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    Subject[PATCH v2 1/1] KVM: arm64: Correctly handle the mmio faulting
    Date
    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(+)

    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
    --
    2.7.4
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