On 2013-12-13 01:36, Christoffer Dall wrote:
> On Thu, Dec 12, 2013 at 02:51:21PM +0000, Marc Zyngier wrote:
>> The THP code in KVM/ARM is a bit restrictive in not allowing a THP
>> to be used if the VMA is not 2MB aligned. Actually, it is not so
>> much
>> the VMA that matters, but the associated memslot:
>>
>> A process can perfectly mmap a region with no particular alignment
>> restriction, and then pass a 2MB aligned address to KVM. In this
>> case, KVM will only use this 2MB aligned region, and will ignore
>> the range between vma->vm_start and memslot->userspace_addr.
>>
>> The fix is then to check the alignment of memslot->userspace_addr.
>
> That's more correct, but I'm wondering if it's enough.
>
> What happens if the base_gfn is not aligned to a 2MB region, will we
> not
> be mapping something completely bogus here?
Indeed. So far, we haven't seen a stupid enough userspace, but I'm sure
it will happen.
I'll update this patch to also check for the base IPA of the memslot.
Thanks,
M.
>
>>
>> Cc: Christoffer Dall <christoffer.dall@xxxxxxxxxx>
>> Signed-off-by: Marc Zyngier <marc.zyngier@xxxxxxx>
>> ---
>> arch/arm/kvm/mmu.c | 4 ++--
>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
>> index 5809069..cec641a 100644
>> --- a/arch/arm/kvm/mmu.c
>> +++ b/arch/arm/kvm/mmu.c
>> @@ -667,14 +667,14 @@ static int user_mem_abort(struct kvm_vcpu
>> *vcpu, phys_addr_t fault_ipa,
>> gfn = (fault_ipa & PMD_MASK) >> PAGE_SHIFT;
>> } else {
>> /*
>> - * Pages belonging to VMAs not aligned to the PMD mapping
>> + * Pages belonging to memslots not aligned to the PMD mapping
>> * granularity cannot be mapped using block descriptors even
>> * if the pages belong to a THP for the process, because the
>> * stage-2 block descriptor will cover more than a single THP
>> * and we loose atomicity for unmapping, updates, and splits
>> * of the THP or other pages in the stage-2 block range.
>> */
>> - if (vma->vm_start & ~PMD_MASK)
>> + if (memslot->userspace_addr & ~PMD_MASK)
>> force_pte = true;
>> }
>> up_read(¤t->mm->mmap_sem);
>> --
>> 1.8.2.3
>>
>>
--
Fast, cheap, reliable. Pick two.
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