From: Sean Christopherson Date: Tue, 30 Dec 2025 21:13:43 +0000 (-0800) Subject: KVM: SVM: Filter out 64-bit exit codes when invoking exit handlers on bare metal X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=405fce694bd1589082a7ffd500b5a4b841c22f0d;p=thirdparty%2Fkernel%2Fstable.git KVM: SVM: Filter out 64-bit exit codes when invoking exit handlers on bare metal 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 Link: https://patch.msgid.link/20251230211347.4099600-5-seanjc@google.com Signed-off-by: Sean Christopherson --- diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c index d2f997965a96..3caf7a21679f 100644 --- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -3467,8 +3467,22 @@ no_vmsa: 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); @@ -3495,7 +3509,7 @@ 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; }