Re: [PATCH v2 1/1] KVM: arm64: Correctly handle the mmio faulting

[Gavin Shan <gshan@redhat.com>]

Hi Santosh,

On 10/26/20 10:24 PM, 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.
> 
> 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 set force_pte=true if kvm_is_device_pfn is true.
> 
> [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-5.10-rc1 tip: 3650b228
> 
> Fixes: 6d674e28 ("KVM: arm/arm64: Properly handle faulting of device mappings")
> Suggested-by: Marc Zyngier <maz@kernel.org>
> Signed-off-by: Santosh Shukla <sashukla@nvidia.com>
> ---
> v2:
> - Per Marc's suggestion - setting force_pte=true.
> - Rebased and tested for 5.10-rc1 commit: 3650b228
> 
> v1: https://lkml.org/lkml/2020/10/21/460
> 
> arch/arm64/kvm/mmu.c | 1 +
>   1 file changed, 1 insertion(+)
> 

Reviewed-by: Gavin Shan <gshan@redhat.com>

> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 19aacc7..d4cd253 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -839,6 +839,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>   
>   	if (kvm_is_device_pfn(pfn)) {
>   		device = true;
> +		force_pte = true;
>   	} else if (logging_active && !write_fault) {
>   		/*
>   		 * Only actually map the page as writable if this was a write
> 

Cheers,
Gavin