KVM: x86: Enable guest SSP read/write interface with new uAPIs

Add a KVM-defined ONE_REG register, KVM_REG_GUEST_SSP, to let userspace
save and restore the guest's Shadow Stack Pointer (SSP).  On both Intel
and AMD, SSP is a hardware register that can only be accessed by software
via dedicated ISA (e.g. RDSSP) or via VMCS/VMCB fields (used by hardware
to context switch SSP at entry/exit).  As a result, SSP doesn't fit in
any of KVM's existing interfaces for saving/restoring state.

Internally, treat SSP as a fake/synthetic MSR, as the semantics of writes
to SSP follow that of several other Shadow Stack MSRs, e.g. the PLx_SSP
MSRs.  Use a translation layer to hide the KVM-internal MSR index so that
the arbitrary index doesn't become ABI, e.g. so that KVM can rework its
implementation as needed, so long as the ONE_REG ABI is maintained.

Explicitly reject accesses to SSP if the vCPU doesn't have Shadow Stack
support to avoid running afoul of ignore_msrs, which unfortunately applies
to host-initiated accesses (which is a discussion for another day).  I.e.
ensure consistent behavior for KVM-defined registers irrespective of
ignore_msrs.

Link: https://lore.kernel.org/all/aca9d389-f11e-4811-90cf-d98e345a5cc2@intel.com
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>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://lore.kernel.org/r/20250919223258.1604852-14-seanjc@google.com
Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>

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
 --------------------
 

arch/x86/include/uapi/asm/kvm.h

@@ -437,6 +437,9 @@ struct kvm_xcrs {
 #define KVM_X86_REG_KVM(index)					\
 	KVM_X86_REG_ID(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)

arch/x86/kvm/x86.c

@@ -6016,9 +6016,27 @@ struct kvm_x86_reg_id {
 	__u8  x86;
 };
 
-static int kvm_translate_kvm_reg(struct kvm_x86_reg_id *reg)
+static int kvm_translate_kvm_reg(struct kvm_vcpu *vcpu,
+				 struct kvm_x86_reg_id *reg)
 {
-	return -EINVAL;
+	switch (reg->index) {
+	case KVM_REG_GUEST_SSP:
+		/*
+		 * FIXME: If host-initiated accesses are ever exempted from
+		 * ignore_msrs (in kvm_do_msr_access()), drop this manual check
+		 * and rely on KVM's standard checks to reject accesses to regs
+		 * that don't exist.
+		 */
+		if (!guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK))
+			return -EINVAL;
+
+		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)
@@ -6067,7 +6085,7 @@ static int kvm_get_set_one_reg(struct kvm_vcpu *vcpu, unsigned int ioctl,
 		return -EINVAL;
 
 	if (reg->type == KVM_X86_REG_TYPE_KVM) {
-		r = kvm_translate_kvm_reg(reg);
+		r = kvm_translate_kvm_reg(vcpu, reg);
 		if (r)
 			return r;
 	}
@@ -6098,11 +6116,22 @@ 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;
 }
 

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)
 {