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

[Ashish Kalra <ashish.kalra@amd.com>]

On Fri, Apr 03, 2020 at 03:55:07PM -0500, Venu Busireddy wrote:
> On 2020-04-03 13:18:52 -0700, Krish Sadhukhan wrote:
> > 
> > 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 ?
> 
> That would need the interface of .get_page_enc_bitmap to change, to pass
> back the local bitmap to the caller for use in copy_to_user() and then
> free it up. I think it is better to call copy_to_user() here and free
> the bitmap before returning.
> 

As i replied in my earlier response to this patch, please note that
as per comments above, here we are checking if bmap->num_pages is not byte
aligned and if not then we read the trailing bits from the userspace and copy
those bits as is.

Thanks,
Ashish

> > 
> > > +		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 */