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Fri, 14 Nov 2025 15:32:09 -0800 (PST) Received: from [192.168.10.48] ([176.206.119.13]) by smtp.googlemail.com with ESMTPSA id ffacd0b85a97d-42b53f17291sm12635481f8f.32.2025.11.14.15.32.08 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 14 Nov 2025 15:32:08 -0800 (PST) Message-ID: <60f7c9b3-312f-41e2-ab47-c4361df1d825@redhat.com> Date: Sat, 15 Nov 2025 00:32:04 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 6/9] KVM: SVM: Filter out 64-bit exit codes when invoking exit handlers on bare metal To: Sean Christopherson , Vitaly Kuznetsov , "K. Y. Srinivasan" , Haiyang Zhang , Wei Liu , Dexuan Cui Cc: kvm@vger.kernel.org, linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org, Jim Mattson , Yosry Ahmed References: <20251113225621.1688428-1-seanjc@google.com> <20251113225621.1688428-7-seanjc@google.com> From: Paolo Bonzini Content-Language: en-US Autocrypt: addr=pbonzini@redhat.com; keydata= xsEhBFRCcBIBDqDGsz4K0zZun3jh+U6Z9wNGLKQ0kSFyjN38gMqU1SfP+TUNQepFHb/Gc0E2 CxXPkIBTvYY+ZPkoTh5xF9oS1jqI8iRLzouzF8yXs3QjQIZ2SfuCxSVwlV65jotcjD2FTN04 hVopm9llFijNZpVIOGUTqzM4U55sdsCcZUluWM6x4HSOdw5F5Utxfp1wOjD/v92Lrax0hjiX DResHSt48q+8FrZzY+AUbkUS+Jm34qjswdrgsC5uxeVcLkBgWLmov2kMaMROT0YmFY6A3m1S P/kXmHDXxhe23gKb3dgwxUTpENDBGcfEzrzilWueOeUWiOcWuFOed/C3SyijBx3Av/lbCsHU Vx6pMycNTdzU1BuAroB+Y3mNEuW56Yd44jlInzG2UOwt9XjjdKkJZ1g0P9dwptwLEgTEd3Fo UdhAQyRXGYO8oROiuh+RZ1lXp6AQ4ZjoyH8WLfTLf5g1EKCTc4C1sy1vQSdzIRu3rBIjAvnC tGZADei1IExLqB3uzXKzZ1BZ+Z8hnt2og9hb7H0y8diYfEk2w3R7wEr+Ehk5NQsT2MPI2QBd wEv1/Aj1DgUHZAHzG1QN9S8wNWQ6K9DqHZTBnI1hUlkp22zCSHK/6FwUCuYp1zcAEQEAAc0j UGFvbG8gQm9uemluaSA8cGJvbnppbmlAcmVkaGF0LmNvbT7CwU0EEwECACMFAlRCcBICGwMH CwkIBwMCAQYVCAIJCgsEFgIDAQIeAQIXgAAKCRB+FRAMzTZpsbceDp9IIN6BIA0Ol7MoB15E 11kRz/ewzryFY54tQlMnd4xxfH8MTQ/mm9I482YoSwPMdcWFAKnUX6Yo30tbLiNB8hzaHeRj jx12K+ptqYbg+cevgOtbLAlL9kNgLLcsGqC2829jBCUTVeMSZDrzS97ole/YEez2qFpPnTV0 VrRWClWVfYh+JfzpXmgyhbkuwUxNFk421s4Ajp3d8nPPFUGgBG5HOxzkAm7xb1cjAuJ+oi/K CHfkuN+fLZl/u3E/fw7vvOESApLU5o0icVXeakfSz0LsygEnekDbxPnE5af/9FEkXJD5EoYG SEahaEtgNrR4qsyxyAGYgZlS70vkSSYJ+iT2rrwEiDlo31MzRo6Ba2FfHBSJ7lcYdPT7bbk9 AO3hlNMhNdUhoQv7M5HsnqZ6unvSHOKmReNaS9egAGdRN0/GPDWr9wroyJ65ZNQsHl9nXBqE AukZNr5oJO5vxrYiAuuTSd6UI/xFkjtkzltG3mw5ao2bBpk/V/YuePrJsnPFHG7NhizrxttB nTuOSCMo45pfHQ+XYd5K1+Cv/NzZFNWscm5htJ0HznY+oOsZvHTyGz3v91pn51dkRYN0otqr