Re: [PATCH v3 8/8] x86: vmx: virtualize split lock detection

Xiaoyao Li
2020-03-05T14:16:58Z

On 3/4/2020 3:30 AM, Sean Christopherson wrote:
> On Thu, Feb 06, 2020 at 03:04:12PM +0800, Xiaoyao Li wrote:
>> Due to the fact that MSR_TEST_CTRL is per-core scope, i.e., the sibling
>> threads in the same physical CPU core share the same MSR, only
>> advertising feature split lock detection to guest when SMT is disabled
>> or unsupported for simplicitly.
>>
>> Below summarizing how guest behaves of different host configuration:
>>
>> sld_fatal - MSR_TEST_CTL.SDL is forced on and is sticky from the guest's
>> perspective (so the guest can detect a forced fatal mode).
>>
>> sld_warn - SLD is exposed to the guest. MSR_TEST_CTRL.SLD is left on
>> until an #AC is intercepted with MSR_TEST_CTRL.SLD=0 in the
>> guest, at which point normal sld_warn rules apply. If a vCPU
>> associated with the task does VM-Enter with
>> MSR_TEST_CTRL.SLD=1, TIF_SLD is reset and the cycle begins
>> anew.
>>
>> sld_off - When set by the guest, MSR_TEST_CTL.SLD is set on VM-Entry
>> and cleared on VM-Exit if guest enables SLD.
>>
>> Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
>> ---
>> arch/x86/include/asm/cpu.h | 2 ++
>> arch/x86/kernel/cpu/intel.c | 7 +++++
>> arch/x86/kvm/vmx/vmx.c | 59 +++++++++++++++++++++++++++++++++++--
>> arch/x86/kvm/vmx/vmx.h | 1 +
>> arch/x86/kvm/x86.c | 14 +++++++--
>> 5 files changed, 79 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/cpu.h b/arch/x86/include/asm/cpu.h
>> index f5172dbd3f01..2920de10e72c 100644
>> --- a/arch/x86/include/asm/cpu.h
>> +++ b/arch/x86/include/asm/cpu.h
>> @@ -46,6 +46,7 @@ unsigned int x86_stepping(unsigned int sig);
>> extern void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c);
>> extern void switch_to_sld(unsigned long tifn);
>> extern bool handle_user_split_lock(unsigned long ip);
>> +extern void sld_turn_back_on(void);
>> extern bool split_lock_detect_enabled(void);
>> extern bool split_lock_detect_fatal(void);
>> #else
>> @@ -55,6 +56,7 @@ static inline bool handle_user_split_lock(unsigned long ip)
>> {
>> return false;
>> }
>> +static inline void sld_turn_back_on(void) {}
>> static inline bool split_lock_detect_enabled(void) { return false; }
>> static inline bool split_lock_detect_fatal(void) { return false; }
>> #endif
>> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
>> index b67b46ea66df..28dc1141152b 100644
>> --- a/arch/x86/kernel/cpu/intel.c
>> +++ b/arch/x86/kernel/cpu/intel.c
>> @@ -1087,6 +1087,13 @@ bool handle_user_split_lock(unsigned long ip)
>> }
>> EXPORT_SYMBOL_GPL(handle_user_split_lock);
>>
>> +void sld_turn_back_on(void)
>> +{
>> + __sld_msr_set(true);
>> + clear_tsk_thread_flag(current, TIF_SLD);
>> +}
>> +EXPORT_SYMBOL_GPL(sld_turn_back_on);
>> +
>> /*
>> * This function is called only when switching between tasks with
>> * different split-lock detection modes. It sets the MSR for the
>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> index 822211975e6c..8735bf0f3dfd 100644
>> --- a/arch/x86/kvm/vmx/vmx.c
>> +++ b/arch/x86/kvm/vmx/vmx.c
>> @@ -1781,6 +1781,25 @@ static int vmx_get_msr_feature(struct kvm_msr_entry *msr)
>> }
>> }
>>
>> +/*
>> + * Note: for guest, feature split lock detection can only be enumerated through
>> + * MSR_IA32_CORE_CAPS_SPLIT_LOCK_DETECT bit. The FMS enumeration is invalid.
>> + */
>> +static inline bool guest_has_feature_split_lock_detect(struct kvm_vcpu *vcpu)
>> +{
>> + return vcpu->arch.core_capabilities & MSR_IA32_CORE_CAPS_SPLIT_LOCK_DETECT;
>> +}
>> +
>> +static inline u64 vmx_msr_test_ctrl_valid_bits(struct kvm_vcpu *vcpu)
>> +{
>> + u64 valid_bits = 0;
>> +
>> + if (guest_has_feature_split_lock_detect(vcpu))
>> + valid_bits |= MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
>> +
>> + return valid_bits;
>> +}
>> +
>> /*
>> * Reads an msr value (of 'msr_index') into 'pdata'.
>> * Returns 0 on success, non-0 otherwise.
>> @@ -1793,6 +1812,12 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>> u32 index;
>>
>> switch (msr_info->index) {
>> + case MSR_TEST_CTRL:
>> + if (!msr_info->host_initiated &&
>> + !guest_has_feature_split_lock_detect(vcpu))
>> + return 1;
>> + msr_info->data = vmx->msr_test_ctrl;
>> + break;
>> #ifdef CONFIG_X86_64
>> case MSR_FS_BASE:
>> msr_info->data = vmcs_readl(GUEST_FS_BASE);
>> @@ -1934,6 +1959,13 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>> u32 index;
>>
>> switch (msr_index) {
>> + case MSR_TEST_CTRL:
>> + if (!msr_info->host_initiated &&
>
> Host initiated writes need to be validated against
> kvm_get_core_capabilities(), otherwise userspace can enable SLD when it's
> supported in hardware and the kernel, but can't be safely exposed to the
> guest due to SMT being on.

