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

[Gavin Shan]

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@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)