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SubjectRe: [PATCH v2 6/6] x86: vmx: virtualize split lock detection
On Mon, Feb 03, 2020 at 11:16:08PM +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.
>
> 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
> ----------------------------------------------------------------------

This table and half the changelog is unnecessary and confusing. State that
SLD is exposed to the guest if and only if SLD is disabled in the host and
SMT is disabled (or the MSR is thread scoped), and leave it at that.

Reiterating everything that was implemented in previous patches does more
harm than good.

> Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.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;

Pretty sure disable_split_lock_detect won't exist, but if it does, don't
reuse it for emulating guest behavior. Keep the two things separate, i.e.
use vmx->msr_test_ctrl to track guest state and use the disable_sld to
track when the feature has been disabled for an ignorant guest.

> + 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);

The "!!" isn't necessary.

> +}
> +
> 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) ||

Again, check for SLD before AC, it's significantly cheaper.

> 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);

Ratelimiting change belongs in the earlier path. Moot point if this routes
through handle_user_split_lock().

> 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);

This will be massively simplified by letting TIF_SLD do the dirty work.
Since SLD will be exposed to the guest if and only if it's disabled in the
host, this becomes:

if (static_cpu_has(...) && vmx->msr_test_control)
wrmsrl(MSR_TEST_CTL,
this_cpu_read(msr_test_ctl_val) | vmx->msr_test_ctl);

__vmx_vcpu_run();


if (static_cpu_has(...) && vmx->msr_test_control)
wrmsrl(MSR_TEST_CTL, this_cpu_read(msr_test_ctl_val));

> + }
>
> /* 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;

And only if SLD is disabled, no?

> +
> + 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;
> case MSR_IA32_BNDCFGS:
> if (!kvm_mpx_supported())
> continue;
> --
> 2.23.0
>

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