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

[Santosh Shukla]

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 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)
-- 
2.7.4