Hutterer: X servers no longer allow byte-swapped clients

Posted Jan 7, 2023 21:40 UTC (Sat) by NYKevin (subscriber, #129325)
In reply to: Hutterer: X servers no longer allow byte-swapped clients by psychon
Parent article: Hutterer: X servers no longer allow byte-swapped clients

Why not just do all the endianness swapping in the BE host? Then the rare code path has to be executed on the same host that generated the possibly-malicious message in the first place, and there's no potential for privilege elevation.

Well, unless someone is running a BE server with a LE client that is potentially malicious. Or the LE server is potentially malicious and we're trying to protect the BE client. Do real users care about either of those problems? The latter seems really weird IMHO because usually the server is "my computer" and the client is "the other computer," and the former implies a rather unusual hardware configuration.

Hutterer: X servers no longer allow byte-swapped clients

Posted Jan 8, 2023 10:12 UTC (Sun) by whot (subscriber, #50317) [Link] (2 responses)

> Why not just do all the endianness swapping in the BE host?

Parts of the X protocol are written so that you don't know what to swap until you've parsed everything else before. e.g. requests with lists of items where each item is of different length (including, iirc, sometimes the length defined by some type field). So basically - you need a full X server to parse enough of the protocol to byte-swap everything. And that's a fair bit of effort that realistically no-one is going to step up to do. libxcb would probably help with that a lot, but that's been on the wishlist for at least 15 years now.

Hutterer: X servers no longer allow byte-swapped clients

Posted Jan 9, 2023 21:14 UTC (Mon) by NYKevin (subscriber, #129325) [Link] (1 responses)

Well, I suppose it depends. When application developers compile code for a big endian architecture, will they prefer to just byte-swap their own X11 messages and pretend to be a LE client, or will they prefer to tell the user to futz with the X server configuration?

Hutterer: X servers no longer allow byte-swapped clients

Posted Jan 10, 2023 9:18 UTC (Tue) by geert (subscriber, #98403) [Link]

Pretending to be an LE client won't help, as that may break connecting to a BE server[1].
So this really divides the world in two distinct universes.
And if distros mitigate this interoperability problem by enabling the support by default, the attack vector stays the same.

The proper way to handle this would be to fix the endianness in the protocol, and have a grace period, like we did for ext2fs (little endian) and XFS (big endian) decades ago. Unfortunately it's a bit late for that, and a short grace period won't work, as there may be clients that cannot be updated easily. Network protocol compatibility is different from file system compatibility, where you can just convert the file system once, and be done with it (there were never many big-endian ext2 file systems in the first place, as the problem was identified and solved early).

[1] None of my BE hardware is supported by contemporary X servers, as Xorg dropped the support for that "exotic" hardware a while ago. So in reality, I only care about using BE clients connecting to an LE server on modern hardware, which is fast enough to make the byte-swapping overhead unnoticeable.

Hutterer: X servers no longer allow byte-swapped clients

Posted Jan 9, 2023 12:47 UTC (Mon) by geert (subscriber, #98403) [Link] (2 responses)

Yeah, let's fix it on all (multiple) clients (assumed you can), instead of on the (single) server ;-)

Seriously, this breaks interoperability with other (non-Xorg) systems. If this was Linux, it would be reverted at the blink of an eye ("Do not break userpace ABI people depend on").

Less-serious: will IPv7 be little-endian?

Hutterer: X servers no longer allow byte-swapped clients

Posted Jan 9, 2023 13:40 UTC (Mon) by geert (subscriber, #98403) [Link] (1 responses)

BTW, if not reverted, I assume distros will enable this by default, as soon as users start complaining.

Hutterer: X servers no longer allow byte-swapped clients

Posted Jan 19, 2023 17:00 UTC (Thu) by mrugiero (guest, #153040) [Link]

There are dozens of us! Dozens!