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SubjectRe: [PATCH v6 09/14] KVM: x86: Introduce KVM_GET_PAGE_ENC_BITMAP ioctl
On Fri, Apr 03, 2020 at 01:18:52PM -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
>
> > };

These are named as per their usage. While the page_enc_status_hc is a
hypercall used by a guest to mark the page encryption bitmap, the other
ones are ioctl interfaces used by Qemu (or Qemu alternative) to get/set
the page encryption bitmaps, so these are named accordingly.

> > 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 ?
>

This is not checking for "num_pages", this is basically checking if
bmap->num_pages is not byte aligned.

> > + unsigned char *bitmap_kernel = (unsigned char *)bitmap;
>
>
> Just trying to understand why you need this extra variable instead of using
> 'bitmap' directly.
>

Makes the code much more readable/understandable.

> > + 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 ?
>

As per the comments above, please note if n is not zero that means
bmap->num_pages is not byte aligned so we read the trailing bits
from the userspace and copy those bits as is. If n is zero, then
bmap->num_pages is correctly aligned and we copy all the bytes back.

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 */

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