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

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Date:   Wed, 13 Dec 2017 16:16:56 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirsky <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Borislav Petkov <bpetkov@...e.de>,
        Greg KH <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...gle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        David Laight <David.Laight@...lab.com>,
        Eduardo Valentin <eduval@...zon.com>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        Will Deacon <will.deacon@....com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [patch 05/16] mm: Allow special mappings with user access cleared

On Wed, Dec 13, 2017 at 4:10 PM, Matthew Wilcox <willy@...radead.org> 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@...radead.org>
>> > > 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.