Re: [PATCH v6 09/14] KVM: x86: Introduce KVM_GET_PAGE_ENC_BITMAP ioctl

[Krish Sadhukhan <krish.sadhukhan@oracle.com>]

On 3/29/20 11:22 PM, Ashish Kalra wrote:
> From: Brijesh Singh <Brijesh.Singh@amd.com>
>
> The ioctl can be used to retrieve page encryption bitmap for a given
> gfn range.
>
> Return the correct bitmap as per the number of pages being requested
> by the user. Ensure that we only copy bmap->num_pages bytes in the
> userspace buffer, if bmap->num_pages is not byte aligned we read
> the trailing bits from the userspace and copy those bits as is.
>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: "Radim Krčmář" <rkrcmar@redhat.com>
> Cc: Joerg Roedel <joro@8bytes.org>
> Cc: Borislav Petkov <bp@suse.de>
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
> Cc: x86@kernel.org
> Cc: kvm@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
> ---
>   Documentation/virt/kvm/api.rst  | 27 +++++++++++++
>   arch/x86/include/asm/kvm_host.h |  2 +
>   arch/x86/kvm/svm.c              | 71 +++++++++++++++++++++++++++++++++
>   arch/x86/kvm/x86.c              | 12 ++++++
>   include/uapi/linux/kvm.h        | 12 ++++++
>   5 files changed, 124 insertions(+)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index ebd383fba939..8ad800ebb54f 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -4648,6 +4648,33 @@ This ioctl resets VCPU registers and control structures according to
>   the clear cpu reset definition in the POP. However, the cpu is not put
>   into ESA mode. This reset is a superset of the initial reset.
>   
> +4.125 KVM_GET_PAGE_ENC_BITMAP (vm ioctl)
> +---------------------------------------
> +
> +:Capability: basic
> +:Architectures: x86
> +:Type: vm ioctl
> +:Parameters: struct kvm_page_enc_bitmap (in/out)
> +:Returns: 0 on success, -1 on error
> +
> +/* for KVM_GET_PAGE_ENC_BITMAP */
> +struct kvm_page_enc_bitmap {
> +	__u64 start_gfn;
> +	__u64 num_pages;
> +	union {
> +		void __user *enc_bitmap; /* one bit per page */
> +		__u64 padding2;
> +	};
> +};
> +
> +The encrypted VMs have concept of private and shared pages. The private
> +page is encrypted with the guest-specific key, while shared page may
> +be encrypted with the hypervisor key. The KVM_GET_PAGE_ENC_BITMAP can
> +be used to get the bitmap indicating whether the guest page is private
> +or shared. The bitmap can be used during the guest migration, if the page
> +is private then userspace need to use SEV migration commands to transmit
> +the page.
> +
>   
>   5. The kvm_run structure
>   ========================
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 90718fa3db47..27e43e3ec9d8 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1269,6 +1269,8 @@ struct kvm_x86_ops {
>   	int (*enable_direct_tlbflush)(struct kvm_vcpu *vcpu);
>   	int (*page_enc_status_hc)(struct kvm *kvm, unsigned long gpa,
>   				  unsigned long sz, unsigned long mode);
> +	int (*get_page_enc_bitmap)(struct kvm *kvm,
> +				struct kvm_page_enc_bitmap *bmap);


Looking back at the previous patch, it seems that these two are 
basically the setter/getter action for page encryption, though one is 
implemented as a hypercall while the other as an ioctl. If we consider 
the setter/getter aspect, isn't it better to have some sort of symmetry 
in the naming of the ops ? For example,

         set_page_enc_hc

         get_page_enc_ioctl

>   };
>   
>   struct kvm_arch_async_pf {
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 1d8beaf1bceb..bae783cd396a 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -7686,6 +7686,76 @@ static int svm_page_enc_status_hc(struct kvm *kvm, unsigned long gpa,
>   	return ret;
>   }
>   
> +static int svm_get_page_enc_bitmap(struct kvm *kvm,
> +				   struct kvm_page_enc_bitmap *bmap)
> +{
> +	struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> +	unsigned long gfn_start, gfn_end;
> +	unsigned long sz, i, sz_bytes;
> +	unsigned long *bitmap;
> +	int ret, n;
> +
> +	if (!sev_guest(kvm))
> +		return -ENOTTY;
> +
> +	gfn_start = bmap->start_gfn;


What if bmap->start_gfn is junk ?

