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

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Message-ID: <20171216003138.GJ21978@ZenIV.linux.org.uk>
Date:   Sat, 16 Dec 2017 00:31:38 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...el.com>,
        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>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v2 01/17] mm/gup: Fixup p*_access_permitted()

On Fri, Dec 15, 2017 at 04:20:31PM -0800, Linus Torvalds wrote:
> On Thu, Dec 14, 2017 at 11:51 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > So we actually need the pte_access_permitted() stuff if we want to
> > ensure we're not stepping on !PAGE_USER things.
> 
> We really don't. Not in that complex and broken format, and not for every level.
> 
> Also, while I think we *should* check the PAGE_USER bit when walking
> the page tables, like we used to, we should
> 
>  (a) do it much more simply, not with that broken interface that takes
> insane and pointless flags
> 
>  (b) not tie it together with this issue at all, since the PAGE_USER
> thing really is largely immaterial.
> 
> The fact is, if we have non-user mappings in the user part of the
> address space, we _need_ to teach access_ok() about them, because
> fundamentally any "get_user()/put_user()" will happily ignore the lack
> of PAGE_USER (since those happen from kernel space).

Details, please - how *can* access_ok() be taught of that?