Re: [PATCH Part2 v6 09/49] x86/fault: Add support to handle the RMP fault for user address

Marc Orr
2022-09-06T13:54:44Z

On Tue, Sep 6, 2022 at 3:25 AM Jarkko Sakkinen <jarkko@kernel.org> wrote:
>
> On Tue, Aug 09, 2022 at 06:55:43PM +0200, Borislav Petkov wrote:
> > On Mon, Jun 20, 2022 at 11:03:43PM +0000, Ashish Kalra wrote:
> > > + pfn = pte_pfn(*pte);
> > > +
> > > + /* If its large page then calculte the fault pfn */
> > > + if (level > PG_LEVEL_4K) {
> > > + unsigned long mask;
> > > +
> > > + mask = pages_per_hpage(level) - pages_per_hpage(level - 1);
> > > + pfn |= (address >> PAGE_SHIFT) & mask;
> >
> > Oh boy, this is unnecessarily complicated. Isn't this
> >
> > pfn |= pud_index(address);
> >
> > or
> > pfn |= pmd_index(address);
>
> I played with this a bit and ended up with
>
> pfn = pte_pfn(*pte) | PFN_DOWN(address & page_level_mask(level - 1));
>
> Unless I got something terribly wrong, this should do the
> same (see the attached patch) as the existing calculations.

Actually, I don't think they're the same. I think Jarkko's version is
correct. Specifically:
- For level = PG_LEVEL_2M they're the same.
- For level = PG_LEVEL_1G:
The current code calculates a garbage mask:
mask = pages_per_hpage(level) - pages_per_hpage(level - 1);
translates to:
>>> hex(262144 - 512)
'0x3fe00'

But I believe Jarkko's version calculates the correct mask (below),
incorporating all 18 offset bits into the 1G page.
>>> hex(262144 -1)
'0x3ffff'