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X Server Prohibits Byte-swapped Clients

Summary

X server implementations (e.g. Xorg and Xwayland) will (by default) no longer allow clients with different endianess to connect.

Owner

Current status

Detailed Description

X server implementations (e.g. Xorg and Xwayland) allow clients with an endianess different to that of the server to connect. Protocol messages to and from these clients are byte-swapped by the X server. However, the code in the X server that does this is virtually untested, providing a large attack surface for malicious clients. One needs to only look at e.g. this X.Org security advisory and count the SProc mentions for an indication on how bad this is. A simple solution to remove this attack surface is to prohibit clients with a different endianess. These clients will simply fail, in a matter similar to failing to authenticate against an X server.

The use-case for clients with different endianess is very niche. The vast majority of users never run a remote client, let alone a remote client on a machine with a different endianess. Fedora only supports one Big Endian architecture (s390x), so the only Fedora users affected by this change are those running remote applications on an s390x.

Note:

  • this only affects use-cases where the server runs on a little endian host and the client on a Big Endian host (or vice versa).
  • this is a change in the default behavior only and can be changed via configuration options (for Xorg) and/or commandline arguments (all X servers).
  • this is a change in the upstream default behavior that Fedora will follow along with. This Change is primarily to increase the exposure of this new behavior.


Feedback

Benefit to Fedora

This change removes a large potential attack surface that can be used by malicious clients to expose memory locations, crash the X server and/or do other entertaining things malicious programs like to do.

Scope

  • Proposal owners:
  1. Merge upstream PR - complete
  2. Backport patch to Fedora's xorg-x11-server and xorg-x11-server-Xwayland packages - complete
  • Other developers:
  1. For systems running /usr/bin/Xorg, no further changes are needed. Users/system administrators can configure the new option in the xorg.conf ServerFlags section.
  2. Wayland compositors that manage /usr/bin/Xwayland (all of them?) that want to (re-)enable support for byte-swapped clients need to provide a configuration option to users that appends the +byteswappedclients option to the Xwayland invocation.
    • GNOME 44 mutter provides the gsettings xwayland-allow-byte-swapped-clients option, see this PR
  1. This feature does not require coordination with release engineering
  • Policies and guidelines: N/A (not needed for this Change)
  • Trademark approval: N/A (not needed for this Change)
  • Alignment with Objectives:

Upgrade/compatibility impact

For the use-case of users that run X clients on a remote machine with a different endianess, these clients will no longer be able to connect by default. Such clients will print an error message to stderr: Prohibited client endianess, see the Xserver man page.

For Xorg, the following xorg.conf.d snippet will re-enable the old behavior: 

Section "ServerFlags"
   Option "AllowByteSwappedClients" "on"
EndSection

Wayland users (and thus Xwayland) need to employ compositor-specific configuration to pass the +byteswappedclients flag to Xwayland. At the time of writing, Wayland compositors do not yet provide such a configuration.

How To Test

To test the impact of this change, you need:

  • an X server running on a little endian architecture and an X client running on a Big Endian architecture (or the other way around)
  • set up the X server to accept remote connections, either via TCP or through SSH
  • run the X client which will fail to connect

Alternatively, a test client is available in the upstream PR. This test client pretends to be BigEndian and will fail to connect when run against a little endian X server.


User Experience

For virtually all users, there is no change in behavior.

Users of Xorg with the server and client on two differently-byte-ordered machines must add the xorg.conf.d snippet shown above on affected systems. For users of a Wayland compositor and thus Xwayland, they need to pass the +byteswappedclients option to Xwayland. This is a configuration option that must be provided by the respective compositor.

Dependencies

No other RPMs depend on this change.


Contingency Plan

This change depends on whether upstream merges this new default behavior. If upstream does not merged the feature in time, this Change will be postponed until the next Fedora version to avoid potential incompatibilities between configurations or commandline options.

  • Contingency mechanism: keep current behavior, try again with next Fedora version
  • Contingency deadline: beta freeze
  • Blocks release? No


Documentation

N/A (not a System Wide Change)

Release Notes

X server implementations (e.g. Xorg and Xwayland) no longer accept connections from remote clients with a byte endianess different to that of the X server **by default**. This removes a large attack surface in the X server that can be exploited by malicious clients. Users that require this particular use-case can re-enable this feature with the AllowByteSwappedClients option (see the xorg.conf man page), or passing +byteswappedclients to the X server invocation (see the Xserver man page). This change does users that do not use X forwarding (with or without ssh).

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