[PATCH v15 02/41] KVM: SEV: Read save fields from GHCB exactly once

Sean Christopherson
2025-09-12T23:23:28Z

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.

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

Fixes: 4e15a0ddc3ff ("KVM: SEV: snapshot the GHCB before accessing it")
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/svm/sev.c | 8 ++++----
arch/x86/kvm/svm/svm.h | 26 ++++++++++++++++----------
2 files changed, 20 insertions(+), 14 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index fe8d148b76c0..37abbda28685 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -3304,16 +3304,16 @@ static void sev_es_sync_from_ghcb(struct vcpu_svm *svm)
svm->vmcb->save.cpl = kvm_ghcb_get_cpl_if_valid(svm, ghcb);

if (kvm_ghcb_xcr0_is_valid(svm)) {
- vcpu->arch.xcr0 = ghcb_get_xcr0(ghcb);
+ vcpu->arch.xcr0 = kvm_ghcb_get_xcr0(ghcb);
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(ghcb);
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);
+ control->exit_info_1 = kvm_ghcb_get_sw_exit_info_1(ghcb);
+ control->exit_info_2 = kvm_ghcb_get_sw_exit_info_2(ghcb);
svm->sev_es.sw_scratch = kvm_ghcb_get_sw_scratch_if_valid(svm, ghcb);

/* Clear the valid entries fields */
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 5d39c0b17988..c2316adde3cc 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -913,16 +913,22 @@ 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 ghcb *ghcb) \
+{ \
+ return READ_ONCE(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, \
+ struct ghcb *ghcb) \
+{ \
+ return kvm_ghcb_##field##_is_valid(svm) ? kvm_ghcb_get_##field(ghcb) : 0; \
+}

DEFINE_KVM_GHCB_ACCESSORS(cpl)
DEFINE_KVM_GHCB_ACCESSORS(rax)
--
2.51.0.384.g4c02a37b29-goog