Re: [PATCH] KVM: arm/arm64: Check pagesize when allocating a hugepage at Stage 2

[Christoffer Dall]

On Thu, Jan 11, 2018 at 3:23 PM, Punit Agrawal <punit.agrawal@xxxxxxx> wrote:
> Christoffer Dall <christoffer.dall@xxxxxxxxxx> writes:
>
>> On Thu, Jan 11, 2018 at 01:01:07PM +0000, Punit Agrawal wrote:
>>> Christoffer Dall <christoffer.dall@xxxxxxxxxx> writes:
>>>
>>> > On Thu, Jan 04, 2018 at 06:24:33PM +0000, Punit Agrawal wrote:
>>> >> KVM only supports PMD hugepages at stage 2 but doesn't actually check
>>> >> that the provided hugepage memory pagesize is PMD_SIZE before populating
>>> >> stage 2 entries.
>>> >>
>>> >> In cases where the backing hugepage size is smaller than PMD_SIZE (such
>>> >> as when using contiguous hugepages),
>>> >
>>> > what are contiguous hugepages and how are they created vs. a normal
>>> > hugetlbfs?  Is this a kernel config thing, or how does it work?
>>>
>>> Contiguous hugepages use the "Contiguous" bit (bit 52) in the page table
>>> entry (pte), to mark successive entries as forming a block mapping.
>>>
>>> The number of successive ptes that can be combined depend on the granule
>>> size. E.g., for 4KB granule, 16 last-level ptes can form a 64KB
>>> hugepage. or 16 adjacent PMD entries can form a 32MB hugepage.
>>>
>>> There's no difference in instantiating contiguous hugepages vs normal
>>> hugepages from a user's perspective other than passing in the
>>> appropriate hugepage size.
>>>
>>> There is no explicit config for contiguous hugepages - instead the
>>> architectural helper to setup "hugepagesz" (see setup_hugepagesz() in
>>> arch/arm64/mm/hugetlbpage.c") dictates the supported sizes.
>>>
>>> Contiguous hugepage support has been enabled/disabled a few times for
>>> arm64 - the latest of which is 5cd028b9d90403b ("arm64: Re-enable
>>> support for contiguous hugepages").
>>>
>>> >
>>> >> KVM can end up creating stage 2
>>> >> mappings that extend beyond the supplied memory.
>>> >>
>>> >> Fix this by checking for the pagesize of userspace vma before creating
>>> >> PMD hugepage at stage 2.
>>> >>
>>> >> Fixes: ad361f093c1e31d ("KVM: ARM: Support hugetlbfs backed huge pages")
>>> >> Signed-off-by: Punit Agrawal <punit.agrawal@xxxxxxx>
>>> >> Cc: Christoffer Dall <christoffer.dall@xxxxxxxxxx>
>>> >> Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
>>> >> ---
>>> >>  virt/kvm/arm/mmu.c | 2 +-
>>> >>  1 file changed, 1 insertion(+), 1 deletion(-)
>>> >>
>>> >> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
>>> >> index b4b69c2d1012..9dea96380339 100644
>>> >> --- a/virt/kvm/arm/mmu.c
>>> >> +++ b/virt/kvm/arm/mmu.c
>>> >> @@ -1310,7 +1310,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>> >>           return -EFAULT;
>>> >>   }
>>> >>
>>> >> - if (is_vm_hugetlb_page(vma) && !logging_active) {
>>> >> + if (vma_kernel_pagesize(vma) == PMD_SIZE && !logging_active) {
>>> >
>>> > Don't we need to also fix this in kvm_send_hwpoison_signal?
>>>
>>> I think we are OK here as the signal is delivered to userspace using the
>>> hva and the lsb_shift is derived from the vma as well, i.e., stage 2 is
>>> not involved here.
>>>
>>> Does that make sense?
>>>
>>
>> Yes, you're right.
>>
>>> >
>>> > (which probably implies this will then need a backport without that for
>>> > older stable kernels.  Has this been an issue from the start or did we
>>> > add contiguous hugepage support at some point?)
>>>
>>> I think kvm was missed out in the first (and subsequent) enabling of
>>> contiguous hugepage support. The functionality didn't start out broken
>>> initially.
>>>
>>> Note that applying the fix as far back as it applies isn't harmful
>>> though.
>>>
>>
>> It's a bit misleading to have the "Fixes: ad361f093c1e31d" tag, in that
>> it may have people running old kernels think this could be affecting
>> their workloads.  I know it's unlikely, but still.  Shouldn't the tag be
>> Fixes 66b3923a1a0f "arm64: hugetlb: add support for PTE contiguous bit"
>> ?
>>
>> That would make it a
>> Cc: <stable@xxxxxxxxxxxxxxx> # v4.5+
>>
>
> Agreed. Makes sense to go only as far back as it really matters.
>
> Can you fix it up when applying? Or I can send a patch with an update as
> well.
>

I'll fix it up.

Thanks,
-Christoffer