linux-kernel - Re: [PATCH v3 19/22] kvm: x86: Get/set expanded xstate buffer

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Date:   Wed, 29 Dec 2021 00:38:32 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jing Liu <jing2.liu@...el.com>
Cc:     x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, pbonzini@...hat.com, corbet@....net,
        shuah@...nel.org, jun.nakajima@...el.com, kevin.tian@...el.com,
        jing2.liu@...ux.intel.com, guang.zeng@...el.com,
        wei.w.wang@...el.com, yang.zhong@...el.com
Subject: Re: [PATCH v3 19/22] kvm: x86: Get/set expanded xstate buffer

Shortlog needs to have a verb somewhere.

On Wed, Dec 22, 2021, Jing Liu wrote:
> From: Guang Zeng <guang.zeng@...el.com>
> 
> When AMX is enabled it requires a larger xstate buffer than
> the legacy hardcoded 4KB one. Exising kvm ioctls

Existing

> (KVM_[G|S]ET_XSAVE under KVM_CAP_XSAVE) are not suitable for
> this purpose.

...

> Reuse KVM_SET_XSAVE for both old/new formats by reimplementing it to
> do properly-sized memdup_user() based on the guest fpu container.

I'm confused, the first sentence says KVM_SET_XSAVE isn't suitable, the second
says it can be reused with minimal effort.

> Also, update the api doc with the new KVM_GET_XSAVE2 ioctl.

...

> @@ -5367,7 +5382,9 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
>  		break;
>  	}
>  	case KVM_SET_XSAVE: {
> -		u.xsave = memdup_user(argp, sizeof(*u.xsave));
> +		int size = vcpu->arch.guest_fpu.uabi_size;

IIUC, reusing KVM_SET_XSAVE works by requiring that userspace use KVM_GET_XSAVE2
if userspace has expanded the guest FPU size by exposing relevant features to
the guest via guest CPUID.  If so, then that needs to be enforced in KVM_GET_XSAVE,
otherwise userspace will get subtle corruption by invoking the wrong ioctl, e.g.

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 2c9606380bca..5d2acbd52df5 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5386,6 +5386,10 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
                break;
        }
        case KVM_GET_XSAVE: {
+               r -EINVAL;
+               if (vcpu->arch.guest_fpu.uabi_size > sizeof(struct kvm_xsave))
+                       break;
+
                u.xsave = kzalloc(sizeof(struct kvm_xsave), GFP_KERNEL_ACCOUNT);
                r = -ENOMEM;
                if (!u.xsave)

> +
> +		u.xsave = memdup_user(argp, size);
>  		if (IS_ERR(u.xsave)) {
>  			r = PTR_ERR(u.xsave);
>  			goto out_nofree;
> @@ -5376,6 +5393,26 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
>  		r = kvm_vcpu_ioctl_x86_set_xsave(vcpu, u.xsave);
>  		break;
>  	}
> +
> +	case KVM_GET_XSAVE2: {
> +		int size = vcpu->arch.guest_fpu.uabi_size;
> +
> +		u.xsave = kzalloc(size, GFP_KERNEL_ACCOUNT);
> +		if (!u.xsave) {
> +			r = -ENOMEM;

I hate the odd patterns in this code as much as anyone, but for better or worse
the style throughout is:

		r = -ENOMEM;
		u.xsave = kzalloc(size, GFP_KERNEL_ACCOUNT);
		if (u.xsave)
			break;

> +			break;
> +		}
> +
> +		kvm_vcpu_ioctl_x86_get_xsave2(vcpu, u.buffer, size);
> +
> +		if (copy_to_user(argp, u.xsave, size)) {
> +			r = -EFAULT;
> +			break;

Same style thing here.

> +		}
> +		r = 0;
> +		break;
> +	}
> +
>  	case KVM_GET_XCRS: {
>  		u.xcrs = kzalloc(sizeof(struct kvm_xcrs), GFP_KERNEL_ACCOUNT);
>  		r = -ENOMEM;
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 1daa45268de2..9d1c01669560 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1131,6 +1131,7 @@ struct kvm_ppc_resize_hpt {
>  #define KVM_CAP_EXIT_ON_EMULATION_FAILURE 204
>  #define KVM_CAP_ARM_MTE 205
>  #define KVM_CAP_VM_MOVE_ENC_CONTEXT_FROM 206
> +#define KVM_CAP_XSAVE2 207
>  
>  #ifdef KVM_CAP_IRQ_ROUTING
>  
> @@ -1610,6 +1611,9 @@ struct kvm_enc_region {
>  #define KVM_S390_NORMAL_RESET	_IO(KVMIO,   0xc3)
>  #define KVM_S390_CLEAR_RESET	_IO(KVMIO,   0xc4)
>  
> +/* Available with KVM_CAP_XSAVE2 */
> +#define KVM_GET_XSAVE2		  _IOR(KVMIO,  0xcf, struct kvm_xsave)
> +
>  struct kvm_s390_pv_sec_parm {
>  	__u64 origin;
>  	__u64 length;
> -- 
> 2.27.0
>