[XFCE] locked xscreensaver displays sensitive notifications

[mfc]

Qubes OS version (e.g., R3.1):

R3.1

Affected TemplateVMs (e.g., fedora-23, if applicable):

xscreensaver on XFCE

---

Expected behavior:

sensitive notifications are not displayed when the screen is locked.

Actual behavior:

sensitive notifications are displayed when the screen is locked.

Steps to reproduce the behavior:

Use Signal Desktop, receive a message. The message will be pushed through xscreensaver and display to the user even if they have locked the screen.

---

Related issues:

#888
#963
#1917

[mfc]

just checked KDE lock screen and it does not leak these same notifications.

[mfc]

also a note that split-gpg notifications of private key access are pushed through the lock screen. This is on R3.2.

[andrewdavidwong]

This would be (hopefully) fixed by changing the default screen locker to physlock (#1917).

[cfcs]

@mfc KDE has the same problems, as outlined by @nvesely in #963 (comment)

(also, this bug is still a problem in Q3.2)

[andrewdavidwong]

I've found that this is not a Qubes-specific issue. I see popup notifications display through xscreensaver all the time on multiple physically distinct machines running an unrelated Linux distro. In light of this, @marmarek, should this issue be closed as notourbug?

[cfcs]

@andrewdavidwong It's a bug that we expose our users to by the choice of software we include, and by choice, since there are solutions available that does not have these security flaws (things leaking, bypassing authentication when the application crashes). Notably physlock, which we have been discussing for years in these Github threads now.

Saying that it's notourbug is IMHO equal to saying meltdown / spectre is not our bug, so lets ignore it, and if that is the strategy that Qubes wants to go with, we could erase most of the Qubes security bulletins already, since even if they affect Qubes users, very few of them are in software maintained by the Qubes team.

[andrewdavidwong]

It's a bug that we expose our users to by the choice of software we include, and by choice, since there are solutions available that does not have these security flaws (things leaking, bypassing authentication when the application crashes). Notably physlock, which we have been discussing for years in these Github threads now.

It's not entirely "by choice." If we could wave a magic wand and switch to physlock with everything working perfectly, I'm sure we would've done so years ago. But switching is not easy, and doing so carries opportunity costs.

Saying that it's notourbug is IMHO equal to saying meltdown / spectre is not our bug, so lets ignore it,

No, it's not. Closing this issue as notourbug does not entail that we'll ignore the screeenlocker topic in general. To be precise, closing #2026 does not entail closing #1917. They're two distinct issues. If #2026 is genuinely a bug in an upstream project's software and not ours, and if our developers cannot or will not fix that bug for the other project, then closing #2026 as notourbug is probably the correct thing to do. It doesn't follow that we're going to ignore the underlying issue or that we don't care about it. On the contrary, if it looks like the upstream project isn't going to fix #2026 (and we can't or won't do it for them), that might be all the more reason to increase the priority of #1917.

and if that is the strategy that Qubes wants to go with, we could erase most of the Qubes security bulletins already, since even if they affect Qubes users, very few of them are in software maintained by the Qubes team.

No. We issue QSBs to notify users of security-critical bugs and how to patch them (among other reasons). Given the nature of Qubes, most bugs (e.g., web browser bugs) are not security critical for Qubes, so we don't publish QSBs about them. As you know, most QSBs are about Xen bugs, because Xen is security critical. The fact that the Qubes team does not maintain Xen (the main project) is irrelevant to whether we issue QSBs for security-critical Xen bugs that affect Qubes.

[cfcs]

• Re: "by choice": Maybe that was a poor/provocative choice of words, I'm sorry. I appreciate that every reconfiguration carries cost, and it's a question of priorities. I personally think it's a somewhat important aspect of a distribution that aims to provide people with a secure desktop, but if a working screen locker is not a priority that the Qubes team shares, and the only security-critical component worth issuing QSBs about is in fact Xen, I feel it would be more honest to be up front about that, or just removing xscreensaver entirely and let people who cares about being able to safely lock their screen seek out their own solution.
  At least this stance comes as a surprise to me, and I think it will be surprising to people who currently use Qubes in shared office spaces, in transit, or anywhere their equipment may conceivably be stolen or momentarily accessed by other people.