How about making the whole check like this:

if (!msr_info->host_initiated &&
(!guest_has_feature_split_lock_detect(vcpu))
return 1;

if (data & ~vmx_msr_test_ctrl_valid_bits(vcpu))
return 1;

>> + (!guest_has_feature_split_lock_detect(vcpu) ||
>> + data & ~vmx_msr_test_ctrl_valid_bits(vcpu)))
>> + return 1;
>> + vmx->msr_test_ctrl = data;
>> + break;
>> case MSR_EFER:
>> ret = kvm_set_msr_common(vcpu, msr_info);
>> break;
>> @@ -4230,6 +4262,7 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>>
>> vmx->rmode.vm86_active = 0;
>> vmx->spec_ctrl = 0;
>> + vmx->msr_test_ctrl = 0;
>>
>> vmx->msr_ia32_umwait_control = 0;
>>
>> @@ -4563,6 +4596,11 @@ static inline bool guest_cpu_alignment_check_enabled(struct kvm_vcpu *vcpu)
>> (kvm_get_rflags(vcpu) & X86_EFLAGS_AC);
>> }
>>
>> +static inline bool guest_cpu_split_lock_detect_enabled(struct vcpu_vmx *vmx)
>> +{
>> + return vmx->msr_test_ctrl & MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
>> +}
>> +
>> static int handle_exception_nmi(struct kvm_vcpu *vcpu)
>> {
>> struct vcpu_vmx *vmx = to_vmx(vcpu);
>> @@ -4658,8 +4696,9 @@ static int handle_exception_nmi(struct kvm_vcpu *vcpu)
>> break;
>> case AC_VECTOR:
>> /*
>> - * Inject #AC back to guest only when guest enables legacy
>> - * alignment check.
>> + * Inject #AC back to guest only when guest is expecting it,
>> + * i.e., guest enables legacy alignment check or split lock
>> + * detection.
>> * Otherwise, it must be an unexpected split lock #AC of guest
>> * since hardware SPLIT_LOCK_DETECT bit keeps unchanged set
>> * when vcpu is running. In this case, treat guest the same as
>> @@ -4670,6 +4709,7 @@ static int handle_exception_nmi(struct kvm_vcpu *vcpu)
>> * similar as sending SIGBUS.
>> */
>> if (!split_lock_detect_enabled() ||
>> + guest_cpu_split_lock_detect_enabled(vmx) ||
>> guest_cpu_alignment_check_enabled(vcpu)) {
>> kvm_queue_exception_e(vcpu, AC_VECTOR, error_code);
>> return 1;
>> @@ -6555,6 +6595,16 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
>> */
>> x86_spec_ctrl_set_guest(vmx->spec_ctrl, 0);
>>
>> + if (static_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT) &&
>> + guest_cpu_split_lock_detect_enabled(vmx)) {
>> + if (test_thread_flag(TIF_SLD))
>> + sld_turn_back_on();
>> + else if (!split_lock_detect_enabled())
>> + wrmsrl(MSR_TEST_CTRL,
