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

[Andy Lutomirski]

On Tue, Dec 12, 2017 at 10:05 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Tue, Dec 12, 2017 at 10:00:08AM -0800, Andy Lutomirski wrote:
>> On Tue, Dec 12, 2017 at 9:32 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>> > From: Peter Zijstra <peterz@xxxxxxxxxxxxx>
>> >
>> > In order to create VMAs that are not accessible to userspace create a new
>> > VM_NOUSER flag. This can be used in conjunction with
>> > install_special_mapping() to inject 'kernel' data into the userspace map.
>> >
>> > Similar to how arch_vm_get_page_prot() allows adding _PAGE_flags to
>> > pgprot_t, introduce arch_vm_get_page_prot_excl() which masks
>> > _PAGE_flags from pgprot_t and use this to implement VM_NOUSER for x86.
>>
>> How does this interact with get_user_pages(), etc?
>
> gup would find the page. These patches do in fact rely on that through
> the populate things.
>

Blech.  So you can write(2) from the LDT to a file and you can even
sendfile it, perhaps.  What happens if it's get_user_page()'d when
modify_ldt() wants to free it?

This patch series scares the crap out of me.