It is my understanding that your argument for refraining from noticing people about bugs in browsers, or the recent LibreOffice exploit (that still works?) is that they expose AppVMs, but not dom0, to danger, and you don't care about what runs in people's AppVMs. The screen locker runs in dom0 and protects the whole system, so I don't see how that argument intuitively extends to mean that screenlocking is not security-critical.

• Re: Is it it a bug? As detailed by upstream in the FAQ this is not so much a bug in XScreenSaver as it is a requirement that was overlooked in the design of X11 (and X.org) decades ago, and neglected in the favor of backwards compatibility ever since. This means [XFCE] locked xscreensaver displays sensitive notifications #2026 will not be fixed upstream since it cannot be fixed.

XScreenSaver in particular has had a few "exciting" bugs that allowed bypassing the lock screen:

• 2015: Plugging in/out a HDMI cable
• 2012: Press Ctrl-Alt-*

But this is not exclusive to XScreenSaver (see #963 for the exact same issue in KDE's screenlocker, also on Qubes; and here @marmarek also reports that it's broken); building an X11 screensaver application is a design flaw in itself. Originally screensavers were meant to - save the screen from burning a permanent image, not to act as a security-critical component. The latter part was bolted onto the solutions as an afterthought, and it kind of shows.

• notourbug: When upstream can't fix a security flaw, and you won't fix it, closing the github issue that collects information about it removes visibility of the bug, which I feel is detrimental to the user experience. If a security-focused software distribution that I am using ships with known security flaws, I would like to know about them so that they can be weighed into my workflow. If people were made aware that their screen content would leak, they could choose not to use Qubes in settings where other people may see their (locked) screen (or install physlock or seek help to mitigate the problem themselves, like installing physlock or similar using the instructions provided in the github issue thread).

[cfcs]

BTW, found this other related thread (now closed for some reason) where @rootkovska and @marmarek also discuss screen locking: #2120

[cfcs]

Was closing the tabs from yesterday when I spotted this quote from the author of XScreenSaver regarding his apparent frustration with the situation (which is that you cannot write a secure X11-based screensaver):

There's little that I can do to make the screen locker secure so long as the kernel and X11 developers are actively working against security. The strength of the lock on your front door doesn't matter much so long as someone else in the house insists on leaving a key under the welcome mat.

I really don't want to sound too bitter or accusatory, please don't read any of this as an attack. I'm just a little weary of having discussed this in Github threads for 3 years now, and it seems like it is very unintuitive to people that xscreensaver, KDE's locker program etc are essentially toys, not security mechanisms (hence all the links and quotes to support this claim).

[andrewdavidwong]

I personally think it's a somewhat important aspect of a distribution that aims to provide people with a secure desktop, but if a working screen locker is not a priority that the Qubes team shares, and the only security-critical component worth issuing QSBs about is in fact Xen, I feel it would be more honest to be up front about that, or just removing xscreensaver entirely and let people who cares about being able to safely lock their screen seek out their own solution.

A secure screen locker is a high priority for us. We care about the security of Qubes OS as a whole, not just Xen. It just happens to be the case that many QSBs have been about Xen because many of the security-critical bugs that have affected Qubes have been in Xen.

When upstream can't fix a security flaw, and you won't fix it, closing the github issue that collects information about it removes visibility of the bug, which I feel is detrimental to the user experience. If a security-focused software distribution that I am using ships with known security flaws, I would like to know about them so that they can be weighed into my workflow. If people were made aware that their screen content would leak, they could choose not to use Qubes in settings where other people may see their (locked) screen (or install physlock or seek help to mitigate the problem themselves, like installing physlock or similar using the instructions provided in the github issue thread).

It sounds like the proper place to communicate such information is in the documentation, release notes, or in the installer. GitHub issues are meant for tracking issues, not for notifying users about things. If the crux of this issue is that users need to be informed about it, then it should be turned into a documentation issue (or similar), and closed once the documentation (or announcement or whatever) is done. The fact that people should be aware of something is not a good argument for leaving a GitHub issue about that thing open, since many users never see the issue tracker, and those who do can easily search through both open and closed issues. More importantly, leaving issues open when they aren't actionable makes the entire issue tracking system less efficient, which hinders our developers from doing their work.

