Date: Thu, 14 Dec 2017 23:24:39 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, X86 ML <x86@kernel.org>,
        Dave Hansen <dave.hansen@intel.com>, Borislav Petkov <bpetkov@suse.de>,
        Greg KH <gregkh@linuxfoundation.org>, Kees Cook <keescook@google.com>,
        Hugh Dickins <hughd@google.com>, Brian Gerst <brgerst@gmail.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <jgross@suse.com>,
        David Laight <David.Laight@aculab.com>,
        Eduardo Valentin <eduval@amazon.com>,
        "Liguori, Anthony" <aliguori@amazon.com>,
        Will Deacon <will.deacon@arm.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Dan Williams <dan.j.williams@intel.com>
Subject: Re: [PATCH v2 11/17] selftests/x86/ldt_gdt: Prepare for access bit
 forced
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On Thu, Dec 14, 2017 at 02:14:00PM -0800, Linus Torvalds wrote:
> On Thu, Dec 14, 2017 at 2:02 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > _Should_ being the operative word, because I cannot currently see it
> > DTRT. But maybe I'm missing the obvious -- I tend to do that at times.
> 
> At least the old get_user_pages_fast() code used to check the USER bit:
> 
>         unsigned long need_pte_bits = _PAGE_PRESENT|_PAGE_USER;
> 
>         if (write)
>                 need_pte_bits |= _PAGE_RW;
> 
> but that may have been lost when we converted over to the generic code.

The generic gup_pte_range() has pte_access_permitted() (which has the
above test) in the right place.

> It shouldn't actually _matter_, since we'd need to change access_ok()
> anyway (and gup had better check that!)

get_user_pages_fast() (both of them) do indeed test access_ok(), but the
regular get_user_pages() does not, I suspect because it can operate on a
foreign mm.

And its the regular old get_user_pages() that's all sorts of broken wrt
!PAGE_USER too.