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diff --git a/widget/windows/docs/blocklist.rst b/widget/windows/docs/blocklist.rst new file mode 100644 index 0000000000..1b7a1a6764 --- /dev/null +++ b/widget/windows/docs/blocklist.rst @@ -0,0 +1,358 @@ +======================== +Windows DLL Blocklisting +======================== + +-------- +Overview +-------- + +There are many applications which interact with another application, which means +they run their code as a DLL in a different process. This technique is used, for +example, when an antivirus software tries to monitor/block navigation to a +malicious website, or a screen reader tries to access UI parts. If such an +application injects their code into Firefox, and if there is a bug in their code +running in our firefox.exe, it will emerge as Firefox’s bug even though it’s +not. + +Firefox for Windows has a feature to prevent DLLs from being loaded into our +processes. If we are aware that a particular DLL causes a problem in our +processes such as a crash or performance degradation, we can stop the problem by +blocking the DLL from being loaded. + +This blocklist is about a third-party application which runs outside Firefox but +interacts with Firefox. For add-ons, there is `a different process +<https://extensionworkshop.com/documentation/publish/add-ons-blocking-process/>`_. + +This page explains how to request to block a DLL which you think we should block +it as well as technical details about the feature. + +----------------------- +Two types of blocklists +----------------------- + +There are two types of blocklists in Firefox: + +1. A static blocklist that is compiled in to Firefox. This consists of DLLs + known to cause problems with Firefox, and this blocklist cannot be disabled + by the user. For more information and instructions on how to add a new DLL + to this list, see :ref:`Process for blocking a DLL in the static blocklist + <how-to-block-dll-in-static-blocklist>` below. +2. A dynamic blocklist that users can use to block DLLs that are giving them + problems. This was added in + `bug 1744362 <https://bugzilla.mozilla.org/show_bug.cgi?id=1744362>`_. + +The static blocklist has ways to specify if only certain versions of a DLL +should be blocked, or only for certain Firefox processes, etc. The dynamic +blocklist does not have this capability; if a DLL is on the list it will always +be blocked. + +Regardless of which blocklist the DLL is on, if it meets the criteria for being +blocked Firefox uses the same mechanism to block it. There are more details +below in :ref:`How the blocklist blocks a DLL <how-the-blocklist-blocks-a-dll>`. + +.. _how-to-block-dll-in-static-blocklist: + +-------------------------------------------------- +Process for blocking a DLL in the static blocklist +-------------------------------------------------- + +But wait, should we really block it? +------------------------------------ + +Blocking a DLL with the static blocklist should be our last resort to fix a +problem because doing it normally breaks functionality of an application which +installed the DLL. If there is another option, we should always go for it. +Sometimes we can safely bypass a third-party’s problem by changing our code even +though its root cause is not on our side. + +When we decide to block it, we must be certain that the issue at hand is so +great that it outweighs the user's choice to install the software, the utility +it provides, and the vendor's freedom to distribute and control their software. + +How to request to block a DLL +----------------------------- + +Our codebase has the file named +`WindowsDllBlocklistDefs.in <https://searchfox.org/mozilla-central/source/toolkit/xre/dllservices/mozglue/WindowsDllBlocklistDefs.in>`_ from which our build process generates DLL blocklists as C++ header files and compiles them. To block a new DLL, you create a patch to update WindowsDllBlocklistDefs.in and land it on our codebase, following our standard development process. Moreover, you need to fill out a form specific to the DLL blockling request so that reviewers can review the impact and risk as well as the patch itself. + +Here are the steps: + +1. File `a bug + <https://bugzilla.mozilla.org/enter_bug.cgi?format=__default__&bug_type=defect&product=Toolkit&component=Blocklist%20Policy%20Requests&op_sys=Windows&short_desc=DLL%20block%20request%3A%20%3CDLL%20name%3E&comment=Please%20go%20through%20https%3A%2F%2Fwiki.mozilla.org%2FBlocklisting%2FDLL%20before%20filing%20a%20new%20bug.