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

[Andy Lutomirski]

On Wed, Dec 13, 2017 at 4:10 PM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> On Wed, Dec 13, 2017 at 11:12:33PM +0100, Peter Zijlstra wrote:
>> On Wed, Dec 13, 2017 at 01:50:22PM -0800, Matthew Wilcox wrote:
>> > On Tue, Dec 12, 2017 at 06:32:26PM +0100, Thomas Gleixner 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.
>> >
>> > Maybe I misunderstand the intent behind this, but I was recently looking
>> > at something kind of similar.  I was calling it VM_NOTLB and it wouldn't
>> > put TLB entries into the userspace map at all.  The idea was to be able
>> > to use the user address purely as a handle for specific kernel pages,
>> > which were guaranteed to never be mapped into userspace, so we didn't
>> > need to send TLB invalidations when we took those pages away from the user
>> > process again.  But we'd be able to pass the address to read() or write().
>>
>> Since the LDT is strictly per process, the idea was to actually inject
>> it into the userspace map. Except of course, userspace must not actually
>> be able to access it. So by mapping it !_PAGE_USER its 'invisible'.
>>
>> But the CPU very much needs the mapping, it will load the LDT entries
>> through them.
>
> So can I use your VM_NOUSER VMAs for my purpose?  That is, can I change
> the page table without flushing the TLB?  The only access to these PTEs
> will be through the kernel mapping, so I don't see why I'd need to.

I doubt it, since if it's in the kernel pagetables at all, then the
mapping can be cached for kernel purposes.

But I still think this discussion is off in the weeds.  x86 does not
actually need any of this stuff.