Subject: [PATCH v4] arm64: fix VTTBR_BADDR_MASK

From: [email-masked] (Joel Schopp)
Date: Tue, 19 Aug 2014 09:53:51 -0500

On 08/19/2014 09:37 AM, Christoffer Dall wrote:
> On Tue, Aug 19, 2014 at 09:05:09AM -0500, Joel Schopp wrote:
>> On 08/19/2014 07:22 AM, Christoffer Dall wrote:
>>> On Mon, Aug 18, 2014 at 03:30:58PM -0500, Joel Schopp wrote:
>>>>  #endif /* __ARM_KVM_MMU_H__ */
>>>> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
>>>> index 16e7994..70f0f02 100644
>>>> --- a/arch/arm/kvm/mmu.c
>>>> +++ b/arch/arm/kvm/mmu.c
>>>> @@ -521,6 +521,7 @@ int create_hyp_io_mappings(void *from, void *to, phys_addr_t phys_addr)
>>>>   */
>>>>  int kvm_alloc_stage2_pgd(struct kvm *kvm)
>>>>  {
>>>> +	unsigned int s2_pgds, s2_pgd_order;
>>>>  	pgd_t *pgd;
>>>>  
>>>>  	if (kvm->arch.pgd != NULL) {
>>>> @@ -528,10 +529,18 @@ int kvm_alloc_stage2_pgd(struct kvm *kvm)
>>>>  		return -EINVAL;
>>>>  	}
>>>>  
>>>> -	pgd = (pgd_t *)__get_free_pages(GFP_KERNEL, S2_PGD_ORDER);
>>>> +	s2_pgds = (1 << (kvm_get_phys_addr_shift() - PGDIR_SHIFT));
>>>> +	s2_pgd_order = get_order(s2_pgds * sizeof(pgd_t));
>>>> +
>>>> +	pgd = (pgd_t *)__get_free_pages(GFP_KERNEL, s2_pgd_order);
>>>>  	if (!pgd)
>>>>  		return -ENOMEM;
>>>>  
>>>> +	if ((unsigned long)pgd & ~vttbr_baddr_mask) {
>>>> +		kvm_err("Stage-2 pgd not correctly aligned: %p\n", pgd);
>>>> +		return -EFAULT;
>>>> +	}
>>>>
>>>>
>>>> There are two problems that I've found here.  The first problem is that
>>>> vttbr_baddr_mask isn't allocated yet at this point in the code.
>>> allocated? you mean assigned?
>>> aren't you setting vttbr_baddr_mask in kvm_arch_init()?  that's
>>> certainly called before kvm_arch_init_vm().
>> Yes, I mean assigned, at least I got the first letter correct :)  All I
>> know is that vttbr_baddr_mask was still zero and checking for zero and
>> calling the set function gave it a value.
> that sounds.... weird and wrong.  Hum.  Mind sticking a few prints in
> there and figuring out what's causing this?
>
>>>
>>>> The
>>>> second problem is that pgd is a virtual address, ie pgd ==
>>>> 0xfffffe03bbb40000 while the vttbr masks off the high bits for a
>>>> physical address, ie vttbr_baddr_mask=0x00007ffffffe0000 .  Even
>>>> correcting for those issues I haven't been able to make this check work
>>>> properly.  I'll resend v5 the patch with all the other suggested changes.
>>>>
>>> What are the issues that you face?  Iow. what is the alignment of the
>>> returned physical address?
>>>
>>> (You should be able to just to virt_to_phys(pgd) and use that to test
>>> for the vttbr_baddr_mask).
>> The addresses above are actually from my system, 64K page aligned on a
>> 64K page kernel.  I did use virt_to_phys() and the kernel got a null
>> dereference and paniced, I didn't trace down where the panic was occuring.
>>
> virt_to_phys() directly caused the null dereference?  That sounds bad!
I don't think it was the virt_to_phys() directly causing the null
dereference, but again I didn't trace it down.

>
> Would you mind trying to trace this down?  I'll be happy to provide as
> much help as I can along the way.
I can break the kvm_alloc_stage2_pgd check into a separate patch on top
of this one and circle back around to it after I finish another
unrelated thing I'm working on.