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

[Keqian Zhu]

Hi Santosh,

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.


BRs,
Keqian


> 
> 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@xxxxxxxxxx>
> ---
>  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)
>