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Message-ID: <20250919223258.1604852-14-seanjc@google.com>
Date: Fri, 19 Sep 2025 15:32:20 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson <seanjc@...gle.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
Tom Lendacky <thomas.lendacky@....com>, Mathias Krause <minipli@...ecurity.net>,
John Allen <john.allen@....com>, Rick Edgecombe <rick.p.edgecombe@...el.com>,
Chao Gao <chao.gao@...el.com>, Binbin Wu <binbin.wu@...ux.intel.com>,
Xiaoyao Li <xiaoyao.li@...el.com>, Maxim Levitsky <mlevitsk@...hat.com>,
Zhang Yi Z <yi.z.zhang@...ux.intel.com>, Xin Li <xin@...or.com>
Subject: [PATCH v16 13/51] KVM: x86: Enable guest SSP read/write interface
with new uAPIs
From: Yang Weijiang <weijiang.yang@...el.com>
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@...gle.com>
Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
Tested-by: Mathias Krause <minipli@...ecurity.net>
Tested-by: John Allen <john.allen@....com>
Tested-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
Signed-off-by: Chao Gao <chao.gao@...el.com>
Co-developed-by: Sean Christopherson <seanjc@...gle.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
Documentation/virt/kvm/api.rst | 8 +++++++
arch/x86/include/uapi/asm/kvm.h | 3 +++
arch/x86/kvm/x86.c | 37 +++++++++++++++++++++++++++++----
arch/x86/kvm/x86.h | 10 +++++++++
4 files changed, 54 insertions(+), 4 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 aae1033c8afa..467116186e71 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_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)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 5245b21168cb..720540f102e1 100644
--- a/arch/x86/kvm/x86.c
+++ b/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;
}
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)
{
--
2.51.0.470.ga7dc726c21-goog
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