Re: [bug report] KVM: s390: vsie: support transactional execution

David Hildenbrand
2018-04-30T15:38:06Z

On 30.04.2018 16:57, David Hildenbrand wrote:
> On 30.04.2018 16:29, Cornelia Huck wrote:
>> On Mon, 30 Apr 2018 17:04:35 +0300
>> Dan Carpenter <dan.carpenter@oracle.com> wrote:
>>
>> [sent to David's current address]
>>
>>> [ Old warnings because this is non-x86 - dan ]
>>>
>>> Hello David Hildenbrand,
>>>
>>> The patch 166ecb3d3cfe: "KVM: s390: vsie: support transactional
>>> execution" from Nov 25, 2015, leads to the following static checker
>>> warning:
>>>
>>> arch/s390/kvm/vsie.c:581 pin_blocks()
>>> warn: was expecting a 64 bit value instead of '~8191'
>>>
>>> arch/s390/kvm/vsie.c
>>> 548 static int pin_blocks(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
>>> 549 {
>>> 550 struct kvm_s390_sie_block *scb_o = vsie_page->scb_o;
>>> 551 struct kvm_s390_sie_block *scb_s = &vsie_page->scb_s;
>>> 552 hpa_t hpa;
>>> 553 gpa_t gpa;
>>> ^^^^^
>>> gpa_t is a u64.
>>>
>>> 554 int rc = 0;
>>> 555
>>> 556 gpa = READ_ONCE(scb_o->scaol) & ~0xfUL;
>>> 557 if (test_kvm_cpu_feat(vcpu->kvm, KVM_S390_VM_CPU_FEAT_64BSCAO))
>>> 558 gpa |= (u64) READ_ONCE(scb_o->scaoh) << 32;
>>> 559 if (gpa) {
>>> 560 if (!(gpa & ~0x1fffUL))
>>> 561 rc = set_validity_icpt(scb_s, 0x0038U);
>>> 562 else if ((gpa & ~0x1fffUL) == kvm_s390_get_prefix(vcpu))
>>> 563 rc = set_validity_icpt(scb_s, 0x0011U);
>>> 564 else if ((gpa & PAGE_MASK) !=
>>> 565 ((gpa + sizeof(struct bsca_block) - 1) & PAGE_MASK))
>>> 566 rc = set_validity_icpt(scb_s, 0x003bU);
>>> 567 if (!rc) {
>>> 568 rc = pin_guest_page(vcpu->kvm, gpa, &hpa);
>>> 569 if (rc)
>>> 570 rc = set_validity_icpt(scb_s, 0x0034U);
>>> 571 }
>>> 572 if (rc)
>>> 573 goto unpin;
>>> 574 vsie_page->sca_gpa = gpa;
>>> 575 scb_s->scaoh = (u32)((u64)hpa >> 32);
>>> 576 scb_s->scaol = (u32)(u64)hpa;
>>> 577 }
>>> 578
>>> 579 gpa = READ_ONCE(scb_o->itdba) & ~0xffUL;
>>> ^^^^^
>>> Here it's UL.
>>>
>>> 580 if (gpa && (scb_s->ecb & ECB_TE)) {
>>> 581 if (!(gpa & ~0x1fffU)) {
>>> ^^^^^^^^^
>>> But here it's u32. So the high 32 bits are not considered. Possibly it
>>> doesn't matter?
>
> /me trying to remember what the young David wanted to achieve here
>
> (Christian, can you double check in the SIE documentation, I assume the
> last 13 bits of the address must not be set because of alignment, right?
> Unfortunately I don't remember)

Looking at it again, this check does not seem to do what I thought it
would do :) Alignment is handled already handled by the defined number
of bits (READ_ONCE(scb_o->itdba) & ~0xffUL).

So this is rather a check that the used address must not lie in the
lower 8k of memory. (low core for real addressing)

This could be triggered by a nested hypervisor but would not do any harm
to us: We simply pin the defined guest (nested hypervisor) address and
forward it.

It sure should be fixed to catch all error cases.

>
> 0x1fffU -> 0x0000000000001fffULL
> ~0x1fffU -> 0x00000000ffffe000ULL
>
> Now, if we have an address like 100001fff
>
> gpa = 100001fff
> !(gpa & ~0x1fffU) -> 1
> !(gpa & ~0x1fffUL) -> 0
>
> So we would not get equal results.
>
> Three years later, I would simply have written this as
> if (gpa & 0x1fffUL)
>
> Then, also the mussing "L" would not have mattered.
>
> But anyhow: While this looks like a BUG, I expect it to not be
> triggerable (is that a word?) because we have a safety net: We forward
> the offset in the page to the SIE. The SIE will perform the same check
> again on our calculated address. It will trigger a validity intercept
> for us that we silently forward.
>
> Dan, whatever tool you're using, nice work! Thanks a lot!
>
>
>>>
>>> 582 rc = set_validity_icpt(scb_s, 0x0080U);
>>> 583 goto unpin;
>>> 584 }
>>> 585 /* 256 bytes cannot cross page boundaries */
>>> 586 rc = pin_guest_page(vcpu->kvm, gpa, &hpa);
>>> 587 if (rc) {
>>> 588 rc = set_validity_icpt(scb_s, 0x0080U);
>>> 589 goto unpin;
>>> 590 }
>>> 591 vsie_page->itdba_gpa = gpa;
>>> 592 scb_s->itdba = hpa;
>>>
>>> regards,
>>> dan carpenter
>>
>
>

--

Thanks,

David / dhildenb