Re: [PATCH 1/3] x86: Honour passed pgprot in track_pfn_insert() and track_pfn_remap()

[Andy Lutomirski]

On Tue, Jan 26, 2016 at 8:40 PM, Matthew Wilcox <willy@xxxxxxxxxxxxxxx> 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@xxxxxxxxx> wrote:
>> > From: Matthew Wilcox <willy@xxxxxxxxxxxxxxx>
>> >
>> > 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 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.

I don't think that whatever_pfn_prot should ever map a page
inconsistently, but I find it surprising that some of the variants
call reserve_memtype to change the memtype and others don't.

Anyway, this is in no way an objection to your patches.

--Andy