linux-kernel - Re: [PATCH v2 1/5] x86/mm/pageattr: Ensure cpa->pfn only contains page frame numbers

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Date:	Tue, 17 Nov 2015 09:50:36 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Borislav Petkov <bp@...e.de>
cc:	Matt Fleming <matt@...eblueprint.co.uk>,
	Ingo Molnar <mingo@...nel.org>,
	"H . Peter Anvin" <hpa@...or.com>, Toshi Kani <toshi.kani@...com>,
	linux-kernel@...r.kernel.org, linux-efi@...r.kernel.org,
	Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
	Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH v2 1/5] x86/mm/pageattr: Ensure cpa->pfn only contains
 page frame numbers

On Mon, 16 Nov 2015, Thomas Gleixner wrote:
> On Mon, 16 Nov 2015, Borislav Petkov wrote:
> > On Mon, Nov 16, 2015 at 09:19:01PM +0100, Thomas Gleixner wrote:
> > > On Sat, 14 Nov 2015, Matt Fleming wrote:
> > > > The x86 pageattr code is confused about the data that is stored
> > > > cpa->pfn, sometimes it's treated as a page frame number, sometimes
> > > > it's treated as an unshifted physical address, and in one place it's
> > > > treated as a pte.
> > > 
> > > Yuck.
> > 
> > This paragraph should read like this instead:
> > 
> > "Boris used cpa->pfn as a scratch variable to contain the physical
> > address. He realizes now that he should've added a separate
> > cpa_data.phys_addr then, instead of confusing everybody."
> > 
> > > > The result of this is that the mapping functions do not map the
> > > > intended physical address.
> > > > 
> > > > This isn't a problem in practice because most of the addresses we're
> > > > mapping in the EFI code paths are already mapped in 'trampoline_pgd'
> > > > and so the pageattr mappings functions don't actually do anything in
> > > > this case. But when we move to using a separate page table for the EFI
> > > > runtime this will be an issue.
> > > 
> > > Are you sure that this does not affect existing kernel versions?
> > 
> > Shouldn't because with this new patchset we're copying all the PGDs from
> > the kernel page table before doing an EFI call, see
> > efi_sync_low_kernel_mappings() in patch 5.
> > 
> > > >  	while (num_pages-- && start < end) {
> > > > -
> > > > -		/* deal with the NX bit */
> > > > -		if (!(pgprot_val(pgprot) & _PAGE_NX))
> > > > -			cpa->pfn &= ~_PAGE_NX;
> > > 
> > > That should be a seperate patch because this is just bogus code and
> > > has nothing to do with the pfn confusion.
> > 
> > Why bogus?
> 
> Even with cpa->pfn used as an address it cannot ever be set as the
> address is page aligned ....

Gah. Misread it. _PAGE_NX is bit 63 and it can be set when cpa->pfn is
abused as an address. So yes, it should go away with that patch.

Thanks,

	tglx
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