On Tue, Jan 26, 2016 at 8:40 PM, Matthew Wilcox [off-list ref] wrote:

On Mon, Jan 25, 2016 at 09:33:35AM -0800, Andy Lutomirski wrote:

quoted

On Mon, Jan 25, 2016 at 9:25 AM, Matthew Wilcox
[off-list ref] wrote:

quoted

From: Matthew Wilcox <redacted>

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

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