Restoration of the host IA32_SPEC_CTRL value is probably too late with respect to the return thunk training sequence.

With respect to the user/kernel boundary, AMD says, "If software chooses to toggle STIBP (e.g., set STIBP on kernel entry, and clear it on kernel exit), software should set STIBP to 1 before executing the return thunk training sequence." I assume the same requirements apply to the guest/host boundary. The return thunk training sequence is in vmenter.S, quite close to the VM-exit. On hosts without V_SPEC_CTRL, however, the host's IA32_SPEC_CTRL value is not restored until much later.

To avoid this, move the restoration of host SPEC_CTRL to assembly and, for consistency, move the restoration of the guest SPEC_CTRL as well. This is not particularly difficult, apart from some care to cover both 32- and 64-bit, and to share code between SEV-ES and normal vmentry.

Cc: stable@vger.kernel.org Fixes: a149180fbcf3 ("x86: Add magic AMD return-thunk") Suggested-by: Jim Mattson jmattson@google.com Signed-off-by: Paolo Bonzini pbonzini@redhat.com --- arch/x86/kernel/cpu/bugs.c | 13 +--- arch/x86/kvm/kvm-asm-offsets.c | 1 + arch/x86/kvm/svm/svm.c | 37 ++++------ arch/x86/kvm/svm/svm.h | 4 +- arch/x86/kvm/svm/vmenter.S | 119 ++++++++++++++++++++++++++++++++- 5 files changed, 136 insertions(+), 38 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c index da7c361f47e0..6ec0b7ce7453 100644 --- a/arch/x86/kernel/cpu/bugs.c +++ b/arch/x86/kernel/cpu/bugs.c @@ -196,22 +196,15 @@ void __init check_bugs(void) }

/* - * NOTE: This function is *only* called for SVM. VMX spec_ctrl handling is - * done in vmenter.S. + * NOTE: This function is *only* called for SVM, since Intel uses + * MSR_IA32_SPEC_CTRL for SSBD. */ void x86_virt_spec_ctrl(u64 guest_spec_ctrl, u64 guest_virt_spec_ctrl, bool setguest) { - u64 msrval, guestval = guest_spec_ctrl, hostval = spec_ctrl_current(); + u64 guestval, hostval; struct thread_info *ti = current_thread_info();

- if (static_cpu_has(X86_FEATURE_MSR_SPEC_CTRL)) { - if (hostval != guestval) { - msrval = setguest ? guestval : hostval; - wrmsrl(MSR_IA32_SPEC_CTRL, msrval); - } - } - /* * If SSBD is not handled in MSR_SPEC_CTRL on AMD, update * MSR_AMD64_L2_CFG or MSR_VIRT_SPEC_CTRL if supported. diff --git a/arch/x86/kvm/kvm-asm-offsets.c b/arch/x86/kvm/kvm-asm-offsets.c index 1b805cd24d66..24a710d37323 100644 --- a/arch/x86/kvm/kvm-asm-offsets.c +++ b/arch/x86/kvm/kvm-asm-offsets.c @@ -16,6 +16,7 @@ static void __used common(void) BLANK(); OFFSET(SVM_vcpu_arch_regs, vcpu_svm, vcpu.arch.regs); OFFSET(SVM_current_vmcb, vcpu_svm, current_vmcb); + OFFSET(SVM_spec_ctrl, vcpu_svm, spec_ctrl); OFFSET(SVM_vmcb01, vcpu_svm, vmcb01); OFFSET(KVM_VMCB_pa, kvm_vmcb_info, pa); OFFSET(SD_save_area_pa, svm_cpu_data, save_area_pa); diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c index 469c1b5617af..cf1aed25f4ab 100644 --- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -720,6 +720,15 @@ static bool msr_write_intercepted(struct kvm_vcpu *vcpu, u32 msr) u32 offset; u32 *msrpm;

+ /* + * For non-nested case: + * If the L01 MSR bitmap does not intercept the MSR, then we need to + * save it. + * + * For nested case: + * If the L02 MSR bitmap does not intercept the MSR, then we need to + * save it. + */ msrpm = is_guest_mode(vcpu) ? to_svm(vcpu)->nested.msrpm: to_svm(vcpu)->msrpm;

@@ -3901,16 +3910,16 @@ static fastpath_t svm_exit_handlers_fastpath(struct kvm_vcpu *vcpu) return EXIT_FASTPATH_NONE; }

-static noinstr void svm_vcpu_enter_exit(struct kvm_vcpu *vcpu) +static noinstr void svm_vcpu_enter_exit(struct kvm_vcpu *vcpu, bool spec_ctrl_intercepted) { struct vcpu_svm *svm = to_svm(vcpu);

guest_state_enter_irqoff();

if (sev_es_guest(vcpu->kvm)) - __svm_sev_es_vcpu_run(svm); + __svm_sev_es_vcpu_run(svm, spec_ctrl_intercepted); else - __svm_vcpu_run(svm); + __svm_vcpu_run(svm, spec_ctrl_intercepted);

guest_state_exit_irqoff(); } @@ -3918,6 +3927,7 @@ static noinstr void svm_vcpu_enter_exit(struct kvm_vcpu *vcpu) static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu) { struct vcpu_svm *svm = to_svm(vcpu); + bool spec_ctrl_intercepted = msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL);

trace_kvm_entry(vcpu);

