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/ -- You received this message because you are subscribed to the Google Groups "qubes-users" group. To unsubscribe from this group and stop receiving emails from it, send an email to qubes-users+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/qubes-users/9cf0dfbf-d2e7-43a7-2951-5c35eb291d8c%40qubes-os.org.
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