Date: Wed, 13 Dec 2017 19:32:09 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Dave Hansen <dave.hansen@intel.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        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>,
        "Aneesh Kumar K. V" <aneesh.kumar@linux.vnet.ibm.com>
Subject: Re: [patch 05/16] mm: Allow special mappings with user access cleared
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On Wed, Dec 13, 2017 at 10:08:30AM -0800, Linus Torvalds wrote:
> On Wed, Dec 13, 2017 at 7:54 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > Which is why get_user_pages() _should_ enforce this.
> >
> > What use are protection keys if you can trivially circumvent them?
> 
> No, we will *not* worry about protection keys in get_user_pages().
> 
> They are not "security". They are a debug aid and safety against random mis-use.
> 
> In particular, they are very much *NOT* about "trivially circumvent
> them". The user could just change their mapping thing, for chrissake!
> 
> We already allow access to PROT_NONE for gdb and friends, very much on purpose.
> 
> We're not going to make the VM more complex for something that
> absolutely nobody cares about, and has zero security issues.

OK, that might have been my phrasing that was off -- mostly because I
was looking at it from the VM_NOUSER angle, but currently:

  - gup_pte_range() has pte_access_permitted()

  - follow_page_pte() has pte_access_permitted() for FOLL_WRITE only

All I'm saying is that that is inconsistent and we should change
follow_page_pte() to use pte_access_permitted() for FOLL_GET, such that
__get_user_pages_fast() and __get_user_pages() have matching semantics.

Now, if VM_NOUSER were to live, the above change would ensure write(2)
cannot read from such VMAs, where the existing test for FOLL_WRITE
already disallows read(2) from writing to them.

But even without VM_NOUSER it makes the VM more consistent than it is
today.