Re: [PATCH v5 12/13] KVM: Expose KVM_MEM_PRIVATE

[Sean Christopherson <seanjc@google.com>]

On Thu, Mar 10, 2022, Chao Peng wrote:
> KVM_MEM_PRIVATE is not exposed by default but architecture code can turn
> on it by implementing kvm_arch_private_memory_supported().
> 
> Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
> Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> ---
>  include/linux/kvm_host.h |  1 +
>  virt/kvm/kvm_main.c      | 24 +++++++++++++++++++-----
>  2 files changed, 20 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 186b9b981a65..0150e952a131 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -1432,6 +1432,7 @@ bool kvm_arch_dy_has_pending_interrupt(struct kvm_vcpu *vcpu);
>  int kvm_arch_post_init_vm(struct kvm *kvm);
>  void kvm_arch_pre_destroy_vm(struct kvm *kvm);
>  int kvm_arch_create_vm_debugfs(struct kvm *kvm);
> +bool kvm_arch_private_memory_supported(struct kvm *kvm);
>  
>  #ifndef __KVM_HAVE_ARCH_VM_ALLOC
>  /*
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 52319f49d58a..df5311755a40 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1485,10 +1485,19 @@ static void kvm_replace_memslot(struct kvm *kvm,
>  	}
>  }
>  
> -static int check_memory_region_flags(const struct kvm_userspace_memory_region *mem)
> +bool __weak kvm_arch_private_memory_supported(struct kvm *kvm)
> +{
> +	return false;
> +}
> +
> +static int check_memory_region_flags(struct kvm *kvm,
> +				const struct kvm_userspace_memory_region *mem)
>  {
>  	u32 valid_flags = KVM_MEM_LOG_DIRTY_PAGES;
>  
> +	if (kvm_arch_private_memory_supported(kvm))
> +		valid_flags |= KVM_MEM_PRIVATE;
> +
>  #ifdef __KVM_HAVE_READONLY_MEM
>  	valid_flags |= KVM_MEM_READONLY;
>  #endif
> @@ -1900,7 +1909,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
>  	int as_id, id;
>  	int r;
>  
> -	r = check_memory_region_flags(mem);
> +	r = check_memory_region_flags(kvm, mem);
>  	if (r)
>  		return r;
>  
> @@ -1913,10 +1922,12 @@ int __kvm_set_memory_region(struct kvm *kvm,
>  		return -EINVAL;
>  	if (mem->guest_phys_addr & (PAGE_SIZE - 1))
>  		return -EINVAL;
> -	/* We can read the guest memory with __xxx_user() later on. */
>  	if ((mem->userspace_addr & (PAGE_SIZE - 1)) ||
> -	    (mem->userspace_addr != untagged_addr(mem->userspace_addr)) ||
> -	     !access_ok((void __user *)(unsigned long)mem->userspace_addr,
> +	    (mem->userspace_addr != untagged_addr(mem->userspace_addr)))
> +		return -EINVAL;
> +	/* We can read the guest memory with __xxx_user() later on. */
> +	if (!(mem->flags & KVM_MEM_PRIVATE) &&
> +	    !access_ok((void __user *)(unsigned long)mem->userspace_addr,

This should sanity check private_offset for private memslots.  At a bare minimum,
wrapping should be disallowed.

>  			mem->memory_size))
>  		return -EINVAL;
>  	if (as_id >= KVM_ADDRESS_SPACE_NUM || id >= KVM_MEM_SLOTS_NUM)
> @@ -1957,6 +1968,9 @@ int __kvm_set_memory_region(struct kvm *kvm,
>  		if ((kvm->nr_memslot_pages + npages) < kvm->nr_memslot_pages)
>  			return -EINVAL;
>  	} else { /* Modify an existing slot. */
> +		/* Private memslots are immutable, they can only be deleted. */
> +		if (mem->flags & KVM_MEM_PRIVATE)
> +			return -EINVAL;

These sanity checks belong in "KVM: Register private memslot to memory backing store",
e.g. that patch is "broken" without the immutability restriction.  It's somewhat moot
because the code is unreachable, but it makes reviewing confusing/difficult.

But rather than move the sanity checks back, I think I'd prefer to pull all of patch 10
here.  I think it also makes sense to drop "KVM: Use memfile_pfn_ops to obtain pfn for
private pages" and add the pointer in "struct kvm_memory_slot" in patch "KVM: Extend the
memslot to support fd-based private memory", with the use of the ops folded into
"KVM: Handle page fault for private memory".  Adding code to KVM and KVM-x86 in a single
patch is ok, and overall makes things easier to review because the new helpers have a
user right away, especially since there will be #ifdeffery.

I.e. end up with something like:

  mm: Introduce memfile_notifier
  mm/shmem: Restrict MFD_INACCESSIBLE memory against RLIMIT_MEMLOCK
  KVM: Extend the memslot to support fd-based private memory
  KVM: Use kvm_userspace_memory_region_ext
  KVM: Add KVM_EXIT_MEMORY_ERROR exit
  KVM: Handle page fault for private memory
  KVM: Register private memslot to memory backing store
  KVM: Zap existing KVM mappings when pages changed in the private fd
  KVM: Enable and expose KVM_MEM_PRIVATE

>  		if ((mem->userspace_addr != old->userspace_addr) ||
>  		    (npages != old->npages) ||
>  		    ((mem->flags ^ old->flags) & KVM_MEM_READONLY))
> -- 
> 2.17.1
>