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NEWS: document the usern/mknod borkage in 4.18 a bit

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Lennart Poettering 2018-12-17 16:46:36 +01:00
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@ -384,6 +384,34 @@ CHANGES WITH 240 in spe:
SD_ID128_ALLF to test if a 128bit ID is set to all 0xFF bytes, and to
initialize one to all 0xFF.
* KERNEL API BREAKAGE: Linux kernel 4.18 changed behaviour regarding
mknod() handling in user namespaces. Previously mknod() would always
fail with EPERM in user namespaces. Since 4.18 mknod() will succeed
but device nodes generated that way cannot be opened, and attempts to
open them result in EPERM. This breaks the "graceful fallback" logic
in systemd's PrivateDevices= sand-boxing option. This option is
implemented defensively, so that when systemd detects it runs in a
restricted environment (such as a user namespace, or an environment
where mknod() is blocked through seccomp or absence of CAP_SYS_MKNOD)
where device nodes cannot be created the effect of PrivateDevices= is
bypassed (following the logic that 2nd-level sand-boxing is not
essential if the system systemd runs in is itself already sand-boxed
as a whole). This logic breaks with 4.18 in container managers where
user namespacing is used: suddenly PrivateDevices= succeeds setting
up a private /dev/ file system containing devices nodes — but when
these are opened they don't work.
At this point is is recommended that container managers utilizing
user namespaces that intend to run systemd in the payload explicitly
block mknod() with seccomp or similar, so that the graceful fallback
logic works again.
We are very sorry for the breakage and the requirement to change
container configurations for newer kernels. It's purely caused by an
incompatible kernel change. The relevant kernel developers have been
notified about this userspace breakage quickly, but they chose to
ignore it.
Contributions from: afg, Alan Jenkins, Aleksei Timofeyev, Alexander
Filippov, Alexander Kurtz, Alexey Bogdanenko, Andreas Henriksson,
Andrew Jorgensen, Anita Zhang, apnix-uk, Arkan49, Arseny Maslennikov,