QSB-104: GUI-related security bugs

system · July 30, 2024, 11:07pm

We have published Qubes Security Bulletin (QSB) 104: GUI-related security bugs. The text of this QSB and its accompanying cryptographic signatures are reproduced below, followed by a general explanation of this announcement and authentication instructions.

Qubes Security Bulletin 104

             ---===[ Qubes Security Bulletin 104 ]===---

                              2024-07-30

                      GUI-related security bugs

User action
------------

Continue to update normally [1] in order to receive the security updates
described in the "Patching" section below. No other user action is
required in response to this QSB.

Summary
--------

This QSB concerns two separate GUI-related bugs with potential security
implications:

1. If the keyboard auto-repeat feature is enabled in a qube, as it is in
   the default templates (due to an error), it can generate unexpected
   key presses. In particular, if the user attempts to select either the
   "Copy to other qube..." or "Move to other qube..." context menu
   option on a file in the qube's graphical file manager using the Enter
   key (rather than clicking on it with the mouse cursor), the keyboard
   auto-repeat feature can cause the selected file to be opened in that
   qube.

2. The Xfce taskbar in dom0 incorrectly handles application icons, which
   can result in some application icons being the wrong color.

Impact
-------

The impact of the first bug depends on the user's habits. If the user
intends to copy a file to another qube before opening it, this bug could
result in that file being opened in the wrong qube, potentially
compromising it. For example, if the user receives a file via email,
saves it in the email qube, then attempts to copy it to another qube,
this bug could result in the file being opened in the email qube.

An attacker who manages to compromise a qube or an application running
inside of a qube could attempt to exploit the second bug in order to
make an application icon from that qube appear in dom0's Xfce taskbar
with a color different from that qube's true color label. For example,
an attacker who controls an application in a qube with the yellow color
label could cause that application's icon to have the color green in the
taskbar in an attempt to deceive or confuse the user. This bug affects
only application icon colors in dom0's Xfce taskbar. It does not affect
other window decorations (i.e., window borders, title bars, or icons in
title bars), nor does it affect icons that appear when switching between
windows with Alt + Tab.

Affected systems
-----------------

The first bug affects both Qubes OS 4.1 and 4.2. The only qubes that are
affected are those in which some program enables the keyboard
auto-repeat feature. The default templates automatically start the
program xfsettingsd, which in turn enables the keyboard auto-repeat
feature. Templates that do not have such a program installed (such as
minimal templates, by default) are not affected.

The second bug affects only Qubes OS 4.2 when using the Xfce desktop
environment, which is the default desktop environment.

Discussion
-----------

The first bug can occur if, and only if, the keyboard auto-repeat
feature is enabled in a qube. (When gui-agent starts, it disables
auto-repeat, but other programs, such as xfsettings in the default
templates, can re-enable it.) By design, qubes do not "see" all keyboard
events. Instead, each qube sees only the keyboard events that are sent
when a window belonging to that qube is in focus. Therefore, if all of a
qube's windows lose focus while a key is being pressed, it may appear to
the qube that the key was pressed for much longer than it actually was.
In particular, the qube may "believe" that the key is being pressed
until a window belonging to that qube regains focus and the key is
released.

Normally, the auto-repeat feature starts generating repeated key press
events after the initial auto-repeat delay has passed. While none of the
qube's windows are in focus, such events are ignored. However, if one of
the qube's windows regains focus while a key is in the pressed state,
the auto-repeat feature in that qube will continue generating repeated
key press events without waiting for the initial auto-repeat delay,
since from its point of view the key was being pressed the whole time.

In this particular case, when the user selects either the "Copy to other
qube..." or "Move to other qube..." option, focus switches to the qrexec
prompt in dom0, and the Enter key may appear to the qube to be pressed
the whole time while the user interacts with the qrexec prompt. When the
dom0 prompt window closes, focus returns to the qube's file manager
window. At this point, the qube believes that the Enter key has been
pressed for a long time, so the auto-repeat feature generates repeated
Enter key press events, which in turn cause the file manager to open the
file normally.

What ought to happen instead is that only dom0 should generate key
repeat events, because only dom0 knows how long a key has actually been
pressed. Meanwhile, the qube should see just one key press event and
(possibly much later) one key release event, and this is exactly what
happens when the auto-repeat feature is disabled in the qube. The qube
would still perceive a long key press while none of its windows have
focus. However, when one of the qube's windows regains focus, there
would not be any repeated key press events, and the qube would get the
current key state. (Note: One might consider sending a synthetic "key
release" event for any key that is still being pressed when an
application loses focus, but that would cause extra key presses to be
delivered when the application regains focus.)

