Re: [RFC Part2 PATCH v3 22/26] KVM: SVM: Pin guest memory when SEV is active

[Borislav Petkov]

On Mon, Jul 24, 2017 at 03:02:59PM -0500, Brijesh Singh wrote:
> The SEV memory encryption engine uses a tweak such that two identical
> plaintexts at different location will have a different ciphertexts.

plaintexts or plaintext pages?		also, s/a //

> So swapping or moving ciphertexts of two pages will not result in
> plaintexts being swapped. Relocating (or migrating) a physical backing

s/a //

> pages for SEV guest will require some additional steps. The current SEV

"for a SEV guest"

> key management spec does not provide commands to swap or migrate (move)
> ciphertexts. For now, we pin the guest memory registered through
> KVM_MEMORY_ENCRYPT_REGISTER_RAM ioctl.
> 
> Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h |   1 +
>  arch/x86/kvm/svm.c              | 113 ++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 114 insertions(+)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 150177e..a91aadf 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -747,6 +747,7 @@ 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 ram_list; /* list of registered ram */

regions_list I guess.

>  };
>  
>  struct kvm_arch {
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 75dcaa9..cdb1cf3 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -333,8 +333,19 @@ static int sev_asid_new(void);
>  static void sev_asid_free(int asid);
>  static void sev_deactivate_handle(struct kvm *kvm, int *error);
>  static void sev_decommission_handle(struct kvm *kvm, int *error);
> +static void sev_unpin_memory(struct page **pages, unsigned long npages);

Unneeded.

> +
>  #define __sme_page_pa(x) __sme_set(page_to_pfn(x) << PAGE_SHIFT)
>  
> +struct kvm_sev_pin_ram {

sev_pinned_region

> +	struct list_head list;
> +	unsigned long npages;
> +	struct page **pages;
> +	struct kvm_memory_encrypt_ram userspace;

That member would need a comment what it is.

> +};
> +
> +static void __mem_encrypt_unregister_ram(struct kvm_sev_pin_ram *ram);

Move code so that you don't need that one.

> +
>  static bool svm_sev_enabled(void)
>  {
>  	return !!max_sev_asid;
> @@ -385,6 +396,11 @@ static inline void sev_set_fd(struct kvm *kvm, int fd)
>  	to_sev_info(kvm)->sev_fd = fd;
>  }
>  
> +static inline struct list_head *sev_get_ram_list(struct kvm *kvm)
> +{
> +	return &to_sev_info(kvm)->ram_list;
> +}
> +
>  static inline void mark_all_dirty(struct vmcb *vmcb)
>  {
>  	vmcb->control.clean = 0;
> @@ -1566,10 +1582,24 @@ static void sev_firmware_uninit(void)
>  static void sev_vm_destroy(struct kvm *kvm)
>  {
>  	int state, error;
> +	struct list_head *pos, *q;
> +	struct kvm_sev_pin_ram *ram;
> +	struct list_head *head = sev_get_ram_list(kvm);

Please sort function local variables declaration in a reverse christmas
tree order:

	<type> longest_variable_name;
	<type> shorter_var_name;
	<type> even_shorter;
	<type> i;

>  
>  	if (!sev_guest(kvm))
>  		return;
>  
> +	/*
> +	 * if userspace was terminated before unregistering the memory region
> +	 * then lets unpin all the registered memory.
> +	 */
> +	if (!list_empty(head)) {
> +		list_for_each_safe(pos, q, head) {
> +			ram = list_entry(pos, struct kvm_sev_pin_ram, list);
> +			__mem_encrypt_unregister_ram(ram);

You don't need the local "ram" varible here:

			__mem_encrypt_unregister_ram(list_entry(pos, struct kvm_sev_pin_ram, list));

> +		}
> +	}
> +
>  	/* release the firmware resources for this guest */
>  	if (sev_get_handle(kvm)) {
>  		sev_deactivate_handle(kvm, &error);
> @@ -5640,6 +5670,7 @@ static int sev_guest_init(struct kvm *kvm, struct kvm_sev_cmd *argp)
>  	sev_set_active(kvm);
>  	sev_set_asid(kvm, asid);
>  	sev_set_fd(kvm, argp->sev_fd);
> +	INIT_LIST_HEAD(sev_get_ram_list(kvm));
>  	ret = 0;
>  e_err:
>  	fdput(f);
> @@ -6437,6 +6468,86 @@ static int svm_memory_encryption_op(struct kvm *kvm, void __user *argp)
>  	return r;
>  }
>  
> +static int mem_encrypt_register_ram(struct kvm *kvm,
> +				    struct kvm_memory_encrypt_ram *ram)
> +{

Please call that arg "regions" or so. "ram" is strange. In the other
functions too.