@@ -3976,26 +3986,7 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu) if (!static_cpu_has(X86_FEATURE_V_SPEC_CTRL)) x86_spec_ctrl_set_guest(svm->spec_ctrl, svm->virt_spec_ctrl);

- svm_vcpu_enter_exit(vcpu); - - /* - * We do not use IBRS in the kernel. If this vCPU has used the - * SPEC_CTRL MSR it may have left it on; save the value and - * turn it off. This is much more efficient than blindly adding - * it to the atomic save/restore list. Especially as the former - * (Saving guest MSRs on vmexit) doesn't even exist in KVM. - * - * For non-nested case: - * If the L01 MSR bitmap does not intercept the MSR, then we need to - * save it. - * - * For nested case: - * If the L02 MSR bitmap does not intercept the MSR, then we need to - * save it. - */ - if (!static_cpu_has(X86_FEATURE_V_SPEC_CTRL) && - unlikely(!msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL))) - svm->spec_ctrl = native_read_msr(MSR_IA32_SPEC_CTRL); + svm_vcpu_enter_exit(vcpu, spec_ctrl_intercepted);

if (!sev_es_guest(vcpu->kvm)) reload_tss(vcpu); diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h index 83955a4e520e..199a2ecef1ce 100644 --- a/arch/x86/kvm/svm/svm.h +++ b/arch/x86/kvm/svm/svm.h @@ -682,7 +682,7 @@ void sev_es_unmap_ghcb(struct vcpu_svm *svm);

/* vmenter.S */

-void __svm_sev_es_vcpu_run(struct vcpu_svm *svm); -void __svm_vcpu_run(struct vcpu_svm *svm); +void __svm_sev_es_vcpu_run(struct vcpu_svm *svm, bool spec_ctrl_intercepted); +void __svm_vcpu_run(struct vcpu_svm *svm, bool spec_ctrl_intercepted);

#endif diff --git a/arch/x86/kvm/svm/vmenter.S b/arch/x86/kvm/svm/vmenter.S index 57440acfc73e..34367dc203f2 100644 --- a/arch/x86/kvm/svm/vmenter.S +++ b/arch/x86/kvm/svm/vmenter.S @@ -32,9 +32,69 @@

.section .noinstr.text, "ax"

+.macro RESTORE_GUEST_SPEC_CTRL + /* No need to do anything if SPEC_CTRL is unset or V_SPEC_CTRL is set */ + ALTERNATIVE_2 "", \ + "jmp 800f", X86_FEATURE_MSR_SPEC_CTRL, \ + "", X86_FEATURE_V_SPEC_CTRL +801: +.endm +.macro RESTORE_GUEST_SPEC_CTRL_BODY +800: + /* + * SPEC_CTRL handling: if the guest's SPEC_CTRL value differs from the + * host's, write the MSR. This is kept out-of-line so that the common + * case does not have to jump. + * + * IMPORTANT: To avoid RSB underflow attacks and any other nastiness, + * there must not be any returns or indirect branches between this code + * and vmentry. + */ + movl SVM_spec_ctrl(%_ASM_DI), %eax + cmp PER_CPU_VAR(x86_spec_ctrl_current), %eax + je 801b + mov $MSR_IA32_SPEC_CTRL, %ecx + xor %edx, %edx + wrmsr + jmp 801b +.endm + +.macro RESTORE_HOST_SPEC_CTRL + /* No need to do anything if SPEC_CTRL is unset or V_SPEC_CTRL is set */ + ALTERNATIVE_2 "", \ + "jmp 900f", X86_FEATURE_MSR_SPEC_CTRL, \ + "", X86_FEATURE_V_SPEC_CTRL +901: +.endm +.macro RESTORE_HOST_SPEC_CTRL_BODY +900: + /* Same for after vmexit. */ + mov $MSR_IA32_SPEC_CTRL, %ecx + + /* + * Load the value that the guest had written into MSR_IA32_SPEC_CTRL, + * if it was not intercepted during guest execution. + */ + cmpb $0, (%_ASM_SP) + jnz 998f + rdmsr + movl %eax, SVM_spec_ctrl(%_ASM_DI) +998: + + /* Now restore the host value of the MSR if different from the guest's. */ + movl PER_CPU_VAR(x86_spec_ctrl_current), %eax + cmp SVM_spec_ctrl(%_ASM_DI), %eax + je 901b + xor %edx, %edx + wrmsr + jmp 901b +.endm + + /** * __svm_vcpu_run - Run a vCPU via a transition to SVM guest mode * @svm: struct vcpu_svm * + * @spec_ctrl_intercepted: bool */ SYM_FUNC_START(__svm_vcpu_run) push %_ASM_BP @@ -54,17 +114,26 @@ SYM_FUNC_START(__svm_vcpu_run) * order compared to when they are needed. */

