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Date: Mon, 3 Feb 2020 23:58:42 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Paolo Bonzini <pbonzini@...hat.com>,
Sean Christopherson <sean.j.christopherson@...el.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
Andy Lutomirski <luto@...capital.net>
Cc: x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
David Laight <David.Laight@...lab.com>
Subject: Re: [PATCH v2 6/6] x86: vmx: virtualize split lock detection
On 2/3/2020 11:16 PM, 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.
>
> Only when host is sld_off, can guest control the hardware value of
> MSR_TEST_CTL, i.e., KVM loads guest's value into hardware when vcpu is
> running.
>
> The vmx->disable_split_lock_detect can be set to true after unhandled
> split_lock #AC in guest only when host is sld_warn mode. It's for not
> burnning old guest, of course malicous guest can exploit it for DoS
> attack.
>
> If want to prevent DoS attack from malicious guest, it must use sld_fatal
> mode in host. When host is sld_fatal, hardware value of
> MSR_TEST_CTL.SPLIT_LOCK_DETECT never cleared.
>
> Below summarizing how guest behaves if SMT is off and it's a linux guest:
>
> -----------------------------------------------------------------------
> Host | Guest | Guest behavior
> -----------------------------------------------------------------------
> 1. off | | same as in bare metal
> -----------------------------------------------------------------------
> 2. warn | off | hardware bit set initially. Once split lock happens,
> | | it sets vmx->disable_split_lock_detect, which leads
> | | hardware bit to be cleared when vcpu is running
> | | So, it's the same as in bare metal
> ---------------------------------------------------------------
> 3. | warn | - user space: get #AC when split lock, then clear
> | | MSR bit, but hardware bit is not cleared. #AC again,
> | | finally sets vmx->disable_split_lock_detect, which
> | | leads hardware bit to be cleared when vcpu is running;
> | | After the userspace process finishes, it sets vcpu's
> | | MSR_TEST_CTRL.SPLIT_LOCK_DETECT bit, which causes
> | | vmx->disable_split_lock_detect to be set false
> | | So it's somehow the same as in bare-metal
> | | - kernel: same as in bare metal.
> --------------------------------------------------------------
> 4. | fatal | same as in bare metal
> ----------------------------------------------------------------------
> 5. fatal| off | #AC reported to userspace
> --------------------------------------------------------------
> 6. | warn | - user space: get #AC when split lock, then clear
> | | MSR bit, but hardware bit is not cleared, #AC again,
> | | #AC reported to userspace
> | | - kernel: same as in bare metal, call die();
> -------------------------------------------------------------
> 7. | fatal | same as in bare metal
> ----------------------------------------------------------------------
>
> Signed-off-by: Xiaoyao Li <xiaoyao.li@...el.com>
> ---
> arch/x86/kvm/vmx/vmx.c | 72 +++++++++++++++++++++++++++++++++++-------
> arch/x86/kvm/vmx/vmx.h | 1 +
> arch/x86/kvm/x86.c | 13 ++++++--
> 3 files changed, 73 insertions(+), 13 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 93e3370c5f84..a0c3f579ecb6 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1781,6 +1781,26 @@ static int vmx_get_msr_feature(struct kvm_msr_entry *msr)
> }
> }
>
> +/*
> + * Note: for guest, feature split lock detection can only be enumerated by
> + * MSR_IA32_CORE_CAPS_SPLIT_LOCK_DETECT. 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 +1813,12 @@ 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 +1960,15 @@ 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 &&
> + (!guest_has_feature_split_lock_detect(vcpu) ||
> + data & ~vmx_msr_test_ctrl_valid_bits(vcpu)))
> + return 1;
> + if (data & MSR_TEST_CTRL_SPLIT_LOCK_DETECT)
> + vmx->disable_split_lock_detect = false;
> + vmx->msr_test_ctrl = data;
> + break;
> case MSR_EFER:
> ret = kvm_set_msr_common(vcpu, msr_info);
> break;
> @@ -4233,6 +4268,7 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>
> vmx->msr_ia32_umwait_control = 0;
>
> + vmx->msr_test_ctrl = 0;
> vmx->disable_split_lock_detect = false;
>
> vcpu->arch.microcode_version = 0x100000000ULL;
> @@ -4565,6 +4601,11 @@ 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);
> @@ -4660,8 +4701,8 @@ static int handle_exception_nmi(struct kvm_vcpu *vcpu)
> break;
> case AC_VECTOR:
> /*
> - * Inject #AC back to guest only when legacy alignment check
> - * enabled.
> + * Inject #AC back to guest only when guest is expecting it,
> + * i.e., legacy alignment check or split lock #AC enabled.
> * Otherwise, it must be an unexpected split-lock #AC for guest
> * since KVM keeps hardware SPLIT_LOCK_DETECT bit unchanged
> * when vcpu is running.
> @@ -4674,12 +4715,13 @@ static int handle_exception_nmi(struct kvm_vcpu *vcpu)
> * similar as sending SIGBUS.
> */
> if (guest_cpu_alignment_check_enabled(vcpu) ||
> + guest_cpu_split_lock_detect_enabled(vmx) ||
> WARN_ON(get_split_lock_detect_state() == sld_off)) {
> kvm_queue_exception_e(vcpu, AC_VECTOR, error_code);
> return 1;
> }
> if (get_split_lock_detect_state() == sld_warn) {
> - pr_warn("kvm: split lock #AC happened in %s [%d]\n",
> + pr_warn_ratelimited("kvm: split lock #AC happened in %s [%d]\n",
> current->comm, current->pid);
> vmx->disable_split_lock_detect = true;
> return 1;
> @@ -6491,6 +6533,7 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
> {
> struct vcpu_vmx *vmx = to_vmx(vcpu);
> unsigned long cr3, cr4;
> + bool host_sld_enabled, guest_sld_enabled;
>
> /* Record the guest's net vcpu time for enforced NMI injections. */
> if (unlikely(!enable_vnmi &&
> @@ -6562,10 +6605,15 @@ 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) &&
> - unlikely(vmx->disable_split_lock_detect) &&
> - !test_tsk_thread_flag(current, TIF_SLD))
> - split_lock_detect_set(false);
> + if (static_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT)) {
> + host_sld_enabled = get_split_lock_detect_state() &&
> + !test_tsk_thread_flag(current, TIF_SLD);
> + guest_sld_enabled = guest_cpu_split_lock_detect_enabled(vmx);
> + if (host_sld_enabled && unlikely(vmx->disable_split_lock_detect))
> + split_lock_detect_set(false);
> + else if (!host_sld_enabled && guest_sld_enabled)
> + split_lock_detect_set(true);
> + }
>
> /* L1D Flush includes CPU buffer clear to mitigate MDS */
> if (static_branch_unlikely(&vmx_l1d_should_flush))
> @@ -6601,10 +6649,12 @@ 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) &&
> - unlikely(vmx->disable_split_lock_detect) &&
> - !test_tsk_thread_flag(current, TIF_SLD))
> - split_lock_detect_set(true);
> + if (static_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT)) {
> + if (host_sld_enabled && unlikely(vmx->disable_split_lock_detect))
> + split_lock_detect_set(true);
> + else if (!host_sld_enabled && guest_sld_enabled)
> + split_lock_detect_set(false);
> + }
>
> /* All fields are clean at this point */
> if (static_branch_unlikely(&enable_evmcs))
> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> index 912eba66c5d5..c36c663f4bae 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 a97a8f5dd1df..56e799981d53 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 +5264,10 @@ 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;
sorry, forget break;
> case MSR_IA32_BNDCFGS:
> if (!kvm_mpx_supported())
> continue;
>
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