes Update Tool or its command-line equivalents. [2]

After installing this update, the XScreenSaver daemon process must be
restarted in order for the changes to take effect. This can be done by
restarting dom0, logging out of dom0 then logging back in, or issuing
the following command in a dom0 terminal:

xscreensaver-command -exit; xscreensaver &


Summary
========

XScreenSaver is the default screen locker in dom0. It tracks which video
outputs are connected to the system in order to blank them properly. In
some specific hardware configurations, disconnecting an output can cause
XScreenSaver to crash, leaving the screen unlocked.

Impact
=======

On hardware configurations with more than 10 video outputs that can be
disconnected, an attacker with physical access to a screen-locked system
may be able to unlock it by physically disconnecting one or more
outputs, bypassing standard screen lock authentication.

Details
========

On X11, screen locking and blanking is done by creating a window that
obscures the whole screen, which is a standard practice. In
XScreenSaver, each such window is assigned a specific property. When a
video output is disconnected, its corresponding blanking window is
destroyed, and its XScreenSaver-specific property is removed so that it
will not be used by `xscreensaver-command` anymore. This is handled by
the `update_screen_layout()` function in the `driver/screens.c` file:

985 /* Synchronize the contents of si->ssi to the current state of
the monitors.
986 Doesn't change anything if nothing has changed; otherwise,
alters and
987 reuses existing saver_screen_info structs as much as possible.
988 Returns True if anything changed.
989 */
990 Bool
991 update_screen_layout (saver_info *si)
992 {
993 monitor **monitors = scan_monitors (si);
994 int count = 0;
995 int good_count = 0;
...
1009 while (monitors[count])
1010 {
1011 if (monitors[count]->sanity == S_SANE)
1012 good_count++;
1013 count++;
1014 }
1015
1016 if (si->ssi_count == 0)
1017 {
1018 si->ssi_count = 10;
1019 si->screens = (saver_screen_info *)
1020 calloc (sizeof(*si->screens), si->ssi_count);
1021 }
1022
1023 if (si->ssi_count <= good_count)
1024 {
1025 si->ssi_count = good_count + 10;
1026 si->screens = (saver_screen_info *)
1027 realloc (si->screens, sizeof(*si->screens) *
si->ssi_count);
1028 memset (si->screens + si->nscreens, 0,
1029 sizeof(*si->screens) * (si->ssi_count -
si->nscreens));
1030 }
...
1092 for (; j < count; j++)
1093 {
1094 saver_screen_info *ssi = &si->screens[j];
1095 if (!ssi->screensaver_window)
1096 continue;
1097 fprintf (stderr, "%s: %d: screen now unused, disabling.\n",
1098 blurb(), j);
1099 /* Undo store_saver_id() so that xscreensaver-command
doesn't attempt
1100 to communicate with us through this window. It might
make more
1101 sense to destroy the window, but I'm not 100% sure
that there are
1102 no outstanding grabs on it that have yet been
transferred.
1103 */
1104 XDeleteProperty (si->dpy, ssi->screensaver_window,
1105 XA_SCREENSAVER_VERSION);
1106 }

The initial portion of the function counts how many outputs are defined
(the `count` variable) and how many of them are connected (the
`good_count` variable). Then, the `si->screens` array is allocated or
re-allocated to fit information about connected outputs, with an extra
margin of 10 entries. However, the loop at the end iterates over the
array up to the total number of outputs, not just the ones that are
connected.

If there are 10 or fewer disconnected outputs, this works fine. However,
if there are more than 10, it will access the array beyond its end,
reading unrelated data from memory. It will interpret this data as an
XScreenSaver window ID. If that unrelated data happens to be non-zero
(which is very likely), then the condition at line 1095 will not skip
it, and the `XDeleteProperty` call will operate on that (most likely
invalid) window ID. This, in turn, will cause the XScreenSaver process
to crash, as that's what the error handler is programmed to do (the
`saver_ehandler()` function in the `driver/xscreensaver.c` file).

The error message will look like this:


##############################################################################

xscreensaver: 11:17:59: X Error! PLEASE REPORT THIS BUG.
xscreensaver: 11:17:59: screen 0/0: 0x2ae, 0x0, 0x6600001
xscreensaver: 11:17:59: screen 0/1: 0x2ae, 0x0, 0x0


##############################################################################

X Error of failed request: BadWindow (invalid Window parameter)
Major opcode of failed request: 19 (X_DeleteProperty)
Resource id in failed request: 0x188dba0
Serial number of failed request: 4284
Current serial number in output stream: 4286

#######################################################################


The issue affects only XScreenSaver version 5.45. Versions 5.44 and
older, as well as 6.00, are not affected. The XScreenSaver author was
notified about this issue and decided not to publish an advisory, as the
issue does not affect the most recent version.

The Qubes Security Team has decided to address this issue in Qubes OS by
patching this specific bug rather than immediately upgrading to the 6.00
version. The reason is that XScreenSaver 6.00 is a major update with
major architectural changes. As such, it poses an increased risk of
introducing unrelated problems. However, this decision does not preclude
the possibility of updating to XScreenSaver 6.00 at some point in the
future, independently of this particular security patch.

Credits
========

The issue was reported by Mustafa Kuscu. [3]

References
===========

[1] https://www.qubes-os.org/doc/testing/
[2] https://www.qubes-os.org/doc/updating-qubes-os/
[3] https://github.com/QubesOS/qubes-issues/issues/6595

--
The Qubes Security Team
https://www.qubes-os.org/security/

```

This announcement is also available on the Qubes website:
https://www.qubes-os.org/news/2021/06/04/qsb-068/