The second bug is caused by the libwnck3 library (which is used by
Xfce's "Window Buttons" widget) interpreting icons incorrectly. Each
window icon is stored in a `_NET_WM_ICON` window property in an ARGB
format. The upstream version of the library correctly converts this ARGB
format into RGBA, then loads it into a `GdkPixbuf` object, which is
later used by the taskbar. However, the Fedora package has extra patches
that add support for HiDPI scaling, which changes `GdkPixbuf` to
`cairo_surface_t`. The latter has native support for the
`CAIRO_FORMAT_ARGB32` format, so the manual conversion was removed.

The problem is that `ARGB` and `CAIRO_FORMAT_ARGB32` have subtle
differences related to transparency. The latter assumes that color
channels are pre-multiplied by the alpha channel, [3] but this isn't the
case in `ARGB`. An attacker could attempt to exploit this vulnerability
in order to manipulate color channels after the icon has been colored in
accordance with the qube's color label, completely changing the icon's
color in some cases.

Specifically, by adjusting the alpha channel, it may be possible to wrap
high color values into low ones. For example, a pixel with the values
R=255, G=250, B=0, and A=254 should appear as yellow. However, due to
the pre-multiplied format, the alpha channel determines the valid range
for color channels. In this case, setting A=254 results in a valid range
of 0-254 for the R, G, and B color channels. Since R=255 is outside of
this range, the red channel would wrap to zero, resulting in a green
pixel. This bug has been present in Fedora's libwnck3 package since
Fedora 34. [4]

Patching
---------

The following packages contain security updates that address the
vulnerabilities described in this bulletin:

  For Qubes 4.1, in templates:
  - qubes-gui-agent version 4.1.34

  For Qubes 4.2, in dom0 and templates:
  - qubes-gui-agent version 4.2.17 (all templates)
  - libwnck3 version 43.0-9 (Fedora templates and dom0)

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. [2] Once available, the packages are to be installed
via the Qubes Update tool or its command-line equivalents. [1]

User session and all qubes must be restarted afterward in order for
the updates to take effect.

Credits
--------

The first bug was reported by Maurice Kayser.

The second bug was reported by Kamil Aronowski.

References
-----------

[1] https://www.qubes-os.org/doc/how-to-update/
[2] https://www.qubes-os.org/doc/testing/
[3] https://www.cairographics.org/manual/cairo-Image-Surfaces.html
[4] https://src.fedoraproject.org/rpms/libwnck3/c/96db18a8fe30eafb7bd3da04d18901723482f97b?branch=f34

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

Source: qsb-104-2024.txt

Marek Marczykowski-Górecki’s PGP signature

Source: qsb-104-2024.txt.sig.marmarek

Simon Gaiser (aka HW42)’s PGP signature

Source: qsb-104-2024.txt.sig.simon

What is the purpose of this announcement?

The purpose of this announcement is to inform the Qubes community that a new Qubes security bulletin (QSB) has been published.

What is a Qubes security bulletin (QSB)?

A Qubes security bulletin (QSB) is a security announcement issued by the Qubes security team. A QSB typically provides a summary and impact analysis of one or more recently-discovered software vulnerabilities, including details about patching to address them. For a list of all QSBs, see Qubes security bulletins (QSBs).

Why should I care about QSBs?

QSBs tell you what actions you must take in order to protect yourself from recently-discovered security vulnerabilities. In most cases, security vulnerabilities are addressed by updating normally. However, in some cases, special user action is required. In all cases, the required actions are detailed in QSBs.

What are the PGP signatures that accompany QSBs?

A PGP signature is a cryptographic digital signature made in accordance with the OpenPGP standard. PGP signatures can be cryptographically verified with programs like GNU Privacy Guard (GPG). The Qubes security team cryptographically signs all QSBs so that Qubes users have a reliable way to check whether QSBs are genuine. The only way to be certain that a QSB is authentic is by verifying its PGP signatures.

Why should I care whether a QSB is authentic?

A forged QSB could deceive you into taking actions that adversely affect the security of your Qubes OS system, such as installing malware or making configuration changes that render your system vulnerable to attack. Falsified QSBs could sow fear, uncertainty, and doubt about the security of Qubes OS or the status of the Qubes OS Project.

How do I verify the PGP signatures on a QSB?

The following command-line instructions assume a Linux system with git and gpg installed. (For Windows and Mac options, see OpenPGP software.)

Obtain the Qubes Master Signing Key (QMSK), e.g.:

$ gpg --fetch-keys https://keys.qubes-os.org/keys/qubes-master-signing-key.asc
gpg: directory '/home/user/.gnupg' created
gpg: keybox '/home/user/.gnupg/pubring.kbx' created
gpg: requesting key from 'https://keys.qubes-os.org/keys/qubes-master-signing-key.asc'
gpg: /home/user/.gnupg/trustdb.gpg: trustdb created
gpg: key DDFA1A3E36879494: public key "Qubes Master Signing Key" imported
gpg: Total number processed: 1
gpg:               imported: 1

(For more ways to obtain the QMSK, see How to import and authenticate the Qubes Master Signing Key.)

