From f1a9761fbb00639c5e73835f7f373ef834bb867f Mon Sep 17 00:00:00 2001 From: Oliver Upton Date: Wed, 16 Mar 2022 00:55:37 +0000 Subject: [PATCH] KVM: x86: Allow userspace to opt out of hypercall patching KVM handles the VMCALL/VMMCALL instructions very strangely. Even though both of these instructions really should #UD when executed on the wrong vendor's hardware (i.e. VMCALL on SVM, VMMCALL on VMX), KVM replaces the guest's instruction with the appropriate instruction for the vendor. Nonetheless, older guest kernels without commit c1118b3602c2 ("x86: kvm: use alternatives for VMCALL vs. VMMCALL if kernel text is read-only") do not patch in the appropriate instruction using alternatives, likely motivating KVM's intervention. Add a quirk allowing userspace to opt out of hypercall patching. If the quirk is disabled, KVM synthesizes a #UD in the guest. Signed-off-by: Oliver Upton Message-Id: <20220316005538.2282772-2-oupton@google.com> Signed-off-by: Paolo Bonzini --- Documentation/virt/kvm/api.rst | 9 +++++++++ arch/x86/include/asm/kvm_host.h | 3 ++- arch/x86/include/uapi/asm/kvm.h | 11 ++++++----- arch/x86/kvm/x86.c | 11 +++++++++++ 4 files changed, 28 insertions(+), 6 deletions(-) diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst index d13fa66004672..700df350332b1 100644 --- a/Documentation/virt/kvm/api.rst +++ b/Documentation/virt/kvm/api.rst @@ -7134,6 +7134,15 @@ The valid bits in cap.args[0] are: Additionally, when this quirk is disabled, KVM clears CPUID.01H:ECX[bit 3] if IA32_MISC_ENABLE[bit 18] is cleared. + + KVM_X86_QUIRK_FIX_HYPERCALL_INSN By default, KVM rewrites guest + VMMCALL/VMCALL instructions to match the + vendor's hypercall instruction for the + system. When this quirk is disabled, KVM + will no longer rewrite invalid guest + hypercall instructions. Executing the + incorrect hypercall instruction will + generate a #UD within the guest. =================================== ============================================ 8. Other capabilities. diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h index 93a2671a57cf2..d9a825182b845 100644 --- a/arch/x86/include/asm/kvm_host.h +++ b/arch/x86/include/asm/kvm_host.h @@ -1987,6 +1987,7 @@ int memslot_rmap_alloc(struct kvm_memory_slot *slot, unsigned long npages); KVM_X86_QUIRK_CD_NW_CLEARED | \ KVM_X86_QUIRK_LAPIC_MMIO_HOLE | \ KVM_X86_QUIRK_OUT_7E_INC_RIP | \ - KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT) + KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT | \ + KVM_X86_QUIRK_FIX_HYPERCALL_INSN) #endif /* _ASM_X86_KVM_HOST_H */ diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h index bf6e96011dfed..21614807a2cbb 100644 --- a/arch/x86/include/uapi/asm/kvm.h +++ b/arch/x86/include/uapi/asm/kvm.h @@ -428,11 +428,12 @@ struct kvm_sync_regs { struct kvm_vcpu_events events; }; -#define KVM_X86_QUIRK_LINT0_REENABLED (1 << 0) -#define KVM_X86_QUIRK_CD_NW_CLEARED (1 << 1) -#define KVM_X86_QUIRK_LAPIC_MMIO_HOLE (1 << 2) -#define KVM_X86_QUIRK_OUT_7E_INC_RIP (1 << 3) -#define KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT (1 << 4) +#define KVM_X86_QUIRK_LINT0_REENABLED (1 << 0) +#define KVM_X86_QUIRK_CD_NW_CLEARED (1 << 1) +#define KVM_X86_QUIRK_LAPIC_MMIO_HOLE (1 << 2) +#define KVM_X86_QUIRK_OUT_7E_INC_RIP (1 << 3) +#define KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT (1 << 4) +#define KVM_X86_QUIRK_FIX_HYPERCALL_INSN (1 << 5) #define KVM_STATE_NESTED_FORMAT_VMX 0 #define KVM_STATE_NESTED_FORMAT_SVM 1 diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 498cd4aaa2cb3..c9964b78fa2e2 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -9267,6 +9267,17 @@ static int emulator_fix_hypercall(struct x86_emulate_ctxt *ctxt) char instruction[3]; unsigned long rip = kvm_rip_read(vcpu); + /* + * If the quirk is disabled, synthesize a #UD and let the guest pick up + * the pieces. + */ + if (!kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_FIX_HYPERCALL_INSN)) { + ctxt->exception.error_code_valid = false; + ctxt->exception.vector = UD_VECTOR; + ctxt->have_exception = true; + return X86EMUL_PROPAGATE_FAULT; + } + static_call(kvm_x86_patch_hypercall)(vcpu, instruction); return emulator_write_emulated(ctxt, rip, instruction, 3, -- 2.30.2