Re: [patch 05/16] mm: Allow special mappings with user access cleared

[Linus Torvalds]

On Wed, Dec 13, 2017 at 10:32 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> Now, if VM_NOUSER were to live, the above change would ensure write(2)
> cannot read from such VMAs, where the existing test for FOLL_WRITE
> already disallows read(2) from writing to them.

So I don't mind at all the notion of disallowing access to some
special mappings at the vma level. So a VM_NOUSER flag that just
disallows get_user_pages entirely I'm ok with.

It's the protection keys in particular that I don't like having to
worry about. They are subtle and have odd architecture-specific
meaning, and needs to be checked at all levels in the page table tree.

               Linus