View the fingerprint of the PGP key you just imported. (Note: gpg> indicates a prompt inside of the GnuPG program. Type what appears after it when prompted.)

$ gpg --edit-key 0x427F11FD0FAA4B080123F01CDDFA1A3E36879494
gpg (GnuPG) 2.2.27; Copyright (C) 2021 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
   
   
pub  rsa4096/DDFA1A3E36879494
     created: 2010-04-01  expires: never       usage: SC
     trust: unknown       validity: unknown
[ unknown] (1). Qubes Master Signing Key
   
gpg> fpr
pub   rsa4096/DDFA1A3E36879494 2010-04-01 Qubes Master Signing Key
 Primary key fingerprint: 427F 11FD 0FAA 4B08 0123  F01C DDFA 1A3E 3687 9494

Important: At this point, you still don’t know whether the key you just imported is the genuine QMSK or a forgery. In order for this entire procedure to provide meaningful security benefits, you must authenticate the QMSK out-of-band. Do not skip this step! The standard method is to obtain the QMSK fingerprint from multiple independent sources in several different ways and check to see whether they match the key you just imported. For more information, see How to import and authenticate the Qubes Master Signing Key.

Tip: After you have authenticated the QMSK out-of-band to your satisfaction, record the QMSK fingerprint in a safe place (or several) so that you don’t have to repeat this step in the future.

Once you are satisfied that you have the genuine QMSK, set its trust level to 5 (“ultimate”), then quit GnuPG with q.

gpg> trust
pub  rsa4096/DDFA1A3E36879494
     created: 2010-04-01  expires: never       usage: SC
     trust: unknown       validity: unknown
[ unknown] (1). Qubes Master Signing Key
   
Please decide how far you trust this user to correctly verify other users' keys
(by looking at passports, checking fingerprints from different sources, etc.)
   
  1 = I don't know or won't say
  2 = I do NOT trust
  3 = I trust marginally
  4 = I trust fully
  5 = I trust ultimately
  m = back to the main menu
   
Your decision? 5
Do you really want to set this key to ultimate trust? (y/N) y
   
pub  rsa4096/DDFA1A3E36879494
     created: 2010-04-01  expires: never       usage: SC
     trust: ultimate      validity: unknown
[ unknown] (1). Qubes Master Signing Key
Please note that the shown key validity is not necessarily correct
unless you restart the program.
   
gpg> q

Use Git to clone the qubes-secpack repo.

$ git clone https://github.com/QubesOS/qubes-secpack.git
Cloning into 'qubes-secpack'...
remote: Enumerating objects: 4065, done.
remote: Counting objects: 100% (1474/1474), done.
remote: Compressing objects: 100% (742/742), done.
remote: Total 4065 (delta 743), reused 1413 (delta 731), pack-reused 2591
Receiving objects: 100% (4065/4065), 1.64 MiB | 2.53 MiB/s, done.
Resolving deltas: 100% (1910/1910), done.

Import the included PGP keys. (See our PGP key policies for important information about these keys.)

