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Date: Sat, 16 Dec 2017 02:48:24 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
Subject: Re: [PATCH v2 01/17] mm/gup: Fixup p*_access_permitted()
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To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Dave Hansen <dave.hansen@intel.com>, Dan Williams <dan.j.williams@intel.com>, Peter Zijlstra <peterz@infradead.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Thomas Gleixner <tglx@linutronix.de>, the arch/x86 maintainers <x86@kernel.org>, Andy Lutomirsky <luto@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 <linux-mm@kvack.org>, "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>

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@intel.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...

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