Linux kernel: userfaultfd bypasses tmpfs file permissions (CVE-2018-18397; since 4.11; fixed in 4.14.87 and 4.19.7)

[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 4.11, userfaultfd can be used to write
arbitrary data into holes in sparse tmpfs files to which an attacker
has read-only access.

This is CVE-2018-18397.

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=29ec90660d68bbdd69507c1c8b4e33aa299278b1
https://cdn.kernel.org/pub/linux/kernel/v4.x/ChangeLog-4.14.87
https://cdn.kernel.org/pub/linux/kernel/v4.x/ChangeLog-4.19.7
https://bugs.chromium.org/p/project-zero/issues/detail?id=1700