> +	gfn_end = gfn_start + bmap->num_pages;
> +
> +	sz = ALIGN(bmap->num_pages, BITS_PER_LONG) / BITS_PER_BYTE;
> +	bitmap = kmalloc(sz, GFP_KERNEL);
> +	if (!bitmap)
> +		return -ENOMEM;
> +
> +	/* by default all pages are marked encrypted */
> +	memset(bitmap, 0xff, sz);
> +
> +	mutex_lock(&kvm->lock);
> +	if (sev->page_enc_bmap) {
> +		i = gfn_start;
> +		for_each_clear_bit_from(i, sev->page_enc_bmap,
> +				      min(sev->page_enc_bmap_size, gfn_end))
> +			clear_bit(i - gfn_start, bitmap);
> +	}
> +	mutex_unlock(&kvm->lock);
> +
> +	ret = -EFAULT;
> +
> +	n = bmap->num_pages % BITS_PER_BYTE;
> +	sz_bytes = ALIGN(bmap->num_pages, BITS_PER_BYTE) / BITS_PER_BYTE;
> +
> +	/*
> +	 * Return the correct bitmap as per the number of pages being
> +	 * requested by the user. Ensure that we only copy bmap->num_pages
> +	 * bytes in the userspace buffer, if bmap->num_pages is not byte
> +	 * aligned we read the trailing bits from the userspace and copy
> +	 * those bits as is.
> +	 */
> +
> +	if (n) {


Is it better to check for 'num_pages' at the beginning of the function 
rather than coming this far if bmap->num_pages is zero ?

> +		unsigned char *bitmap_kernel = (unsigned char *)bitmap;


Just trying to understand why you need this extra variable instead of 
using 'bitmap' directly.

> +		unsigned char bitmap_user;
> +		unsigned long offset, mask;
> +
> +		offset = bmap->num_pages / BITS_PER_BYTE;
> +		if (copy_from_user(&bitmap_user, bmap->enc_bitmap + offset,
> +				sizeof(unsigned char)))
> +			goto out;
> +
> +		mask = GENMASK(n - 1, 0);
> +		bitmap_user &= ~mask;
> +		bitmap_kernel[offset] &= mask;
> +		bitmap_kernel[offset] |= bitmap_user;
> +	}
> +
> +	if (copy_to_user(bmap->enc_bitmap, bitmap, sz_bytes))


If 'n' is zero, we are still copying stuff back to the user. Is that 
what is expected from userland ?

Another point. Since copy_from_user() was done in the caller, isn't it 
better to move this to the caller to keep a symmetry ?

> +		goto out;
> +
> +	ret = 0;
> +out:
> +	kfree(bitmap);
> +	return ret;
> +}
> +
>   static int svm_mem_enc_op(struct kvm *kvm, void __user *argp)
>   {
>   	struct kvm_sev_cmd sev_cmd;
> @@ -8090,6 +8160,7 @@ static struct kvm_x86_ops svm_x86_ops __ro_after_init = {
>   	.apic_init_signal_blocked = svm_apic_init_signal_blocked,
>   
>   	.page_enc_status_hc = svm_page_enc_status_hc,
> +	.get_page_enc_bitmap = svm_get_page_enc_bitmap,
>   };
>   
>   static int __init svm_init(void)
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 68428eef2dde..3c3fea4e20b5 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5226,6 +5226,18 @@ long kvm_arch_vm_ioctl(struct file *filp,
>   	case KVM_SET_PMU_EVENT_FILTER:
>   		r = kvm_vm_ioctl_set_pmu_event_filter(kvm, argp);
>   		break;
> +	case KVM_GET_PAGE_ENC_BITMAP: {
> +		struct kvm_page_enc_bitmap bitmap;
> +
> +		r = -EFAULT;
> +		if (copy_from_user(&bitmap, argp, sizeof(bitmap)))
> +			goto out;
> +
> +		r = -ENOTTY;
> +		if (kvm_x86_ops->get_page_enc_bitmap)
> +			r = kvm_x86_ops->get_page_enc_bitmap(kvm, &bitmap);
> +		break;
> +	}
>   	default:
>   		r = -ENOTTY;
>   	}
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 4e80c57a3182..db1ebf85e177 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -500,6 +500,16 @@ struct kvm_dirty_log {
>   	};
>   };
>   
> +/* for KVM_GET_PAGE_ENC_BITMAP */
> +struct kvm_page_enc_bitmap {
> +	__u64 start_gfn;
> +	__u64 num_pages;
> +	union {
> +		void __user *enc_bitmap; /* one bit per page */
> +		__u64 padding2;
> +	};
> +};
> +
>   /* for KVM_CLEAR_DIRTY_LOG */
>   struct kvm_clear_dirty_log {
>   	__u32 slot;
> @@ -1478,6 +1488,8 @@ struct kvm_enc_region {
>   #define KVM_S390_NORMAL_RESET	_IO(KVMIO,   0xc3)
>   #define KVM_S390_CLEAR_RESET	_IO(KVMIO,   0xc4)
>   
> +#define KVM_GET_PAGE_ENC_BITMAP	_IOW(KVMIO, 0xc5, struct kvm_page_enc_bitmap)
> +
>   /* Secure Encrypted Virtualization command */
>   enum sev_cmd_id {
>   	/* Guest initialization commands */