bQ4tlFFuVjArBZcapSIe6NV8C4cEiSTOwE0EVEJx7gEIAMeHcVzuv2bp9HlWDp6+RkZe+vtl KwAHplb/WH59j2wyG8V6i33+6MlSSJMOFnYUCCL77bucx9uImI5nX24PIlqT+zasVEEVGSRF m8dgkcJDB7Tps0IkNrUi4yof3B3shR+vMY3i3Ip0e41zKx0CvlAhMOo6otaHmcxr35sWq1Jk tLkbn3wG+fPQCVudJJECvVQ//UAthSSEklA50QtD2sBkmQ14ZryEyTHQ+E42K3j2IUmOLriF dNr9NvE1QGmGyIcbw2NIVEBOK/GWxkS5+dmxM2iD4Jdaf2nSn3jlHjEXoPwpMs0KZsgdU0pP JQzMUMwmB1wM8JxovFlPYrhNT9MAEQEAAcLBMwQYAQIACQUCVEJx7gIbDAAKCRB+FRAMzTZp sadRDqCctLmYICZu4GSnie4lKXl+HqlLanpVMOoFNnWs9oRP47MbE2wv8OaYh5pNR9VVgyhD OG0AU7oidG36OeUlrFDTfnPYYSF/mPCxHttosyt8O5kabxnIPv2URuAxDByz+iVbL+RjKaGM GDph56ZTswlx75nZVtIukqzLAQ5fa8OALSGum0cFi4ptZUOhDNz1onz61klD6z3MODi0sBZN Aj6guB2L/+2ZwElZEeRBERRd/uommlYuToAXfNRdUwrwl9gRMiA0WSyTb190zneRRDfpSK5d usXnM/O+kr3Dm+Ui+UioPf6wgbn3T0o6I5BhVhs4h4hWmIW7iNhPjX1iybXfmb1gAFfjtHfL xRUr64svXpyfJMScIQtBAm0ihWPltXkyITA92ngCmPdHa6M1hMh4RDX+Jf1fiWubzp1voAg0 JBrdmNZSQDz0iKmSrx8xkoXYfA3bgtFN8WJH2xgFL28XnqY4M6dLhJwV3z08tPSRqYFm4NMP dRsn0/7oymhneL8RthIvjDDQ5ktUjMe8LtHr70OZE/TT88qvEdhiIVUogHdo4qBrk41+gGQh b906Dudw5YhTJFU3nC6bbF2nrLlB4C/XSiH76ZvqzV0Z/cAMBo5NF/w= In-Reply-To: <20251113225621.1688428-7-seanjc@google.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 11/13/25 23:56, Sean Christopherson wrote: > 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. > > Signed-off-by: Sean Christopherson > --- > 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 202a4d8088a2..3b05476296d0 100644 > --- a/arch/x86/kvm/svm/svm.c > +++ b/arch/x86/kvm/svm/svm.c > @@ -3433,8 +3433,22 @@ 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; I reviewed the series and it looks good, but with respect to this patch and patch 8, is it really worth it? While there is a possibility that code 0x00000000ffffffff is used, or that any high 32-bit values other than all-zeros or all-ones are used, they'd be presumably enabled by some control bits in the VMCB or some paravirt thing in the hypervisor. What really matters is that SEV-ES's kvm_get_cached_sw_exit_code() is reading the full 64 bits and discarding invalid codes before reaching svm_invoke_exit_handler(). I totally agree, of course, with passing __exit_code as u64 and adding a comment explaining what's going on with "u32 exit_code == (u32)__exit_code". Paolo > #ifdef CONFIG_MITIGATION_RETPOLINE > if (exit_code == SVM_EXIT_MSR) > return msr_interception(vcpu); > @@ -3461,7 +3475,7 @@ 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; > } >