$ gpg --import qubes-secpack/keys/*/*
gpg: key 063938BA42CFA724: public key "Marek Marczykowski-Górecki (Qubes OS signing key)" imported
gpg: qubes-secpack/keys/core-devs/retired: read error: Is a directory
gpg: no valid OpenPGP data found.
gpg: key 8C05216CE09C093C: 1 signature not checked due to a missing key
gpg: key 8C05216CE09C093C: public key "HW42 (Qubes Signing Key)" imported
gpg: key DA0434BC706E1FCF: public key "Simon Gaiser (Qubes OS signing key)" imported
gpg: key 8CE137352A019A17: 2 signatures not checked due to missing keys
gpg: key 8CE137352A019A17: public key "Andrew David Wong (Qubes Documentation Signing Key)" imported
gpg: key AAA743B42FBC07A9: public key "Brennan Novak (Qubes Website & Documentation Signing)" imported
gpg: key B6A0BB95CA74A5C3: public key "Joanna Rutkowska (Qubes Documentation Signing Key)" imported
gpg: key F32894BE9684938A: public key "Marek Marczykowski-Górecki (Qubes Documentation Signing Key)" imported
gpg: key 6E7A27B909DAFB92: public key "Hakisho Nukama (Qubes Documentation Signing Key)" imported
gpg: key 485C7504F27D0A72: 1 signature not checked due to a missing key
gpg: key 485C7504F27D0A72: public key "Sven Semmler (Qubes Documentation Signing Key)" imported
gpg: key BB52274595B71262: public key "unman (Qubes Documentation Signing Key)" imported
gpg: key DC2F3678D272F2A8: 1 signature not checked due to a missing key
gpg: key DC2F3678D272F2A8: public key "Wojtek Porczyk (Qubes OS documentation signing key)" imported
gpg: key FD64F4F9E9720C4D: 1 signature not checked due to a missing key
gpg: key FD64F4F9E9720C4D: public key "Zrubi (Qubes Documentation Signing Key)" imported
gpg: key DDFA1A3E36879494: "Qubes Master Signing Key" not changed
gpg: key 1848792F9E2795E9: public key "Qubes OS Release 4 Signing Key" imported
gpg: qubes-secpack/keys/release-keys/retired: read error: Is a directory
gpg: no valid OpenPGP data found.
gpg: key D655A4F21830E06A: public key "Marek Marczykowski-Górecki (Qubes security pack)" imported
gpg: key ACC2602F3F48CB21: public key "Qubes OS Security Team" imported
gpg: qubes-secpack/keys/security-team/retired: read error: Is a directory
gpg: no valid OpenPGP data found.
gpg: key 4AC18DE1112E1490: public key "Simon Gaiser (Qubes Security Pack signing key)" imported
gpg: Total number processed: 17
gpg:               imported: 16
gpg:              unchanged: 1
gpg: marginals needed: 3  completes needed: 1  trust model: pgp
gpg: depth: 0  valid:   1  signed:   6  trust: 0-, 0q, 0n, 0m, 0f, 1u
gpg: depth: 1  valid:   6  signed:   0  trust: 6-, 0q, 0n, 0m, 0f, 0u

Verify signed Git tags.

$ cd qubes-secpack/
$ git tag -v `git describe`
object 266e14a6fae57c9a91362c9ac784d3a891f4d351
type commit
tag marmarek_sec_266e14a6
tagger Marek Marczykowski-Górecki 1677757924 +0100
   
Tag for commit 266e14a6fae57c9a91362c9ac784d3a891f4d351
gpg: Signature made Thu 02 Mar 2023 03:52:04 AM PST
gpg:                using RSA key 2D1771FE4D767EDC76B089FAD655A4F21830E06A
gpg: Good signature from "Marek Marczykowski-Górecki (Qubes security pack)" [full]

The exact output will differ, but the final line should always start with gpg: Good signature from... followed by an appropriate key. The [full] indicates full trust, which this key inherits in virtue of being validly signed by the QMSK.

Verify PGP signatures, e.g.:

$ cd QSBs/
$ gpg --verify qsb-087-2022.txt.sig.marmarek qsb-087-2022.txt
gpg: Signature made Wed 23 Nov 2022 04:05:51 AM PST
gpg:                using RSA key 2D1771FE4D767EDC76B089FAD655A4F21830E06A
gpg: Good signature from "Marek Marczykowski-Górecki (Qubes security pack)" [full]
$ gpg --verify qsb-087-2022.txt.sig.simon qsb-087-2022.txt
gpg: Signature made Wed 23 Nov 2022 03:50:42 AM PST
gpg:                using RSA key EA18E7F040C41DDAEFE9AA0F4AC18DE1112E1490
gpg: Good signature from "Simon Gaiser (Qubes Security Pack signing key)" [full]
$ cd ../canaries/
$ gpg --verify canary-034-2023.txt.sig.marmarek canary-034-2023.txt
gpg: Signature made Thu 02 Mar 2023 03:51:48 AM PST
gpg:                using RSA key 2D1771FE4D767EDC76B089FAD655A4F21830E06A
gpg: Good signature from "Marek Marczykowski-Górecki (Qubes security pack)" [full]
$ gpg --verify canary-034-2023.txt.sig.simon canary-034-2023.txt
gpg: Signature made Thu 02 Mar 2023 01:47:52 AM PST
gpg:                using RSA key EA18E7F040C41DDAEFE9AA0F4AC18DE1112E1490
gpg: Good signature from "Simon Gaiser (Qubes Security Pack signing key)" [full]

Again, the exact output will differ, but the final line of output from each gpg --verify command should always start with gpg: Good signature from... followed by an appropriate key.

For this announcement (QSB-104), the commands are:

$ gpg --verify qsb-104-2024.txt.sig.marmarek qsb-104-2024.txt
$ gpg --verify qsb-104-2024.txt.sig.simon qsb-104-2024.txt

You can also verify the signatures directly from this announcement in addition to or instead of verifying the files from the qubes-secpack. Simply copy and paste the QSB-104 text into a plain text file and do the same for both signature files. Then, perform the same authentication steps as listed above, substituting the filenames above with the names of the files you just created.

This is a companion discussion topic for the original entry at https://www.qubes-os.org/news/2024/07/30/qsb-104/