Re: [PATCH v10 08/16] KVM: X86: Introduce KVM_HC_PAGE_ENC_STATUS hypercall

From: Ashish Kalra
Date: Thu Feb 04 2021 - 22:33:16 EST

Hello Steve,

On Thu, Feb 04, 2021 at 05:44:27PM -0800, Steve Rutherford wrote:
> On Wed, Feb 3, 2021 at 4:38 PM Ashish Kalra <Ashish.Kalra@xxxxxxx> wrote:
> >
> > From: Brijesh Singh <brijesh.singh@xxxxxxx>
> >
> > This hypercall is used by the SEV guest to notify a change in the page
> > encryption status to the hypervisor. The hypercall should be invoked
> > only when the encryption attribute is changed from encrypted -> decrypted
> > and vice versa. By default all guest pages are considered encrypted.
> >
> > The patch introduces a new shared pages list implemented as a
> > sorted linked list to track the shared/unencrypted regions marked by the
> > guest hypercall.
> >
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > Cc: "Radim Krčmář" <rkrcmar@xxxxxxxxxx>
> > Cc: Joerg Roedel <joro@xxxxxxxxxx>
> > Cc: Borislav Petkov <bp@xxxxxxx>
> > Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
> > Cc: x86@xxxxxxxxxx
> > Cc: kvm@xxxxxxxxxxxxxxx
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> > Co-developed-by: Ashish Kalra <ashish.kalra@xxxxxxx>
> > Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
> > ---
> > Documentation/virt/kvm/hypercalls.rst | 15 +++
> > arch/x86/include/asm/kvm_host.h | 2 +
> > arch/x86/kvm/svm/sev.c | 150 ++++++++++++++++++++++++++
> > arch/x86/kvm/svm/svm.c | 2 +
> > arch/x86/kvm/svm/svm.h | 5 +
> > arch/x86/kvm/vmx/vmx.c | 1 +
> > arch/x86/kvm/x86.c | 6 ++
> > include/uapi/linux/kvm_para.h | 1 +
> > 8 files changed, 182 insertions(+)
> >
> > diff --git a/Documentation/virt/kvm/hypercalls.rst b/Documentation/virt/kvm/hypercalls.rst
> > index ed4fddd364ea..7aff0cebab7c 100644
> > --- a/Documentation/virt/kvm/hypercalls.rst
> > +++ b/Documentation/virt/kvm/hypercalls.rst
> > @@ -169,3 +169,18 @@ a0: destination APIC ID
> >
> > :Usage example: When sending a call-function IPI-many to vCPUs, yield if
> > any of the IPI target vCPUs was preempted.
> > +
> > +
> > +8. KVM_HC_PAGE_ENC_STATUS
> > +-------------------------
> > +:Architecture: x86
> > +:Status: active
> > +:Purpose: Notify the encryption status changes in guest page table (SEV guest)
> > +
> > +a0: the guest physical address of the start page
> > +a1: the number of pages
> > +a2: encryption attribute
> > +
> > + Where:
> > + * 1: Encryption attribute is set
> > + * 0: Encryption attribute is cleared
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 3d6616f6f6ef..2da5f5e2a10e 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -1301,6 +1301,8 @@ struct kvm_x86_ops {
> > int (*complete_emulated_msr)(struct kvm_vcpu *vcpu, int err);
> >
> > void (*vcpu_deliver_sipi_vector)(struct kvm_vcpu *vcpu, u8 vector);
> > + int (*page_enc_status_hc)(struct kvm *kvm, unsigned long gpa,
> > + unsigned long sz, unsigned long mode);
> > };
> >
> > struct kvm_x86_nested_ops {
> > diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> > index 25eaf35ba51d..55c628df5155 100644
> > --- a/arch/x86/kvm/svm/sev.c
> > +++ b/arch/x86/kvm/svm/sev.c
> > @@ -45,6 +45,11 @@ struct enc_region {
> > unsigned long size;
> > };
> >
> > +struct shared_region {
> > + struct list_head list;
> > + unsigned long gfn_start, gfn_end;
> > +};
> > +
> > static int sev_flush_asids(void)
> > {
> > int ret, error = 0;
> > @@ -196,6 +201,8 @@ static int sev_guest_init(struct kvm *kvm, struct kvm_sev_cmd *argp)
> > sev->active = true;
> > sev->asid = asid;
> > INIT_LIST_HEAD(&sev->regions_list);
> > + INIT_LIST_HEAD(&sev->shared_pages_list);
> > + sev->shared_pages_list_count = 0;
> >
> > return 0;
> >
> > @@ -1473,6 +1480,148 @@ static int sev_receive_finish(struct kvm *kvm, struct kvm_sev_cmd *argp)
> > return ret;
> > }
> >
> > +static int remove_shared_region(unsigned long start, unsigned long end,
> > + struct list_head *head)
> > +{
> > + struct shared_region *pos;
> > +
> > + list_for_each_entry(pos, head, list) {
> > + if (pos->gfn_start == start &&
> > + pos->gfn_end == end) {
> > + list_del(&pos->list);
> > + kfree(pos);
> > + return -1;
> > + } else if (start >= pos->gfn_start && end <= pos->gfn_end) {
> > + if (start == pos->gfn_start)
> > + pos->gfn_start = end + 1;
> > + else if (end == pos->gfn_end)
> > + pos->gfn_end = start - 1;
> > + else {
> > + /* Do a de-merge -- split linked list nodes */
> > + unsigned long tmp;
> > + struct shared_region *shrd_region;
> > +
> > + tmp = pos->gfn_end;
> > + pos->gfn_end = start-1;
> > + shrd_region = kzalloc(sizeof(*shrd_region), GFP_KERNEL_ACCOUNT);
> > + if (!shrd_region)
> > + return -ENOMEM;
> > + shrd_region->gfn_start = end + 1;
> > + shrd_region->gfn_end = tmp;
> > + list_add(&shrd_region->list, &pos->list);
> > + return 1;
> > + }
> > + return 0;
> > + }
> > + }
>
> This doesn't handle the case where the region being marked as
> encrypted is larger than than the unencrypted region under
> consideration, which (I believe) can happen with the current kexec
> handling (since it is oblivious to the prior state).
> I would probably break this down into the "five cases": no
> intersection (skip), entry is completely contained (drop), entry
> completely contains the removed region (split), intersect start
> (chop), and intersect end (chop).
>

