Explicitly filter out 64-bit exit codes when invoking exit handlers, as svm_exit_handlers[] will never be sized with entries that use bits 63:32.
Processing the non-failing exit code as a 32-bit value will allow tracking exit_code as a single 64-bit value (which it is, architecturally). This will also allow hardening KVM against Spectre-like attacks without needing to do silly things to avoid build failures on 32-bit kernels (array_index_nospec() rightly asserts that the index fits in an "unsigned long"). Omit the check when running as a VM, as KVM has historically failed to set bits 63:32 appropriately when synthesizing VM-Exits, i.e. KVM could get false positives when running as a VM on an older, broken KVM/kernel. From a functional perspective, omitting the check is "fine", as any unwanted collision between e.g. VMEXIT_INVALID and a 32-bit exit code will be fatal to KVM-on-KVM regardless of what KVM-as-L1 does. Reviewed-by: Yosry Ahmed <[email protected]> Signed-off-by: Sean Christopherson <[email protected]> --- arch/x86/kvm/svm/svm.c | 18 ++++++++++++++++-- 1 file changed, 16 insertions(+), 2 deletions(-) diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c index e24bedf1fc81..1ffe922e95fd 100644 --- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -3443,8 +3443,22 @@ static void dump_vmcb(struct kvm_vcpu *vcpu) sev_free_decrypted_vmsa(vcpu, save); } -int svm_invoke_exit_handler(struct kvm_vcpu *vcpu, u64 exit_code) +int svm_invoke_exit_handler(struct kvm_vcpu *vcpu, u64 __exit_code) { + u32 exit_code = __exit_code; + + /* + * SVM uses negative values, i.e. 64-bit values, to indicate that VMRUN + * failed. Report all such errors to userspace (note, VMEXIT_INVALID, + * a.k.a. SVM_EXIT_ERR, is special cased by svm_handle_exit()). Skip + * the check when running as a VM, as KVM has historically left garbage + * in bits 63:32, i.e. running KVM-on-KVM would hit false positives if + * the underlying kernel is buggy. + */ + if (!cpu_feature_enabled(X86_FEATURE_HYPERVISOR) && + (u64)exit_code != __exit_code) + goto unexpected_vmexit; + #ifdef CONFIG_MITIGATION_RETPOLINE if (exit_code == SVM_EXIT_MSR) return msr_interception(vcpu); @@ -3471,7 +3485,7 @@ int svm_invoke_exit_handler(struct kvm_vcpu *vcpu, u64 exit_code) unexpected_vmexit: dump_vmcb(vcpu); - kvm_prepare_unexpected_reason_exit(vcpu, exit_code); + kvm_prepare_unexpected_reason_exit(vcpu, __exit_code); return 0; } -- 2.52.0.351.gbe84eed79e-goog
