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

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Date:   Wed, 13 Dec 2017 15:34:55 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        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>, aliguori@...zon.com,
        Will Deacon <will.deacon@....com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        kirill.shutemov@...ux.intel.com, aneesh.kumar@...ux.vnet.ibm.com
Subject: Re: [patch 05/16] mm: Allow special mappings with user access cleared

On Wed, Dec 13, 2017 at 03:57:40PM +0300, Kirill A. Shutemov wrote:
> On Wed, Dec 13, 2017 at 01:22:11PM +0100, Peter Zijlstra wrote:

> > get_user_pages_fast() will ultimately end up doing
> > pte_access_permitted() before getting the page, follow_page OTOH does
> > not do this, which makes for a curious difference between the two.
> > 
> > So I'm thinking we want the below irrespective of the VM_NOUSER patch,
> > but with VM_NOUSER it would mean write(2) will no longer be able to
> > access the page.
> 
> Oh..
> 
> We do call pte_access_permitted(), but only for write access.
> See can_follow_write_pte().

My can_follow_write_pte() looks like:

static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
{
	return pte_write(pte) ||
		((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
}

am I perchance looking at the wrong tee?

> The issue seems bigger: we also need such calls for other page table levels :-/

Sure.