+ /* Accessed directly from the stack in RESTORE_HOST_SPEC_CTRL. */ + push %_ASM_ARG2 + /* Needed to restore access to percpu variables. */ __ASM_SIZE(push) PER_CPU_VAR(svm_data + SD_save_area_pa)

- /* Save @svm. */ + /* Finally save @svm. */ push %_ASM_ARG1

.ifnc _ASM_ARG1, _ASM_DI - /* Move @svm to RDI. */ + /* + * Stash @svm in RDI early. On 32-bit, arguments are in RAX, RCX + * and RDX which are clobbered by RESTORE_GUEST_SPEC_CTRL. + */ mov %_ASM_ARG1, %_ASM_DI .endif

+ /* Clobbers RAX, RCX, RDX. */ + RESTORE_GUEST_SPEC_CTRL + /* * Use a single vmcb (vmcb01 because it's always valid) for * context switching guest state via VMLOAD/VMSAVE, that way @@ -142,6 +211,9 @@ SYM_FUNC_START(__svm_vcpu_run) FILL_RETURN_BUFFER %_ASM_AX, RSB_CLEAR_LOOPS, X86_FEATURE_RETPOLINE #endif

+ /* Clobbers RAX, RCX, RDX. */ + RESTORE_HOST_SPEC_CTRL + /* * Mitigate RETBleed for AMD/Hygon Zen uarch. RET should be * untrained as soon as we exit the VM and are back to the @@ -177,6 +249,9 @@ SYM_FUNC_START(__svm_vcpu_run) xor %r15d, %r15d #endif

+ /* "Pop" @spec_ctrl_intercepted. */ + pop %_ASM_BX + pop %_ASM_BX

#ifdef CONFIG_X86_64 @@ -191,6 +266,9 @@ SYM_FUNC_START(__svm_vcpu_run) pop %_ASM_BP RET

+ RESTORE_GUEST_SPEC_CTRL_BODY + RESTORE_HOST_SPEC_CTRL_BODY + 10: cmpb $0, kvm_rebooting jne 2b ud2 @@ -214,6 +292,7 @@ SYM_FUNC_END(__svm_vcpu_run) /** * __svm_sev_es_vcpu_run - Run a SEV-ES vCPU via a transition to SVM guest mode * @svm: struct vcpu_svm * + * @spec_ctrl_intercepted: bool */ SYM_FUNC_START(__svm_sev_es_vcpu_run) push %_ASM_BP @@ -228,8 +307,30 @@ SYM_FUNC_START(__svm_sev_es_vcpu_run) #endif push %_ASM_BX

+ /* + * Save variables needed after vmexit on the stack, in inverse + * order compared to when they are needed. + */ + + /* Accessed directly from the stack in RESTORE_HOST_SPEC_CTRL. */ + push %_ASM_ARG2 + + /* Save @svm. */ + push %_ASM_ARG1 + +.ifnc _ASM_ARG1, _ASM_DI + /* + * Stash @svm in RDI early. On 32-bit, arguments are in RAX, RCX + * and RDX which are clobbered by RESTORE_GUEST_SPEC_CTRL. + */ + mov %_ASM_ARG1, %_ASM_DI +.endif + + /* Clobbers RAX, RCX, RDX. */ + RESTORE_GUEST_SPEC_CTRL + /* Get svm->current_vmcb->pa into RAX. */ - mov SVM_current_vmcb(%_ASM_ARG1), %_ASM_AX + mov SVM_current_vmcb(%_ASM_DI), %_ASM_AX mov KVM_VMCB_pa(%_ASM_AX), %_ASM_AX

/* Enter guest mode */ @@ -239,11 +340,17 @@ SYM_FUNC_START(__svm_sev_es_vcpu_run)

2: cli

+ /* Pop @svm to RDI, guest registers have been saved already. */ + pop %_ASM_DI + #ifdef CONFIG_RETPOLINE /* IMPORTANT: Stuff the RSB immediately after VM-Exit, before RET! */ FILL_RETURN_BUFFER %_ASM_AX, RSB_CLEAR_LOOPS, X86_FEATURE_RETPOLINE #endif

+ /* Clobbers RAX, RCX, RDX. */ + RESTORE_HOST_SPEC_CTRL + /* * Mitigate RETBleed for AMD/Hygon Zen uarch. RET should be * untrained as soon as we exit the VM and are back to the @@ -253,6 +360,9 @@ SYM_FUNC_START(__svm_sev_es_vcpu_run) */ UNTRAIN_RET

+ /* "Pop" @spec_ctrl_intercepted. */ + pop %_ASM_BX + pop %_ASM_BX

#ifdef CONFIG_X86_64 @@ -267,6 +377,9 @@ SYM_FUNC_START(__svm_sev_es_vcpu_run) pop %_ASM_BP RET

+ RESTORE_GUEST_SPEC_CTRL_BODY + RESTORE_HOST_SPEC_CTRL_BODY + 3: cmpb $0, kvm_rebooting jne 2b ud2