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Fri, 28 Jul 2023 17:36:52 -0700 (PDT) Date: Fri, 28 Jul 2023 17:36:12 -0700 In-Reply-To: <20230729003643.1053367-1-seanjc@google.com> Mime-Version: 1.0 References: <20230729003643.1053367-1-seanjc@google.com> X-Mailer: git-send-email 2.41.0.487.g6d72f3e995-goog Message-ID: <20230729003643.1053367-4-seanjc@google.com> Subject: [PATCH v4 03/34] KVM: selftests: Add a shameful hack to preserve/clobber GPRs across ucall From: Sean Christopherson To: Paolo Bonzini , Marc Zyngier , Oliver Upton , Christian Borntraeger , Janosch Frank , Claudio Imbrenda Cc: kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org, Sean Christopherson , Thomas Huth , " =?utf-8?q?Philippe_Mathieu-Daud=C3=A9?= " , Aaron Lewis X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20230728_173658_325426_0F1519BC X-CRM114-Status: GOOD ( 14.10 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Sean Christopherson Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Preserve or clobber all GPRs (except RIP and RSP, as they're saved and restored via the VMCS) when performing a ucall on x86 to fudge around a horrific long-standing bug in selftests' nested VMX support where L2's GPRs are not preserved across a nested VM-Exit. I.e. if a test triggers a nested VM-Exit to L1 in response to a ucall, e.g. GUEST_SYNC(), then L2's GPR state can be corrupted. The issues manifests as an unexpected #GP in clear_bit() when running the hyperv_evmcs test due to RBX being used to track the ucall object, and RBX being clobbered by the nested VM-Exit. The problematic hyperv_evmcs testcase is where L0 (test's host userspace) injects an NMI in response to GUEST_SYNC(8) from L2, but the bug could "randomly" manifest in any test that induces a nested VM-Exit from L0. The bug hasn't caused failures in the past due to sheer dumb luck. The obvious fix is to rework the nVMX helpers to save/restore L2 GPRs across VM-Exit and VM-Enter, but that is a much bigger task and carries its own risks, e.g. nSVM does save/restore GPRs, but not in a thread-safe manner, and there is a _lot_ of cleanup that can be done to unify code for doing VM-Enter on nVMX, nSVM, and eVMCS. Signed-off-by: Sean Christopherson --- .../testing/selftests/kvm/lib/x86_64/ucall.c | 32 +++++++++++++++++-- 1 file changed, 30 insertions(+), 2 deletions(-) diff --git a/tools/testing/selftests/kvm/lib/x86_64/ucall.c b/tools/testing/selftests/kvm/lib/x86_64/ucall.c index 4d41dc63cc9e..a53df3ece2f8 100644 --- a/tools/testing/selftests/kvm/lib/x86_64/ucall.c +++ b/tools/testing/selftests/kvm/lib/x86_64/ucall.c @@ -14,8 +14,36 @@ void ucall_arch_init(struct kvm_vm *vm, vm_paddr_t mmio_gpa) void ucall_arch_do_ucall(vm_vaddr_t uc) { - asm volatile("in %[port], %%al" - : : [port] "d" (UCALL_PIO_PORT), "D" (uc) : "rax", "memory"); + /* + * FIXME: Revert this hack (the entire commit that added it) once nVMX + * preserves L2 GPRs across a nested VM-Exit. If a ucall from L2, e.g. + * to do a GUEST_SYNC(), lands the vCPU in L1, any and all GPRs can be + * clobbered by L1. Save and restore non-volatile GPRs (clobbering RBP + * in particular is problematic) along with RDX and RDI (which are + * inputs), and clobber volatile GPRs. *sigh* + */ +#define HORRIFIC_L2_UCALL_CLOBBER_HACK \ + "rcx", "rsi", "r8", "r9", "r10", "r11" + + asm volatile("push %%rbp\n\t" + "push %%r15\n\t" + "push %%r14\n\t" + "push %%r13\n\t" + "push %%r12\n\t" + "push %%rbx\n\t" + "push %%rdx\n\t" + "push %%rdi\n\t" + "in %[port], %%al\n\t" + "pop %%rdi\n\t" + "pop %%rdx\n\t" + "pop %%rbx\n\t" + "pop %%r12\n\t" + "pop %%r13\n\t" + "pop %%r14\n\t" + "pop %%r15\n\t" + "pop %%rbp\n\t" + : : [port] "d" (UCALL_PIO_PORT), "D" (uc) : "rax", "memory", + HORRIFIC_L2_UCALL_CLOBBER_HACK); } void *ucall_arch_get_ucall(struct kvm_vcpu *vcpu)