kernel: Dangerous interaction between clear_child_tid, set_fs(), and kernel oopses

[Nelson Elhage <nelhage () ksplice com>]

I've discovered an interesting interaction in the Linux kernel between the
clear_child_tid feature of clone(2), and the set_fs() function used internally
in the kernel to temporarily disable access_ok() checking of userspace pointers.

Under some (not totally uncommon) circumstances, it is possible for a user to
leverage this interaction to turn a kernel oops or BUG() into a write of an
integer 0 to a user-controlled address in kernel memory.

I'm not sure if this merits a CVE or not; It is (as far as I can tell) only a
problem in the presence of another security bug, but it potentially makes a
large class of bugs significantly more dangerous (DoS -> privesc).

Reference:
https://lkml.org/lkml/2010/12/1/543

- Nelson