Re: [patch 2/2] x86/kvm/vmx: Prevent split lock detection induced #AC wreckage

[Sean Christopherson]

On Thu, Apr 02, 2020 at 02:33:00PM +0200, Thomas Gleixner wrote:
> Without at least minimal handling for split lock detection induced #AC, VMX
> will just run into the same problem as the VMWare hypervisor, which was
> reported by Kenneth.
> 
> It will inject the #AC blindly into the guest whether the guest is prepared
> or not.
> 
> Add the minimal required handling for it:
> 
>   - Check guest state whether CR0.AM is enabled and EFLAGS.AC is set.  If
>     so, then the #AC originated from CPL3 and the guest has is prepared to
>     handle it. In this case it does not matter whether the #AC is due to a
>     split lock or a regular unaligned check.
> 
>  - Invoke a minimal split lock detection handler. If the host SLD mode is
>    sld_warn, then handle it in the same way as user space handling works:
>    Emit a warning, disable SLD and mark the current task with TIF_SLD.
>    With that resume the guest without injecting #AC.
> 
>    If the host mode is sld_fatal or sld_off, emit a warning and deliver
>    the exception to user space which can crash and burn itself.
> 
> Mark the module with MOD_INFO(sld_safe, "Y") so the module loader does not
> force SLD off.

Some comments below.  But, any objection to taking Xiaoyao's patches that
do effectively the same things, minus the MOD_INFO()?  I'll repost them in
reply to this thread.
 
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: "Kenneth R. Crudup" <kenny@xxxxxxxxx>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Fenghua Yu <fenghua.yu@xxxxxxxxx>
> Cc: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
> Cc: Nadav Amit <namit@xxxxxxxxxx>
> Cc: Thomas Hellstrom <thellstrom@xxxxxxxxxx>
> Cc: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> Cc: Tony Luck <tony.luck@xxxxxxxxx>
> ---
>  arch/x86/include/asm/cpu.h  |    1 +
>  arch/x86/kernel/cpu/intel.c |   28 +++++++++++++++++++++++-----
>  arch/x86/kvm/vmx/vmx.c      |   40 +++++++++++++++++++++++++++++++++++++---
>  3 files changed, 61 insertions(+), 8 deletions(-)
> 
> --- a/arch/x86/include/asm/cpu.h
> +++ b/arch/x86/include/asm/cpu.h
> @@ -44,6 +44,7 @@ unsigned int x86_stepping(unsigned int s
>  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(struct pt_regs *regs, long error_code);
> +extern bool handle_guest_split_lock(unsigned long ip);
>  extern void split_lock_validate_module_text(struct module *me, void *text, void *text_end);
>  #else
>  static inline void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c) {}
> --- a/arch/x86/kernel/cpu/intel.c
> +++ b/arch/x86/kernel/cpu/intel.c
> @@ -1102,13 +1102,10 @@ static void split_lock_init(void)
>  	split_lock_verify_msr(sld_state != sld_off);
>  }
>  
> -bool handle_user_split_lock(struct pt_regs *regs, long error_code)
> +static void split_lock_warn(unsigned long ip)
>  {
> -	if ((regs->flags & X86_EFLAGS_AC) || sld_state == sld_fatal)
> -		return false;
> -
>  	pr_warn_ratelimited("#AC: %s/%d took a split_lock trap at address: 0x%lx\n",
> -			    current->comm, current->pid, regs->ip);
> +			    current->comm, current->pid, ip);
>  
>  	/*
>  	 * Disable the split lock detection for this task so it can make
> @@ -1117,6 +1114,27 @@ bool handle_user_split_lock(struct pt_re
>  	 */
>  	sld_update_msr(false);
>  	set_tsk_thread_flag(current, TIF_SLD);
> +}
> +
> +bool handle_guest_split_lock(unsigned long ip)
> +{
> +	if (sld_state == sld_warn) {
> +		split_lock_warn(ip);
> +		return true;
> +	}
> +
> +	pr_warn_once("#AC: %s/%d %s split_lock trap at address: 0x%lx\n",
> +		     current->comm, current->pid,
> +		     sld_state == sld_fatal ? "fatal" : "bogus", ip);
> +	return false;
> +}
> +EXPORT_SYMBOL_GPL(handle_guest_split_lock);
> +
> +bool handle_user_split_lock(struct pt_regs *regs, long error_code)
> +{
> +	if ((regs->flags & X86_EFLAGS_AC) || sld_state == sld_fatal)
> +		return false;
> +	split_lock_warn(regs->ip);
>  	return true;
>  }
>  
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -65,6 +65,7 @@
>  
>  MODULE_AUTHOR("Qumranet");
>  MODULE_LICENSE("GPL");
> +MODULE_INFO(sld_safe, "Y");
>  
>  #ifdef MODULE
>  static const struct x86_cpu_id vmx_cpu_id[] = {
> @@ -4623,6 +4624,22 @@ static int handle_machine_check(struct k
>  	return 1;
>  }
>  
> +static bool guest_handles_ac(struct kvm_vcpu *vcpu)
> +{
> +	/*
> +	 * If guest has alignment checking enabled in CR0 and activated in
> +	 * eflags, then the #AC originated from CPL3 and the guest is able
> +	 * to handle it. It does not matter whether this is a regular or
> +	 * a split lock operation induced #AC.
> +	 */
> +	if (vcpu->arch.cr0 & X86_CR0_AM &&

Technically not required since KVM doesn't let the gets toggle CR0.AM at
will, but going through kvm_read_cr0{_bits}() is preferred.

> +	    vmx_get_rflags(vcpu) & X86_EFLAGS_AC)

I don't think this is correct.  A guest could trigger a split-lock #AC at
CPL0 with EFLAGS.AC=1 and CR0.AM=1, and then panic because it didn't expect
#AC at CPL0.

> +		return true;
> +
> +	/* Add guest SLD handling checks here once it's supported */
> +	return false;
> +}
> +
>  static int handle_exception_nmi(struct kvm_vcpu *vcpu)
>  {
>  	struct vcpu_vmx *vmx = to_vmx(vcpu);
> @@ -4688,9 +4705,6 @@ static int handle_exception_nmi(struct k
>  		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 &
> @@ -4719,6 +4733,26 @@ static int handle_exception_nmi(struct k
>  		kvm_run->debug.arch.pc = vmcs_readl(GUEST_CS_BASE) + rip;
>  		kvm_run->debug.arch.exception = ex_no;
>  		break;
> +	case AC_VECTOR:
> +		if (guest_handles_ac(vcpu)) {
> +			kvm_queue_exception_e(vcpu, AC_VECTOR, error_code);
> +			return 1;
> +		}
> +		/*
> +		 * Handle #AC caused by split lock detection. If the host
> +		 * mode is sld_warn, then it warns, marks current with
> +		 * TIF_SLD and disables split lock detection. So the guest
> +		 * can just continue.
> +		 *
> +		 * If the host mode is fatal, the handling code warned. Let
> +		 * qemu kill itself.
> +		 *
> +		 * If the host mode is off, then this #AC is bonkers and
> +		 * something is badly wrong. Let it fail as well.
> +		 */
> +		if (handle_guest_split_lock(kvm_rip_read(vcpu)))
> +			return 1;
> +		/* fall through */
>  	default:
>  		kvm_run->exit_reason = KVM_EXIT_EXCEPTION;
>  		kvm_run->ex.exception = ex_no;
>