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

From: Marc Zyngier <[email-masked]>
Date: Fri, 23 Oct 2020 12:29:28 +0100

Hi Santosh,

Thanks for this.

On 2020-10-21 17:16, 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.
> 
> 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].
>  }

Hmmm. Nice. Any chance you could provide us with an actual reproducer?

> 
> 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 <[email-masked]>
> ---
>  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)

Why don't you directly set force_pte to true at the point where we 
update
the flags? It certainly would be a bit more readable:

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 3d26b47a1343..7a4ad984d54e 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1920,6 +1920,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, 
phys_addr_t fault_ipa,
  	if (kvm_is_device_pfn(pfn)) {
  		mem_type = PAGE_S2_DEVICE;
  		flags |= KVM_S2PTE_FLAG_IS_IOMAP;
+		force_pte = true;
  	} else if (logging_active) {
  		/*
  		 * Faults on pages in a memslot with logging enabled

and almost directly applies to what we have queued for 5.10.

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...

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