Re: [PATCH] KVM: arm64: Correctly handle the mmio faulting

[Gavin Shan <gshan@redhat.com>]

Hi Keqian and Santosh,

On 4/21/21 12:59 PM, Keqian Zhu wrote:
> On 2020/10/22 0:16, Santosh Shukla wrote:
>> The Commit:6d674e28 introduces a notion to detect and handle the
>> device mapping. The commit checks for the VM_PFNMAP flag is set
>> in vma->flags and if set then marks force_pte to true such that
>> if force_pte is true then ignore the THP function check
>> (/transparent_hugepage_adjust()).
>>
>> There could be an issue with the VM_PFNMAP flag setting and checking.
>> For example consider a case where the mdev vendor driver register's
>> the vma_fault handler named vma_mmio_fault(), which maps the
>> host MMIO region in-turn calls remap_pfn_range() and maps
>> the MMIO's vma space. Where, remap_pfn_range implicitly sets
>> the VM_PFNMAP flag into vma->flags.
> Could you give the name of the mdev vendor driver that triggers this issue?
> I failed to find one according to your description. Thanks.
> 

I think it would be fixed in driver side to set VM_PFNMAP in
its mmap() callback (call_mmap()), like vfio PCI driver does.
It means it won't be delayed until page fault is issued and
remap_pfn_range() is called. It's determined from the beginning
that the vma associated the mdev vendor driver is serving as
PFN remapping purpose. So the vma should be populated completely,
including the VM_PFNMAP flag before it becomes visible to user
space.

The example can be found from vfio driver in drivers/vfio/pci/vfio_pci.c:
     vfio_pci_mmap:       VM_PFNMAP is set for the vma
     vfio_pci_mmap_fault: remap_pfn_range() is called

Thanks,
Gavin

>>
>> Now lets assume a mmio fault handing flow where guest first access
>> the MMIO region whose 2nd stage translation is not present.
>> So that results to arm64-kvm hypervisor executing guest abort handler,
>> like below:
>>
>> kvm_handle_guest_abort() -->
>>   user_mem_abort()--> {
>>
>>      ...
>>      0. checks the vma->flags for the VM_PFNMAP.
>>      1. Since VM_PFNMAP flag is not yet set so force_pte _is_ false;
>>      2. gfn_to_pfn_prot() -->
>>          __gfn_to_pfn_memslot() -->
>>              fixup_user_fault() -->
>>                  handle_mm_fault()-->
>>                      __do_fault() -->
>>                         vma_mmio_fault() --> // vendor's mdev fault handler
>>                          remap_pfn_range()--> // Here sets the VM_PFNMAP
>> 						flag into vma->flags.
>>      3. Now that force_pte is set to false in step-2),
>>         will execute transparent_hugepage_adjust() func and
>>         that lead to Oops [4].
>>   }
>>
>> The proposition is to check is_iomap flag before executing the THP
>> function transparent_hugepage_adjust().
>>
>> [4] THP Oops:
>>> pc: kvm_is_transparent_hugepage+0x18/0xb0
>>> ...
>>> ...
>>> user_mem_abort+0x340/0x9b8
>>> kvm_handle_guest_abort+0x248/0x468
>>> handle_exit+0x150/0x1b0
>>> kvm_arch_vcpu_ioctl_run+0x4d4/0x778
>>> kvm_vcpu_ioctl+0x3c0/0x858
>>> ksys_ioctl+0x84/0xb8
>>> __arm64_sys_ioctl+0x28/0x38
>>
>> Tested on Huawei Kunpeng Taishan-200 arm64 server, Using VFIO-mdev device.
>> Linux tip: 583090b1
>>
>> Fixes: 6d674e28 ("KVM: arm/arm64: Properly handle faulting of device mappings")
>> Signed-off-by: Santosh Shukla <sashukla@nvidia.com>
>> ---
>>   arch/arm64/kvm/mmu.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
>> index 3d26b47..ff15357 100644
>> --- a/arch/arm64/kvm/mmu.c
>> +++ b/arch/arm64/kvm/mmu.c
>> @@ -1947,7 +1947,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>   	 * If we are not forced to use page mapping, check if we are
>>   	 * backed by a THP and thus use block mapping if possible.
>>   	 */
>> -	if (vma_pagesize == PAGE_SIZE && !force_pte)
>> +	if (vma_pagesize == PAGE_SIZE && !force_pte && !is_iomap(flags))
>>   		vma_pagesize = transparent_hugepage_adjust(memslot, hva,
>>   							   &pfn, &fault_ipa);
>>   	if (writable)
>>
> 

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