>`_ + if it does not exist. +2. Answer all the questions in `this questionnaire + <https://msmania.github.io/assets/mozilla/third-party-modules/questionnaire.txt>`_, + and attach it to the bug as a plaintext. +3. Make a patch and start a code review via Phabricator as usual. + +How to edit WindowsDllBlocklistDefs.in +-------------------------------------- + +WindowsDllBlocklistDefs.in defines several variables as a Python Array. When you +add a new entry in the blocklists, you pick one of the variables and add an +entry in the following syntax: + +Syntax +****** + +:: + + Variable += [ + ... + # One-liner comment including a bug number + EntryType(Name, Version, Flags), + ... + ] + +Parameters +********** + ++-----------+--------------------------------------------------------------------------------+ +| Parameter | Value | ++===========+================================================================================+ +| Variable | ALL_PROCESSES \| BROWSER_PROCESS \| CHILD_PROCESSES \| GMPLUGIN_PROCESSES \| | +| | GPU_PROCESSES \| SOCKET_PROCESSES \| UTILITY_PROCESSES | ++-----------+--------------------------------------------------------------------------------+ +| EntryType | DllBlocklistEntry \| A11yBlocklistEntry \| RedirectToNoOpEntryPoint | ++-----------+--------------------------------------------------------------------------------+ +| Name | A case-insensitive string representing a DLL's filename to block | ++-----------+--------------------------------------------------------------------------------+ +| Version | One of the following formats: | +| | | +| | - ALL_VERSIONS \| UNVERSIONED | +| | - A tuple consisting of four digits | +| | - A 32-bit integer representing a Unix timestamp with PETimeStamp | ++-----------+--------------------------------------------------------------------------------+ +| Flags | BLOCK_WIN8_AND_OLDER \| BLOCK_WIN7_AND_OLDER | ++-----------+--------------------------------------------------------------------------------+ + +Variable +******** + +Choose one of the following predefined variables. + +- **ALL_PROCESSES**: DLLs defined here are blocked in BROWSER_PROCESS + + CHILD_PROCESSES +- **BROWSER_PROCESS**: DLLs defined here are blocked in the browser process +- **CHILD_PROCESSES**: DLLs defined here are blocked in non-browser processes +- **GMPLUGIN_PROCESSES**: DLLs defined here are blocked in GMPlugin processes +- **GPU_PROCESSES**: DLLs defined here are blocked in GPU processes +- **SOCKET_PROCESSES**: DLLs defined here are blocked in socket processes +- **UTILITY_PROCESSES**: DLLs defined here are blocked in utility processes + +EntryType +********* +Choose one of the following predefined EntryTypes. + +- **DllBlocklistEntry**: Use this EntryType unless your case matches the other + EntryTypes. +- **A11yBlocklistEntry**: If you want to block a module only when it’s loaded by + an accessibility application such as a screen reader, you can use this + EntryType. +- **RedirectToNoOpEntryPoint**: If a modules is injected via Import Directory + Table, adding the module as DllBlocklistEntry breaks process launch, meaning + DllBlocklistEntry is not an option. You can use RedirectToNoOpEntryPoint + instead. + +Name +**** +A case-insensitive string representing a DLL's filename to block. Don’t include a directory name. + +Version +******* + +A maximum version to be blocked. If you specify a value, a module with the +specified version, older versions, and a module with no version are blocked. + +| If you want to block a module regardless of its version, use ALL_VERSIONS. +| If you want to block a module with no version, use UNVERSIONED. + + +To specify a version, you can use either of the following formats: + +- | A tuple consisting of four digits. This is compared to the version that is embedded in a DLL as a version resource. + | Example: (1, 2, 3, 4) +- | A 32-bit integer representing a Unix timestamp with PETimeStamp. This is compared to an integer