linux-kernel - Re: [PATCH 1/3] x86: Honour passed pgprot in track_pfn_insert() and track_pfn

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Date:	Fri, 29 Jan 2016 09:49:09 -0500
From:	Matthew Wilcox <willy@...ux.intel.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	Ingo Molnar <mingo@...hat.com>,
	Kees Cook <keescook@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 1/3] x86: Honour passed pgprot in track_pfn_insert() and
 track_pfn_remap()

On Tue, Jan 26, 2016 at 09:44:24PM -0800, Andy Lutomirski wrote:
> On Tue, Jan 26, 2016 at 8:40 PM, Matthew Wilcox <willy@...ux.intel.com> wrote:
> > On Mon, Jan 25, 2016 at 09:33:35AM -0800, Andy Lutomirski wrote:
> >> On Mon, Jan 25, 2016 at 9:25 AM, Matthew Wilcox
> >> <matthew.r.wilcox@...el.com> wrote:
> >> > From: Matthew Wilcox <willy@...ux.intel.com>
> >> >
> >> > track_pfn_insert() overwrites the pgprot that is passed in with a value
> >> > based on the VMA's page_prot.  This is a problem for people trying to
> >> > do clever things with the new vm_insert_pfn_prot() as it will simply
> >> > overwrite the passed protection flags.  If we use the current value of
> >> > the pgprot as the base, then it will behave as people are expecting.
> >> >
> >> > Also fix track_pfn_remap() in the same way.
> >>
> >> Well that's embarrassing.  Presumably it worked for me because I only
> >> overrode the cacheability bits and lookup_memtype did the right thing.
> >>
> >> But shouldn't the PAT code change the memtype if vm_insert_pfn_prot
> >> requests it?  Or are there no callers that actually need that?  (HPET
> >> doesn't, because there's a plain old ioremapped mapping.)
> >
> > I'm confused.  Here's what I understand:
> >
> >  - on x86, the bits in pgprot can be considered as two sets of bits;
> >    the 'cacheability bits' -- those in _PAGE_CACHE_MASK and the
> >    'protection bits' -- PRESENT, RW, USER, ACCESSED, NX
> >  - The purpose of track_pfn_insert() is to ensure that the cacheability bits
> >    are the same on all mappings of a given page, as strongly advised by the
> >    Intel manuals [1].  So track_pfn_insert() is really only supposed to
> >    modify _PAGE_CACHE_MASK of the passed pgprot, but in fact it ends up
> >    modifying the protection bits as well, due to the bug.
> >
> > I don't think you overrode the cacheability bits at all.  It looks to
> > me like your patch ends up mapping the HPET into userspace writable.
> 
> I sure hope not.  If vm_page_prot was writable, something was already
> broken, because this is the vvar mapping, and the vvar mapping is
> VM_READ (and not even VM_MAYREAD).

I do beg yor pardon.  I thought you were inserting a readonly page
into the middle of a writable mapping.  Instead you're inserting a
non-executable page into the middle of a VM_READ | VM_EXEC mapping.
Sorry for the confusion.  I should have written:

"like your patch ends up mapping the HPET into userspace executable"

which is far less exciting.

> > I don't think the vm_insert_pfn_prot() call gets to change the memtype.
> > For one, that page may already be mapped into a differet userspace using
> > the pre-existing memtype, and [1] continues to bite you.  Then there
> > may be outstanding kernel users of the page that's being mapped in.
> 
> So why was remap_pfn_range different?  I'm sure there was a reason.

Yeah, doesn't make sense to me either.