[PATCH v15 13/41] KVM: x86: Enable guest SSP read/write interface with new uAPIs

Sean Christopherson
2025-09-12T23:23:48Z

From: Yang Weijiang <weijiang.yang@intel.com>

Enable guest shadow stack pointer(SSP) access interface with new uAPIs.
CET guest SSP is HW register which has corresponding VMCS field to save
and restore guest values when VM-{Exit,Entry} happens. KVM handles SSP
as a fake/synthetic MSR for userspace access.

Use a translation helper to set up mapping for SSP synthetic index and
KVM-internal MSR index so that userspace doesn't need to take care of
KVM's management for synthetic MSRs and avoid conflicts.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
Documentation/virt/kvm/api.rst | 8 ++++++++
arch/x86/include/uapi/asm/kvm.h | 3 +++
arch/x86/kvm/x86.c | 23 +++++++++++++++++++++--
arch/x86/kvm/x86.h | 10 ++++++++++
4 files changed, 42 insertions(+), 2 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index abd02675a24d..6ae24c5ca559 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -2911,6 +2911,14 @@ such as set vcpu counter or reset vcpu, and they have the following id bit patte
x86 MSR registers have the following id bit patterns::
0x2030 0002 <msr number:32>

+Following are the KVM-defined registers for x86:
+
+======================= ========= =============================================
+ Encoding Register Description
+======================= ========= =============================================
+ 0x2030 0003 0000 0000 SSP Shadow Stack Pointer
+======================= ========= =============================================
+
4.69 KVM_GET_ONE_REG
--------------------

diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
index 508b713ca52e..8cc79eca34b2 100644
--- a/arch/x86/include/uapi/asm/kvm.h
+++ b/arch/x86/include/uapi/asm/kvm.h
@@ -437,6 +437,9 @@ struct kvm_xcrs {
#define KVM_X86_REG_KVM(index) \
KVM_X86_REG_ENCODE(KVM_X86_REG_TYPE_KVM, index)

+/* KVM-defined registers starting from 0 */
+#define KVM_REG_GUEST_SSP 0
+
#define KVM_SYNC_X86_REGS (1UL << 0)
#define KVM_SYNC_X86_SREGS (1UL << 1)
#define KVM_SYNC_X86_EVENTS (1UL << 2)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 2c9908bc8b32..460ceae11495 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6017,7 +6017,15 @@ struct kvm_x86_reg_id {

static int kvm_translate_kvm_reg(struct kvm_x86_reg_id *reg)
{
- return -EINVAL;
+ switch (reg->index) {
+ case KVM_REG_GUEST_SSP:
+ reg->type = KVM_X86_REG_TYPE_MSR;
+ reg->index = MSR_KVM_INTERNAL_GUEST_SSP;
+ break;
+ default:
+ return -EINVAL;
+ }
+ return 0;
}

static int kvm_get_one_msr(struct kvm_vcpu *vcpu, u32 msr, u64 __user *user_val)
@@ -6097,11 +6105,22 @@ static int kvm_get_set_one_reg(struct kvm_vcpu *vcpu, unsigned int ioctl,
static int kvm_get_reg_list(struct kvm_vcpu *vcpu,
struct kvm_reg_list __user *user_list)
{
- u64 nr_regs = 0;
+ u64 nr_regs = guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK) ? 1 : 0;
+ u64 user_nr_regs;
+
+ if (get_user(user_nr_regs, &user_list->n))
+ return -EFAULT;

if (put_user(nr_regs, &user_list->n))
return -EFAULT;

+ if (user_nr_regs < nr_regs)
+ return -E2BIG;
+
+ if (nr_regs &&
+ put_user(KVM_X86_REG_KVM(KVM_REG_GUEST_SSP), &user_list->reg[0]))
+ return -EFAULT;
+
return 0;
}

diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index 786e36fcd0fb..a7c9c72fca93 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -101,6 +101,16 @@ do { \
#define KVM_SVM_DEFAULT_PLE_WINDOW_MAX USHRT_MAX
#define KVM_SVM_DEFAULT_PLE_WINDOW 3000

+/*
+ * KVM's internal, non-ABI indices for synthetic MSRs. The values themselves
+ * are arbitrary and have no meaning, the only requirement is that they don't
+ * conflict with "real" MSRs that KVM supports. Use values at the upper end
+ * of KVM's reserved paravirtual MSR range to minimize churn, i.e. these values
+ * will be usable until KVM exhausts its supply of paravirtual MSR indices.
+ */
+
+#define MSR_KVM_INTERNAL_GUEST_SSP 0x4b564dff
+
static inline unsigned int __grow_ple_window(unsigned int val,
unsigned int base, unsigned int modifier, unsigned int max)
{
--
2.51.0.384.g4c02a37b29-goog