[PATCH v16 02/51] KVM: SEV: Read save fields from GHCB exactly once

Sean Christopherson
2025-09-19T22:33:07Z

Wrap all reads of GHCB save fields with READ_ONCE() via a KVM-specific
GHCB get() utility to help guard against TOCTOU bugs. Using READ_ONCE()
doesn't completely prevent such bugs, e.g. doesn't prevent KVM from
redoing get() after checking the initial value, but at least addresses
all potential TOCTOU issues in the current KVM code base.

To prevent unintentional use of the generic helpers, take only @svm for
the kvm_ghcb_get_xxx() helpers and retrieve the ghcb instead of explicitly
passing it in.

Opportunistically reduce the indentation of the macro-defined helpers and
clean up the alignment.

Fixes: 4e15a0ddc3ff ("KVM: SEV: snapshot the GHCB before accessing it")
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/svm/sev.c | 22 +++++++++++-----------
arch/x86/kvm/svm/svm.h | 25 +++++++++++++++----------
2 files changed, 26 insertions(+), 21 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index f046a587ecaf..8d057dbd8a71 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -3343,26 +3343,26 @@ static void sev_es_sync_from_ghcb(struct vcpu_svm *svm)
BUILD_BUG_ON(sizeof(svm->sev_es.valid_bitmap) != sizeof(ghcb->save.valid_bitmap));
memcpy(&svm->sev_es.valid_bitmap, &ghcb->save.valid_bitmap, sizeof(ghcb->save.valid_bitmap));

- vcpu->arch.regs[VCPU_REGS_RAX] = kvm_ghcb_get_rax_if_valid(svm, ghcb);
- vcpu->arch.regs[VCPU_REGS_RBX] = kvm_ghcb_get_rbx_if_valid(svm, ghcb);
- vcpu->arch.regs[VCPU_REGS_RCX] = kvm_ghcb_get_rcx_if_valid(svm, ghcb);
- vcpu->arch.regs[VCPU_REGS_RDX] = kvm_ghcb_get_rdx_if_valid(svm, ghcb);
- vcpu->arch.regs[VCPU_REGS_RSI] = kvm_ghcb_get_rsi_if_valid(svm, ghcb);
+ vcpu->arch.regs[VCPU_REGS_RAX] = kvm_ghcb_get_rax_if_valid(svm);
+ vcpu->arch.regs[VCPU_REGS_RBX] = kvm_ghcb_get_rbx_if_valid(svm);
+ vcpu->arch.regs[VCPU_REGS_RCX] = kvm_ghcb_get_rcx_if_valid(svm);
+ vcpu->arch.regs[VCPU_REGS_RDX] = kvm_ghcb_get_rdx_if_valid(svm);
+ vcpu->arch.regs[VCPU_REGS_RSI] = kvm_ghcb_get_rsi_if_valid(svm);

- svm->vmcb->save.cpl = kvm_ghcb_get_cpl_if_valid(svm, ghcb);
+ svm->vmcb->save.cpl = kvm_ghcb_get_cpl_if_valid(svm);

if (kvm_ghcb_xcr0_is_valid(svm)) {
- vcpu->arch.xcr0 = ghcb_get_xcr0(ghcb);
+ vcpu->arch.xcr0 = kvm_ghcb_get_xcr0(svm);
vcpu->arch.cpuid_dynamic_bits_dirty = true;
}

/* Copy the GHCB exit information into the VMCB fields */
- exit_code = ghcb_get_sw_exit_code(ghcb);
+ exit_code = kvm_ghcb_get_sw_exit_code(svm);
control->exit_code = lower_32_bits(exit_code);
control->exit_code_hi = upper_32_bits(exit_code);
- control->exit_info_1 = ghcb_get_sw_exit_info_1(ghcb);
- control->exit_info_2 = ghcb_get_sw_exit_info_2(ghcb);
- svm->sev_es.sw_scratch = kvm_ghcb_get_sw_scratch_if_valid(svm, ghcb);
+ control->exit_info_1 = kvm_ghcb_get_sw_exit_info_1(svm);
+ control->exit_info_2 = kvm_ghcb_get_sw_exit_info_2(svm);
+ svm->sev_es.sw_scratch = kvm_ghcb_get_sw_scratch_if_valid(svm);

/* Clear the valid entries fields */
memset(ghcb->save.valid_bitmap, 0, sizeof(ghcb->save.valid_bitmap));
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 5d39c0b17988..5365984e82e5 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -913,16 +913,21 @@ void __svm_sev_es_vcpu_run(struct vcpu_svm *svm, bool spec_ctrl_intercepted,
void __svm_vcpu_run(struct vcpu_svm *svm, bool spec_ctrl_intercepted);

#define DEFINE_KVM_GHCB_ACCESSORS(field) \
- static __always_inline bool kvm_ghcb_##field##_is_valid(const struct vcpu_svm *svm) \
- { \
- return test_bit(GHCB_BITMAP_IDX(field), \
- (unsigned long *)&svm->sev_es.valid_bitmap); \
- } \
- \
- static __always_inline u64 kvm_ghcb_get_##field##_if_valid(struct vcpu_svm *svm, struct ghcb *ghcb) \
- { \
- return kvm_ghcb_##field##_is_valid(svm) ? ghcb->save.field : 0; \
- } \
+static __always_inline u64 kvm_ghcb_get_##field(struct vcpu_svm *svm) \
+{ \
+ return READ_ONCE(svm->sev_es.ghcb->save.field); \
+} \
+ \
+static __always_inline bool kvm_ghcb_##field##_is_valid(const struct vcpu_svm *svm) \
+{ \
+ return test_bit(GHCB_BITMAP_IDX(field), \
+ (unsigned long *)&svm->sev_es.valid_bitmap); \
+} \
+ \
+static __always_inline u64 kvm_ghcb_get_##field##_if_valid(struct vcpu_svm *svm) \
+{ \
+ return kvm_ghcb_##field##_is_valid(svm) ? kvm_ghcb_get_##field(svm) : 0; \
+}

DEFINE_KVM_GHCB_ACCESSORS(cpl)
DEFINE_KVM_GHCB_ACCESSORS(rax)
--
2.51.0.470.ga7dc726c21-goog