linux-kernel - Re: [PATCH v2 01/17] mm/gup: Fixup p*_access

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Message-ID: <20171216024824.GK21978@ZenIV.linux.org.uk>
Date:   Sat, 16 Dec 2017 02:48:24 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        Andy Lutomirsky <luto@...nel.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>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        Will Deacon <will.deacon@....com>,
        linux-mm <linux-mm@...ck.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v2 01/17] mm/gup: Fixup p*_access_permitted()

On Fri, Dec 15, 2017 at 06:28:36PM -0800, Linus Torvalds wrote:
> On Fri, Dec 15, 2017 at 5:25 PM, Dave Hansen <dave.hansen@...el.com> wrote:
> >
> > I think the reason we needed VMA and PTE checks was the
> > get_user_pages_fast() path not having a VMA.
> 
> That is indeed the point of get_user_pages_fast(): no vma lookup, no
> locking, just "do the default case as streamlined as possible".
> 
> But part of it is also that we should fall back to the slow case if
> the fast case doesn't work (eg because the page isn't there or
> whatever).
> 
> So what we could do - perhaps - is to just make get_user_pages_fast()
> check whether any of the protection key bits are set, and fail for
> that case.

FWIW, a good description of fast path in get_user_pages_fast() is
"simulate a TLB miss", the slow path being "... and go for simulated
page fault if TLB miss would have escalated to #PF".

Treating protection key bits as "escalate to page fault and let that
deal with the checks" should be fine - page fault handler must
cope with the page actually being present in page tables anyway, for
obvious reasons...