> +	struct list_head *head = sev_get_ram_list(kvm);
> +	struct kvm_sev_pin_ram *pin_ram;
> +
> +	if (!sev_guest(kvm))
> +		return -ENOTTY;
> +
> +	pin_ram = kzalloc(sizeof(*pin_ram), GFP_KERNEL);
> +	if (!pin_ram)
> +		return -ENOMEM;
> +
> +	pin_ram->pages = sev_pin_memory(ram->address, ram->size,
> +					&pin_ram->npages, 1);

Let it stick out.

> +	if (!pin_ram->pages)
> +		goto e_free;
> +
> +	/*
> +	 * Guest may change the memory encryption attribute from C=0 -> C=1

"The guest"

> +	 * for this memory range. Lets make sure caches are flushed to ensure
> +	 * that guest data gets written into memory with correct C-bit.
> +	 */
> +	sev_clflush_pages(pin_ram->pages, pin_ram->npages);
> +
> +	pin_ram->userspace.address = ram->address;
> +	pin_ram->userspace.size = ram->size;
> +	list_add_tail(&pin_ram->list, head);
> +	return 0;

<---- newline here.

> +e_free:
> +	kfree(pin_ram);
> +	return 1;
> +}
> +
> +static struct kvm_sev_pin_ram *sev_find_pinned_ram(struct kvm *kvm,
> +					struct kvm_memory_encrypt_ram *ram)

So this function signature is almost impossible to read: you have "kvm"
"sev" "pin" "ram" and those long structure names.

Now look how something like this:

static struct regions_list *
sev_find_pinned_memory(struct kvm *kvm, struct enc_range *range)

tells you exactly what the function does.

> +{
> +	struct list_head *head = sev_get_ram_list(kvm);
> +	struct kvm_sev_pin_ram *i;
> +
> +	list_for_each_entry(i, head, list) {
> +		if (i->userspace.address == ram->address &&
> +			i->userspace.size == ram->size)

		if (i->usr.addr == reg->addr &&
		    i->usr.size == reg->size)

reads much better to me.

> +			return i;
> +	}
> +
> +	return NULL;
> +}
> +
> +static void __mem_encrypt_unregister_ram(struct kvm_sev_pin_ram *ram)
> +{
> +	/*
> +	 * Guest may have changed the memory encryption attribute from

"The guest"

> +	 * C=0 -> C=1. Lets make sure caches are flushed to ensure in data

Both comments talk about the 0 -> 1 case for the C-bit. What about the
reverse: 1->0? Do we not flush there or we don't have cases where a
guest doesn't decrypt its memory?

> +	 * gets written into memory with correct C-bit.
> +	 */
> +	sev_clflush_pages(ram->pages, ram->npages);
> +
> +	sev_unpin_memory(ram->pages, ram->npages);
> +	list_del(&ram->list);
> +	kfree(ram);
> +}
> +
> +static int mem_encrypt_unregister_ram(struct kvm *kvm,
> +				      struct kvm_memory_encrypt_ram *ram)
> +{
> +	struct kvm_sev_pin_ram *pinned_ram;
> +
> +	if (!sev_guest(kvm))
> +		return -ENOTTY;
> +
> +	pinned_ram = sev_find_pinned_ram(kvm, ram);
> +	if (!pinned_ram)
> +		return -EINVAL;
> +
> +	__mem_encrypt_unregister_ram(pinned_ram);
> +
> +	return 0;
> +}
> +
>  static struct kvm_x86_ops svm_x86_ops __ro_after_init = {
>  	.cpu_has_kvm_support = has_svm,
>  	.disabled_by_bios = is_disabled,
> @@ -6551,6 +6662,8 @@ static struct kvm_x86_ops svm_x86_ops __ro_after_init = {
>  	.setup_mce = svm_setup_mce,
>  
>  	.memory_encryption_op = svm_memory_encryption_op,
> +	.memory_encryption_register_ram = mem_encrypt_register_ram,
> +	.memory_encryption_unregister_ram = mem_encrypt_unregister_ram,

Names are too long. mem_encrypt_reg_memory or so I guess. In general,
choose a prefix and stick with it. mem_enc, mem_encrypt, mem_crypt...

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix ImendÃrffer, Jane Smithard, Graham Norton, HRB 21284 (AG NÃrnberg)
--