Re: [PATCH v2 4/6] kvm: vmx: Extend VMX's #AC handding for split lock in guest

[Sean Christopherson]

On Mon, Feb 03, 2020 at 11:16:06PM +0800, Xiaoyao Li wrote:
> There are two types of #AC can be generated in Intel CPUs:
>  1. legacy alignment check #AC;
>  2. split lock #AC;
> 
> Legacy alignment check #AC can be injected to guest if guest has enabled
> alignemnet check.
> 
> When host enables split lock detection, i.e., split_lock_detect != off,
> guest will receive an unexpected #AC when there is a split lock happens
> since KVM doesn't virtualize this feature to guest hardware value of
> MSR_TEST_CTRL.SPLIT_LOCK_DETECT bit stays unchanged when vcpu is running.
> 
> Since old guests lack split_lock #AC handler and may have split lock buges.
> To make them survive from split lock, applying the similar policy
> as host's split lock detect configuration:
>  - host split lock detect is sld_warn:
>    warn the split lock happened in guest, and disabling split lock
>    detect during vcpu is running to allow the guest to continue running.
>  - host split lock detect is sld_fatal:
>    forwarding #AC to userspace, somewhat similar as sending SIGBUS.
> 
> Please note:
> 1. If sld_warn and SMT is enabled, the split lock in guest's vcpu
> leads to disable split lock detect on the sibling CPU thread during
> the vcpu is running.
> 
> 2. When host is sld_warn, it allows guest to generate split lock which also
> opens the door for malicious guest to do DoS attack. It is same that in
> sld_warn mode, userspace application can do DoS attack.
> 
> 3. If want to prevent DoS attack from guest, host must use sld_fatal mode.
> 
> Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
> ---
>  arch/x86/kvm/vmx/vmx.c | 48 +++++++++++++++++++++++++++++++++++++++---
>  arch/x86/kvm/vmx/vmx.h |  3 +++
>  2 files changed, 48 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index c475fa2aaae0..93e3370c5f84 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -4233,6 +4233,8 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>  
>  	vmx->msr_ia32_umwait_control = 0;
>  
> +	vmx->disable_split_lock_detect = false;
> +

I see no reason to give special treatment to RESET/INIT, i.e. leave the
flag set.  vCPUs are zeroed on allocation.

>  	vcpu->arch.microcode_version = 0x100000000ULL;
>  	vmx->vcpu.arch.regs[VCPU_REGS_RDX] = get_rdx_init_val();
>  	vmx->hv_deadline_tsc = -1;
> @@ -4557,6 +4559,12 @@ static int handle_machine_check(struct kvm_vcpu *vcpu)
>  	return 1;
>  }
>  
> +static inline bool guest_cpu_alignment_check_enabled(struct kvm_vcpu *vcpu)
> +{
> +	return vmx_get_cpl(vcpu) == 3 && kvm_read_cr0_bits(vcpu, X86_CR0_AM) &&
> +	       (kvm_get_rflags(vcpu) & X86_EFLAGS_AC);
> +}
> +
>  static int handle_exception_nmi(struct kvm_vcpu *vcpu)
>  {
>  	struct vcpu_vmx *vmx = to_vmx(vcpu);
> @@ -4622,9 +4630,6 @@ static int handle_exception_nmi(struct kvm_vcpu *vcpu)
>  		return handle_rmode_exception(vcpu, ex_no, error_code);
>  
>  	switch (ex_no) {
> -	case AC_VECTOR:
> -		kvm_queue_exception_e(vcpu, AC_VECTOR, error_code);
> -		return 1;
>  	case DB_VECTOR:
>  		dr6 = vmcs_readl(EXIT_QUALIFICATION);
>  		if (!(vcpu->guest_debug &
> @@ -4653,6 +4658,33 @@ static int handle_exception_nmi(struct kvm_vcpu *vcpu)
>  		kvm_run->debug.arch.pc = vmcs_readl(GUEST_CS_BASE) + rip;
>  		kvm_run->debug.arch.exception = ex_no;
>  		break;
> +	case AC_VECTOR:
> +		/*
> +		 * Inject #AC back to guest only when legacy alignment check
> +		 * 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.
> +		 *  - If sld_state == sld_warn, treat it similar as user space
> +		 *    process that warn and allow it to continue running.
> +		 *    In this case, setting vmx->diasble_split_lock_detect to
> +		 *    true so that it will toggle MSR_TEST.SPLIT_LOCK_DETECT
> +		 *    bit during every following VM Entry and Exit;
> +		 *  - If sld_state == sld_fatal, it forwards #AC to userspace,
> +		 *    similar as sending SIGBUS.
> +		 */
> +		if (guest_cpu_alignment_check_enabled(vcpu) ||
> +		    WARN_ON(get_split_lock_detect_state() == sld_off)) {

Eh, I'd omit the WARN.  And invert the ordering to avoid multiple VMREADs
when SLD is disabled, which will be the common case.

> +			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",
> +				current->comm, current->pid);

Set TIF_SLD and the MSR bit, then __switch_to_xtra() will automatically
handle writing the MSR when necessary.

Even better would be to export handle_user_split_lock() and call that
directly.  The EFLAGS.AC logic in handle_user_split_lock() can be moved out
to do_alignment_check() to avoid that complication; arguably that should be
done in the initial SLD patch.

> +			vmx->disable_split_lock_detect = true;
> +			return 1;
> +		}
> +		/* fall through*/
>  	default:
>  		kvm_run->exit_reason = KVM_EXIT_EXCEPTION;
>  		kvm_run->ex.exception = ex_no;
> @@ -6530,6 +6562,11 @@ 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);
> +
>  	/* L1D Flush includes CPU buffer clear to mitigate MDS */
>  	if (static_branch_unlikely(&vmx_l1d_should_flush))
>  		vmx_l1d_flush(vcpu);
> @@ -6564,6 +6601,11 @@ 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) &&
> +	    unlikely(vmx->disable_split_lock_detect) &&
> +	    !test_tsk_thread_flag(current, TIF_SLD))
> +		split_lock_detect_set(true);

Manually calling split_lock_detect_set() in vmx_vcpu_run() is unnecessary.
The MSR only needs to be written on the initial #AC, after that KVM can
rely on the stickiness of TIF_SLD to ensure the MSR is set correctly when
control transfer to/from this vCPU.

> +
>  	/* 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..912eba66c5d5 100644
> --- a/arch/x86/kvm/vmx/vmx.h
> +++ b/arch/x86/kvm/vmx/vmx.h
> @@ -274,6 +274,9 @@ struct vcpu_vmx {
>  
>  	bool req_immediate_exit;
>  
> +	/* Disable split-lock detection when running the vCPU */
> +	bool disable_split_lock_detect;
> +
>  	/* Support for PML */
>  #define PML_ENTITY_NUM		512
>  	struct page *pml_pg;
> -- 
> 2.23.0
>