Re: [RFC PATCH v2 32/32] x86: kvm: Pin the guest memory when SEV is active

[Paolo Bonzini <pbonzini@redhat.com>]

On 02/03/2017 16:18, Brijesh Singh wrote:
> The SEV memory encryption engine uses a tweak such that two identical
> plaintexts at different location will have a different ciphertexts.
> So swapping or moving ciphertexts of two pages will not result in
> plaintexts being swapped. Relocating (or migrating) a physical backing pages
> for SEV guest will require some additional steps. The current SEV key
> management spec [1] does not provide commands to swap or migrate (move)
> ciphertexts. For now we pin the memory allocated for the SEV guest. In
> future when SEV key management spec provides the commands to support the
> page migration we can update the KVM code to remove the pinning logical
> without making any changes into userspace (qemu).
> 
> The patch pins userspace memory when a new slot is created and unpin the
> memory when slot is removed.
> 
> [1] http://support.amd.com/TechDocs/55766_SEV-KM%20API_Spec.pdf

This is not enough, because memory can be hidden temporarily from the
guest and remapped later.  Think of a PCI BAR that is backed by RAM, or
also SMRAM.  The pinning must be kept even in that case.

You need to add a pair of KVM_MEMORY_ENCRYPT_OPs (one that doesn't map
to a PSP operation), such as KVM_REGISTER/UNREGISTER_ENCRYPTED_RAM.  In
QEMU you can use a RAMBlockNotifier to invoke the ioctls.

