Linux kernel: broken uid/gid mapping for nested user namespaces with >5 ranges (CVE-2018-18955; since 4.15; fixed in 4.18.19 and 4.19.2)

[Jann Horn <jannh () google com>]

NOTE: I have requested a CVE identifier, and I'm sending this message,
to make tracking of the fix easier; however, to avoid missing security
fixes without CVE identifiers, you should *NOT* be cherry-picking a
specific patch in response to a notification about a kernel security
bug.

In Linux kernel versions since 4.15, map_write() in
kernel/user_namespace.c handles nested user namespaces with more than
5 UID or GID ranges incorrectly. This can allow a user who has
CAP_SYS_ADMIN in a user namespace which maps at least 6 UIDs or GIDs
to bypass access controls on resources outside the namespace.

This is CVE-2018-18955.

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d2f007dbe7e4c9583eea6eb04d60001e85c6f1bd
https://cdn.kernel.org/pub/linux/kernel/v4.x/ChangeLog-4.18.19
https://cdn.kernel.org/pub/linux/kernel/v4.x/ChangeLog-4.19.2
https://bugs.chromium.org/p/project-zero/issues/detail?id=1712