>> + this_cpu_read(msr_test_ctrl_cache) |
>> + MSR_TEST_CTRL_SPLIT_LOCK_DETECT);
>> + }
>> +
>> /* L1D Flush includes CPU buffer clear to mitigate MDS */
>> if (static_branch_unlikely(&vmx_l1d_should_flush))
>> vmx_l1d_flush(vcpu);
>> @@ -6589,6 +6639,11 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
>>
>> x86_spec_ctrl_restore_host(vmx->spec_ctrl, 0);
>>
>> + if (static_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT) &&
>> + guest_cpu_split_lock_detect_enabled(vmx) &&
>> + !split_lock_detect_enabled())
>> + wrmsrl(MSR_TEST_CTRL, this_cpu_read(msr_test_ctrl_cache));
>> +
>> /* All fields are clean at this point */
>> if (static_branch_unlikely(&enable_evmcs))
>> current_evmcs->hv_clean_fields |=
>> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
>> index 7f42cf3dcd70..4cb8075e0b2a 100644
>> --- a/arch/x86/kvm/vmx/vmx.h
>> +++ b/arch/x86/kvm/vmx/vmx.h
>> @@ -222,6 +222,7 @@ struct vcpu_vmx {
>> #endif
>>
>> u64 spec_ctrl;
>> + u64 msr_test_ctrl;
>> u32 msr_ia32_umwait_control;
>>
>> u32 secondary_exec_control;
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index ed16644289a3..a3bb09319526 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -1163,7 +1163,7 @@ static const u32 msrs_to_save_all[] = {
>> #endif
>> MSR_IA32_TSC, MSR_IA32_CR_PAT, MSR_VM_HSAVE_PA,
>> MSR_IA32_FEAT_CTL, MSR_IA32_BNDCFGS, MSR_TSC_AUX,
>> - MSR_IA32_SPEC_CTRL,
>> + MSR_IA32_SPEC_CTRL, MSR_TEST_CTRL,
>> MSR_IA32_RTIT_CTL, MSR_IA32_RTIT_STATUS, MSR_IA32_RTIT_CR3_MATCH,
>> MSR_IA32_RTIT_OUTPUT_BASE, MSR_IA32_RTIT_OUTPUT_MASK,
>> MSR_IA32_RTIT_ADDR0_A, MSR_IA32_RTIT_ADDR0_B,
>> @@ -1345,7 +1345,12 @@ static u64 kvm_get_arch_capabilities(void)
>>
>> static u64 kvm_get_core_capabilities(void)
>> {
>> - return 0;
>> + u64 data = 0;
>> +
>> + if (boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT) && !cpu_smt_possible())
>> + data |= MSR_IA32_CORE_CAPS_SPLIT_LOCK_DETECT;
>> +
>> + return data;
>> }
>>
>> static int kvm_get_msr_feature(struct kvm_msr_entry *msr)
>> @@ -5259,6 +5264,11 @@ static void kvm_init_msr_list(void)
>> * to the guests in some cases.
>> */
>> switch (msrs_to_save_all[i]) {
>> + case MSR_TEST_CTRL:
>> + if (!(kvm_get_core_capabilities() &
>> + MSR_IA32_CORE_CAPS_SPLIT_LOCK_DETECT))
>> + continue;
>> + break;
>> case MSR_IA32_BNDCFGS:
>> if (!kvm_mpx_supported())
>> continue;
>> --
>> 2.23.0
>>