Paolo

> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> ---
>  arch/x86/include/asm/kvm_host.h |    6 +++
>  arch/x86/kvm/svm.c              |   93 +++++++++++++++++++++++++++++++++++++++
>  arch/x86/kvm/x86.c              |    3 +
>  3 files changed, 102 insertions(+)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index fcc4710..9dc59f0 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -723,6 +723,7 @@ struct kvm_sev_info {
>  	unsigned int handle;	/* firmware handle */
>  	unsigned int asid;	/* asid for this guest */
>  	int sev_fd;		/* SEV device fd */
> +	struct list_head pinned_memory_slot;
>  };
>  
>  struct kvm_arch {
> @@ -1043,6 +1044,11 @@ struct kvm_x86_ops {
>  	void (*setup_mce)(struct kvm_vcpu *vcpu);
>  
>  	int (*memory_encryption_op)(struct kvm *kvm, void __user *argp);
> +
> +	void (*prepare_memory_region)(struct kvm *kvm,
> +			struct kvm_memory_slot *memslot,
> +			const struct kvm_userspace_memory_region *mem,
> +			enum kvm_mr_change change);
>  };
>  
>  struct kvm_arch_async_pf {
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 13996d6..ab973f9 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -498,12 +498,21 @@ static inline bool gif_set(struct vcpu_svm *svm)
>  }
>  
>  /* Secure Encrypted Virtualization */
> +struct kvm_sev_pinned_memory_slot {
> +	struct list_head list;
> +	unsigned long npages;
> +	struct page **pages;
> +	unsigned long userspace_addr;
> +	short id;
> +};
> +
>  static unsigned int max_sev_asid;
>  static unsigned long *sev_asid_bitmap;
>  static void sev_deactivate_handle(struct kvm *kvm);
>  static void sev_decommission_handle(struct kvm *kvm);
>  static int sev_asid_new(void);
>  static void sev_asid_free(int asid);
> +static void sev_unpin_memory(struct page **pages, unsigned long npages);
>  #define __sev_page_pa(x) ((page_to_pfn(x) << PAGE_SHIFT) | sme_me_mask)
>  
>  static bool kvm_sev_enabled(void)
> @@ -1544,9 +1553,25 @@ static inline int avic_free_vm_id(int id)
>  
>  static void sev_vm_destroy(struct kvm *kvm)
>  {
> +	struct list_head *pos, *q;
> +	struct kvm_sev_pinned_memory_slot *pinned_slot;
> +	struct list_head *head = &kvm->arch.sev_info.pinned_memory_slot;
> +
>  	if (!sev_guest(kvm))
>  		return;
>  
> +	/* if guest memory is pinned then unpin it now */
> +	if (!list_empty(head)) {
> +		list_for_each_safe(pos, q, head) {
> +			pinned_slot = list_entry(pos,
> +				struct kvm_sev_pinned_memory_slot, list);
> +			sev_unpin_memory(pinned_slot->pages,
> +					pinned_slot->npages);
> +			list_del(pos);
> +			kfree(pinned_slot);
> +		}
> +	}
> +
>  	/* release the firmware resources */
>  	sev_deactivate_handle(kvm);
>  	sev_decommission_handle(kvm);
> @@ -5663,6 +5688,8 @@ static int sev_pre_start(struct kvm *kvm, int *asid)
>  		}
>  		*asid = ret;
>  		ret = 0;
> +
> +		INIT_LIST_HEAD(&kvm->arch.sev_info.pinned_memory_slot);
>  	}
>  
>  	return ret;
> @@ -6189,6 +6216,71 @@ static int sev_launch_measure(struct kvm *kvm, struct kvm_sev_cmd *argp)
>  	return ret;
>  }
>  
> +static struct kvm_sev_pinned_memory_slot *sev_find_pinned_memory_slot(
> +		struct kvm *kvm, struct kvm_memory_slot *slot)
> +{
> +	struct kvm_sev_pinned_memory_slot *i;
> +	struct list_head *head = &kvm->arch.sev_info.pinned_memory_slot;
> +
> +	list_for_each_entry(i, head, list) {
> +		if (i->userspace_addr == slot->userspace_addr &&
> +			i->id == slot->id)
> +			return i;
> +	}
> +
> +	return NULL;
> +}
> +
> +static void amd_prepare_memory_region(struct kvm *kvm,
> +				struct kvm_memory_slot *memslot,
> +				const struct kvm_userspace_memory_region *mem,
> +				enum kvm_mr_change change)
> +{
> +	struct kvm_sev_pinned_memory_slot *pinned_slot;
> +	struct list_head *head = &kvm->arch.sev_info.pinned_memory_slot;
> +
> +	mutex_lock(&kvm->lock);
> +
> +	if (!sev_guest(kvm))
> +		goto unlock;
> +
> +	if (change == KVM_MR_CREATE) {
> +
> +		if (!mem->memory_size)
> +			goto unlock;
> +
> +		pinned_slot = kmalloc(sizeof(*pinned_slot), GFP_KERNEL);
> +		if (pinned_slot == NULL)
> +			goto unlock;
> +
> +		pinned_slot->pages = sev_pin_memory(mem->userspace_addr,
> +				mem->memory_size, &pinned_slot->npages);
> +		if (pinned_slot->pages == NULL) {
> +			kfree(pinned_slot);
> +			goto unlock;
> +		}
> +
> +		sev_clflush_pages(pinned_slot->pages, pinned_slot->npages);
> +
> +		pinned_slot->id = memslot->id;
> +		pinned_slot->userspace_addr = mem->userspace_addr;
> +		list_add_tail(&pinned_slot->list, head);
> +
> +	} else if  (change == KVM_MR_DELETE) {
> +
> +		pinned_slot = sev_find_pinned_memory_slot(kvm, memslot);
> +		if (!pinned_slot)
> +			goto unlock;
> +
> +		sev_unpin_memory(pinned_slot->pages, pinned_slot->npages);
> +		list_del(&pinned_slot->list);
> +		kfree(pinned_slot);
> +	}
> +
> +unlock:
> +	mutex_unlock(&kvm->lock);
> +}
> +
>  static int amd_memory_encryption_cmd(struct kvm *kvm, void __user *argp)
>  {
>  	int r = -ENOTTY;
> @@ -6355,6 +6447,7 @@ static struct kvm_x86_ops svm_x86_ops __ro_after_init = {
>  	.update_pi_irte = svm_update_pi_irte,
>  
>  	.memory_encryption_op = amd_memory_encryption_cmd,
> +	.prepare_memory_region = amd_prepare_memory_region,
>  };
>  
>  static int __init svm_init(void)
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 6a737e9..e05069d 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8195,6 +8195,9 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
>  				const struct kvm_userspace_memory_region *mem,
>  				enum kvm_mr_change change)
>  {
> +	if (kvm_x86_ops->prepare_memory_region)
> +		kvm_x86_ops->prepare_memory_region(kvm, memslot, mem, change);
> +
>  	return 0;
>  }
>  
>