Re: [kernel-hardening] Re: [RFC v2][PATCH 04/11] x86: Implement __arch_rare_write_begin/unmap()

[Mark Rutland <mark.rutland@arm.com>]

On Fri, Apr 07, 2017 at 09:14:29AM -0700, Andy Lutomirski wrote:
> I think we're approaching this all wrong, actually.  The fact that x86
> has this CR0.WP thing is arguably a historical accident, and the fact
> that PaX uses it doesn't mean that PaX is doing it the best way for
> upstream Linux.
> 
> Why don't we start at the other end and do a generic non-arch-specific
> implementation: set up an mm_struct that contains an RW alias of the
> relevant parts of rodata and use use_mm to access it.  (That is,
> get_fs() to back up the old fs, set_fs(USER_DS),
> use_mm(&rare_write_mm), do the write using copy_to_user, undo
> everything.)

FWIW, I completely agree with this approach.

That's largely the approach arm64 would have to take regardless (as per
Hoeun's patches), and having a consistent common implementation would be
desireable.

There are a couple of other complications to handle (e.g. a perf
interrupt coming in and trying to read from the mapping), but I think we
can handle that generically.

Thanks,
Mark.