Re: [PATCH v2 11/17] selftests/x86/ldt_gdt: Prepare for access bit forced

[Linus Torvalds]

On Thu, Dec 14, 2017 at 1:22 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>
> Which kind of kills the whole thing.  There's no way the idea of
> putting the LDT in a VMA is okay if it's RW.

Sure there is.

I really don't understand why you guys think it has to be RO.

All it has to be is not _user_ accessible. And that's a requirement
regardless, because no way in hell should users be able to read the
damn thing.

So it clearly needs to have the PAGE_USER bit clear (to avoid users
accessing it directly), and it needs to be marked somehow for
get_user_pages() to refuse it too, and access_ok() needs to fail it so
that we can't do get_user/put_user on it.

But the whole RO vs RW is not fundamentally critical.

Now, I do agree that RO is much much better in general, and it avoids
the requirement to play games with "access_ok()" and friends (assuming
we're just ok with users reading it), but I disagree with the whole
"this is fundamental".

                 Linus