of IMAGE_FILE_HEADER::TimeDateStamp. + | Example: PETimeStamp(0x12345678) + +Flags +***** + +If you know a problem happens only on older Windows versions, you can use one of +the following flags to narrow down the affected platform. + +- BLOCK_WIN8_AND_OLDER +- BLOCK_WIN7_AND_OLDER + + +----------------- +Technical details +----------------- + +.. _how-the-blocklist-blocks-a-dll: + +How the blocklist blocks a DLL +------------------------------ + +Briefly speaking, we make ntdll!NtMapViewOfSection return +``STATUS_ACCESS_DENIED`` if a given module is on the blocklist, thereby a +third-party’s code, or even Firefox’s legitimate code, which tries to load a DLL +in our processes in any way such as LoadLibrary API fails and receives an +access-denied error. + +Cases where we should not block a module +---------------------------------------- + +As our blocklist works as explained above, there are the cases where we should not block a module. + +- | A module is loaded via `Import Directory Table <https://docs.microsoft.com/en-us/windows/win32/debug/pe-format#import-directory-table>`_ + | Blocking this type of module blocks even a process from launching. You may be able to block this type of module with RedirectToNoOpEntryPoint. +- | A module is loaded as a `Layered Service Provider <https://docs.microsoft.com/en-us/windows/win32/winsock/categorizing-layered-service-providers-and-applications>`_ + | Blocking this type of module on Windows 8 or newer breaks networking. Blocking a LSP on Windows 7 is ok. + +(we used to have to avoid blocking modules loaded via a +`Window hook <https://docs.microsoft.com/en-us/windows/win32/winmsg/hooks>`_ because blocking this type of +module would cause repetitive attempts to load a module, resulting in slow performance +like `Bug 1633718 <https://bugzilla.mozilla.org/show_bug.cgi?id=1633718>`_, but this should be fixed +as of `Bug 1823412 <https://bugzilla.mozilla.org/show_bug.cgi?id=1823412>`_.) + +Third-party-module ping +----------------------- + +We’re collecting the :ref:`third-party-module ping <third-party-modules-ping>` +which captures a moment when a third-party module is loaded into the +Browser/Tab/RDD process. As it’s asked in the request form, it’s important to +check the third-party-module ping and see whether a module we want to block +appears in the ping or not. If it appears, you may be able to know how a module +is loaded by looking at a callstack in the ping. + +How to view callstacks in the ping +********************************** + +1. You can run a query on BigQuery console or STMO. (BigQuery console is much + faster and can handle larger data.) + + - BigQuery console (visit + `here <https://docs.telemetry.mozilla.org/cookbooks/bigquery.html#gcp-bigquery-console>`_ + to request access): https://console.cloud.google.com/bigquery + - STMO: https://sql.telemetry.mozilla.org/ + +2. Make your own query based on `this template + <https://msmania.github.io/assets/mozilla/third-party-modules/query-template.txt>`_. +3. Run the query. +4. Save the result as a JSON file. + + - In BigQuery console, click [SAVE RESULTS] and choose [JSON (local file)]. + - In STMO, click [...] at the right-top corner and select [Show API Key], + then you can download a JSON from a URL shown in the [Results in JSON format]. + +5. | Go to https://msmania.github.io/assets/mozilla/third-party-modules/ + | (A temporal link. Need to find a permanent place.) +6. Click [Upload JSON] and select the file you saved at the step 4. +7. Click a row in the table to view a callstack + + +How to see the versions of a specific module in the ping +******************************************************** + +You can use `this template query +<https://msmania.github.io/assets/mozilla/third-party-modules/query-groupby-template.txt>`_ +to query which versions of a specific module are captured in the ping. This +tells the product versions which are actively used including the crashing +versions and the working versions. + +You can also get the crashing versions by querying the crash reports or the +Socorro table. Having two version lists, you can decide whether you can specify +the Version parameter in a blocklist entry. + +Initialization +-------------- + +In order to