Re: [PATCH] KVM: arm/arm64: Revert to old way of checking for device mapping in stage2_flush_ptes().

[Christoffer Dall <christoffer.dall@linaro.org>]

On Wed, Dec 02, 2015 at 08:04:42PM +0100, Ard Biesheuvel wrote:
> On 2 December 2015 at 19:50, Christoffer Dall
> <christoffer.dall@linaro.org> wrote:
> > On Tue, Dec 01, 2015 at 04:03:52PM +0300, Pavel Fedin wrote:
> >> This function takes stage-II physical addresses (A.K.A. IPA), on input, not
> >> real physical addresses. This causes kvm_is_device_pfn() to return wrong
> >> values, depending on how much guest and host memory maps match. This
> >> results in completely broken KVM on some boards. The problem has been
> >> caught on Samsung proprietary hardware.
> >>
> >> Cc: stable@vger.kernel.org
> >
> > cc'ing stable doesn't make sense here as the bug was introduced in
> > v4.4-rc3 and we didn't release v4.4 yet...
> >
> >> Fixes: e6fab5442345 ("ARM/arm64: KVM: test properly for a PTE's uncachedness")
> >>
> >> Signed-off-by: Pavel Fedin <p.fedin@samsung.com>
> >> ---
> >>  arch/arm/kvm/mmu.c | 3 ++-
> >>  1 file changed, 2 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
> >> index 7dace90..51ad98f 100644
> >> --- a/arch/arm/kvm/mmu.c
> >> +++ b/arch/arm/kvm/mmu.c
> >> @@ -310,7 +310,8 @@ static void stage2_flush_ptes(struct kvm *kvm, pmd_t *pmd,
> >>
> >>       pte = pte_offset_kernel(pmd, addr);
> >>       do {
> >> -             if (!pte_none(*pte) && !kvm_is_device_pfn(__phys_to_pfn(addr)))
> >> +             if (!pte_none(*pte) &&
> >> +                 (pte_val(*pte) & PAGE_S2_DEVICE) != PAGE_S2_DEVICE)
> >>                       kvm_flush_dcache_pte(*pte);
> >>       } while (pte++, addr += PAGE_SIZE, addr != end);
> >>  }
> >
> > You are right that there was a bug in the fix, but your fix is not the
> > right one.
> >
> > Either we have to apply an actual mask and the compare against the value
> > (yes, I know, because of the UXN bit we get lucky so far, but that's too
> > brittle), or we should do a translation fo the gfn to a pfn.  Is there
> > anything preventing us to do the following?
> >
> > if (!pte_none(*pte) && !kvm_is_device_pfn(pte_pfn(*pte)))
> >
> 
> Yes, that looks better. I got confused by addr being a 'phys_addr_t'

Yeah, that's what I thought when I saw this.  Admittedly we could have a
typedef for the IPA, but oh well...

> but obviously, the address inside the PTE is the one we need to test
> for device-ness, so I think we should replace both instances with this
> 

care to send a patch by any chance?

-Christoffer