[cfcs]

@andrewdavidwong I think it would be very good if this was detailed in the documentation (I guess release notes are more for information specific to a certain release), and/or in the installer indeed.
But right now this is not mentioned anywhere else, which is why I'm concerned that by closing the issue we would lose the only place where users can currently learn about the issue.
Information dissemination aside, this is an issue that affects current versions of Qubes, and therefore I do think it belongs in the issue tracker. It is a technical issue to which concrete solutions have been proposed, so I would prefer the fix being implemented in Qubes rather than shipping affected releases and informing that user that it comes pre-installed with security vulnerabilities. The latter is a perfectly valid approach to dealing with security vulnerabilities that are impossible to solve, but where solutions are available I find it suboptimal.

It is an actionable issue; the fix is simple: Use physlock instead.

[andrewdavidwong]

I think it would be very good if this was detailed in the documentation (I guess release notes are more for information specific to a certain release), and/or in the installer indeed.
But right now this is not mentioned anywhere else, which is why I'm concerned that by closing the issue we would lose the only place where users can currently learn about the issue.
Information dissemination aside, this is an issue that affects current versions of Qubes, and therefore I do think it belongs in the issue tracker.

Please read my comment above again. Specifically:

"If the crux of this issue is that users need to be informed about it, then it should be turned into a documentation issue (or similar), and closed once the documentation (or announcement or whatever) is done."

It is a technical issue to which concrete solutions have been proposed, so I would prefer the fix being implemented in Qubes rather than shipping affected releases and informing that user that it comes pre-installed with security vulnerabilities. The latter is a perfectly valid approach to dealing with security vulnerabilities that are impossible to solve, but where solutions are available I find it suboptimal.

It is an actionable issue; the fix is simple: Use physlock instead.

That's precisely what #1917 is about.

[cfcs]

@andrewdavidwong Ah, sorry: This thread is about the glitching, and #1917 is about the fix.
Then I agree with you. :-)

[andrewdavidwong]

This issue is being closed because:

• This issue has the "Release 3.2 updates" milestone.
• Qubes OS 3.2 recently reached end-of-life (EOL).
• This issue has not been updated in over a year.

If anyone believes that this issue should be reopened, please let us know in a comment here.

[iamahuman]

This issue should be reopened since:

• It persists into Qubes OS 4.0.
• It probably also persists into Qubes OS 4.1.
• Opening a new issue seems unnecessary since there is nothing new to add to the problem.

[iamahuman]

Edit: I didn't thoroughly looked into the code. Please see @marmarek's comment below.

For users that considers this issue to be a very serious security problem and don't need pop-up windows, a temporary workaround is to set override_redirect_protection = true;, introduced in this PR which actually aims to fix #5731.

[qubesos-bot]

Automated announcement from builder-github

The package qubes-gui-dom0-4.0.12-1.fc25 has been pushed to the r4.0 testing repository for dom0.
To test this update, please install it with the following command:

sudo qubes-dom0-update --enablerepo=qubes-dom0-current-testing

Changes included in this update

[qubesos-bot]

Automated announcement from builder-github

The package qubes-gui-daemon_4.1.8-1 has been pushed to the r4.1 testing repository for the Debian template.
To test this update, first enable the testing repository in /etc/apt/sources.list.d/qubes-*.list by uncommenting the line containing stretch-testing (or appropriate equivalent for your template version), then use the standard update command:

sudo apt-get update && sudo apt-get dist-upgrade

Changes included in this update

[qubesos-bot]

Automated announcement from builder-github

The package qubes-gui-dom0-4.0.12-1.fc25 has been pushed to the r4.0 stable repository for dom0.
To install this update, please use the standard update command:

sudo qubes-dom0-update

Or update dom0 via Qubes Manager.

Changes included in this update

[qubesos-bot]

Automated announcement from builder-github

The package qubes-audio-daemon-4.1.8-1.fc32 has been pushed to the r4.1 stable repository for dom0.
To install this update, please use the standard update command:

sudo qubes-dom0-update

Or update dom0 via Qubes Manager.

Changes included in this update

[qubesos-bot]

Automated announcement from builder-github

The package qubes-gui-daemon_4.1.8-1+deb9u1 has been pushed to the r4.1 stable repository for the Debian template.
To install this update, please use the standard update command:

sudo apt-get update && sudo apt-get dist-upgrade

Changes included in this update