I believe that the above is already handling these cases :

1). no intersection (skip) : handled
2). entry is completely contained (drop) : handled
3). entry completely contains the removed region (split) : handled
4). intersect start in case of #3 (chop) : handled
5). intersect end in case of #3 (chop) : handled.

The one case it does not handle currently is where the region being marked
as encrypted is larger than the unencrypted region under consideration.

> >
> > + return 0;
> > +}
> > +
> > +static int add_shared_region(unsigned long start, unsigned long end,
> > + struct list_head *shared_pages_list)
> > +{
> > + struct list_head *head = shared_pages_list;
> > + struct shared_region *shrd_region;
> > + struct shared_region *pos;
> > +
> > + if (list_empty(head)) { > > + shrd_region = kzalloc(sizeof(*shrd_region), GFP_KERNEL_ACCOUNT);
> > + if (!shrd_region)
> > + return -ENOMEM;
> > + shrd_region->gfn_start = start;
> > + shrd_region->gfn_end = end;
> > + list_add_tail(&shrd_region->list, head);
> > + return 1;
> > + }
> > +
> > + /*
> > + * Shared pages list is a sorted list in ascending order of
> > + * guest PA's and also merges consecutive range of guest PA's
> > + */
> > + list_for_each_entry(pos, head, list) {
> > + if (pos->gfn_end < start)
> > + continue;
> > + /* merge consecutive guest PA(s) */
> > + if (pos->gfn_start <= start && pos->gfn_end >= start) {
> > + pos->gfn_end = end;
>
> I'm not sure this correctly avoids having duplicate overlapping
> elements in the list. It also doesn't merge consecutive contiguous
> regions. Current guest implementation should never call the hypercall
> with C=0 for the same region twice, without calling with c=1 in
> between, but this API should be compatible with that model.

Yes it does not handle duplicate overlapping elements being added in
the list, i will fix that.

And merging consecutive contigious regions is supported,
the code is merging consecutive GPAs as the example below shows :

...
[ 50.894254] marking as unencrypted, gfn_start = c0000, gfn_end = fc000
[ 50.894255] inserting new node @guest PA = c0000
[ 50.894259] marking as unencrypted, gfn_start = fc000, gfn_end = fec00
[ 50.894260] merging consecutive GPA start = c0000, end = fc000
[ 50.894267] marking as unencrypted, gfn_start = fec00, gfn_end = fec01
[ 50.894267] merging consecutive GPA start = c0000, end = fec00
[ 50.894274] marking as unencrypted, gfn_start = fec01, gfn_end = fed00
[ 50.894274] merging consecutive GPA start = c0000, end = fec01
[ 50.894278] marking as unencrypted, gfn_start = fed00, gfn_end = fed01
[ 50.894279] merging consecutive GPA start = c0000, end = fed00
[ 50.894283] marking as unencrypted, gfn_start = fed00, gfn_end = fed1c
[ 50.894283] merging consecutive GPA start = c0000, end = fed01
[ 50.894287] marking as unencrypted, gfn_start = fed1c, gfn_end = fed20
[ 50.894288] merging consecutive GPA start = c0000, end = fed1c
[ 50.894294] marking as unencrypted, gfn_start = fed20, gfn_end = fee00
[ 50.894294] merging consecutive GPA start = c0000, end = fed20
[ 50.894303] marking as unencrypted, gfn_start = fee00, gfn_end = fef00
[ 50.894304] merging consecutive GPA start = c0000, end = fee00
[ 50.894669] marking as unencrypted, gfn_start = fef00, gfn_end = 1000000
...

The above is merged into a single entry, as shown below:

qemu-system-x86_64: gfn start = c0000, gfn_end = 1000000

>
> The easiest pattern would probably be to:
> 1) find (or insert) the node that will contain the added region.

Except the find part, the above is handled.

> 2) remove the contents of the added region from the tail (will
> typically do nothing).
> 3) merge the head of the tail into the current node, if the end of the
> current node matches the start of that head.

This is also being handled.

Thanks,
Ashish

> >
> > + return 0;
> > + }
> > + break;
> > + }
> >
> > + /*
> > + * Add a new node, allocate nodes using GFP_KERNEL_ACCOUNT so that
> > + * kernel memory can be tracked/throttled in case a
> > + * malicious guest makes infinite number of hypercalls to
> > + * exhaust host kernel memory and cause a DOS attack.
> > + */
> > + shrd_region = kzalloc(sizeof(*shrd_region), GFP_KERNEL_ACCOUNT);
> > + if (!shrd_region)
> > + return -ENOMEM;
> > + shrd_region->gfn_start = start;
> > + shrd_region->gfn_end = end;
> > + list_add_tail(&shrd_region->list, &pos->list);
> > + return 1;
> >
> > +}
> > +
>
> Thanks!
> Steve