have the most effective blocking of DLLs, the blocklist is +initialized very early during browser startup. If the :ref:`launcher process +<launcher-process>` is available, the steps are: + +- Launcher process loads dynamic blocklist from disk (see + `DynamicBlocklist::LoadFile() + <https://searchfox.org/mozilla-central/search?q=DynamicBlocklist%3A%3ALoadFile&path=&case=false®exp=false>`_) +- Launcher process puts dynamic blocklist data in shared section (see + `SharedSection::AddBlocklist() + <https://searchfox.org/mozilla-central/search?q=SharedSection%3A%3AAddBlocklist&path=&case=false®exp=false>`_) +- Launcher process creates the browser process in a suspended mode, sets up its + dynamic blocklist, then starts it. (see `LauncherMain() + <https://searchfox.org/mozilla-central/search?q=LauncherMain&path=&case=false®exp=false>`_) + + - This is so (ideally) no DLLs can be injected before the blocklist is set up. + +If the launcher process is not available, a different blocklist is used, defined +in `mozglue/WindowsDllBlocklist.cpp +<https://searchfox.org/mozilla-central/source/toolkit/xre/dllservices/mozglue/WindowsDllBlocklist.cpp>`_. +This code does not currently support the dynamic blocklist. This is intended to +only be used in testing and other non-deployed scenarios, so this shouldn't be +a problem for users. + +Note that the mozglue blocklist also has a feature to block threads that start +in ``LoadLibrary`` and variants. This code is currently only turned on in +Nightly builds because it breaks some third-party DLP products. + +Dynamic blocklist file location +------------------------------- + +Because the blocklist is loaded so early during startup, we don't have access to +what profile is going to be loaded, so the blocklist file can't be stored there. +Instead, by default the blocklist file is stored in the Windows user's roaming +app data directory, specifically + +``<Roaming AppData directory>\Mozilla\Firefox\blocklist-<install hash>`` + +Note that the install hash here is what is returned by `GetInstallHash() +<https://searchfox.org/mozilla-central/source/toolkit/mozapps/update/common/commonupdatedir.cpp#404>`_, +and is suitable for uniquely identifying the particular Firefox installation +that is running. + +On first launch, this location will be written to the registry, and can be +overriden by setting that key to a different file location. The registry key is +``HKEY_CURRENT_USER\Software\Mozilla\Firefox\Launcher``, and the name is the +full path to firefox.exe with "\|Blocklist" appended. This code is in +`LauncherRegistryInfo +<https://searchfox.org/mozilla-central/source/toolkit/xre/LauncherRegistryInfo.cpp>`_. + +Adding to and removing from the dynamic blocklist +------------------------------------------------- + +Users can add or remove DLLs from the dynamic blocklist by navigating to +``about:third-party``, finding the entry for the DLL they are interested in, and +clicking on the dash icon. They will then be prompted to restart the browser, as +the change will only take effect after the browser restarts. + +Disabling the dynamic blocklist +------------------------------- + +It is possible that users can get Firefox into a bad state by putting a DLL on +the dynamic blocklist. One possibility is that the user blocks only one of a set +of DLLs that interact, which could make Firefox behave in unpredictable ways or +crash. + +By launching Firefox with ``--disableDynamicBlocklist``\, the dynamic blocklist +will be loaded but not used to block DLLs. This lets the user go to +``about:third-party`` and attempt to fix the problem by unblocking or blocking +DLLs. + +Similarly, in safe mode the dynamic blocklist is also disabled. + +Enterprise policy +----------------- + +The dynamic blocklist can be disabled by setting a registry key at +``HKEY_CURRENT_USER\Software\Policies\Mozilla\Firefox`` with a name of +DisableThirdPartyModuleBlocking and a DWORD value of 1. This will have the +effect of not loading the dynamic blocklist, and no icons will show up in +``about:third-party`` to allow blocking DLLs. + +------- +Contact +------- + +Any questions or feedback are welcome! + +**Matrix**: `#hardening <https://app.element.io/#/room/#hardening:mozilla.org>`_ |