linux-kernel - Re: [PATCH v5 06/13] KVM: Use kvm_userspace_memory_region

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Date:   Mon, 28 Mar 2022 22:26:55 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Chao Peng <chao.p.peng@...ux.intel.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-api@...r.kernel.org, qemu-devel@...gnu.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jeff Layton <jlayton@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...nel.org>,
        Steven Price <steven.price@....com>,
        "Maciej S . Szmigiero" <mail@...iej.szmigiero.name>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        luto@...nel.org, jun.nakajima@...el.com, dave.hansen@...el.com,
        ak@...ux.intel.com, david@...hat.com
Subject: Re: [PATCH v5 06/13] KVM: Use kvm_userspace_memory_region_ext

On Thu, Mar 10, 2022, Chao Peng wrote:
> @@ -4476,14 +4477,23 @@ static long kvm_vm_ioctl(struct file *filp,
>  		break;
>  	}
>  	case KVM_SET_USER_MEMORY_REGION: {
> -		struct kvm_userspace_memory_region kvm_userspace_mem;
> +		struct kvm_userspace_memory_region_ext region_ext;

It's probably a good idea to zero initialize the full region to avoid consuming
garbage stack data if there's a bug and an _ext field is accessed without first
checking KVM_MEM_PRIVATE.  I'm usually opposed to unnecessary initialization, but
this seems like something we could screw up quite easily.

>  		r = -EFAULT;
> -		if (copy_from_user(&kvm_userspace_mem, argp,
> -						sizeof(kvm_userspace_mem)))
> +		if (copy_from_user(&region_ext, argp,
> +				sizeof(struct kvm_userspace_memory_region)))
>  			goto out;
> +		if (region_ext.region.flags & KVM_MEM_PRIVATE) {
> +			int offset = offsetof(
> +				struct kvm_userspace_memory_region_ext,
> +				private_offset);
> +			if (copy_from_user(&region_ext.private_offset,
> +					   argp + offset,
> +					   sizeof(region_ext) - offset))

In this patch, KVM_MEM_PRIVATE should result in an -EINVAL as it's not yet
supported.  Copying the _ext on KVM_MEM_PRIVATE belongs in the "Expose KVM_MEM_PRIVATE"
patch.

Mechnically, what about first reading flags via get_user(), and then doing a single
copy_from_user()?  It's technically more work in the common case, and requires an
extra check to guard against TOCTOU attacks, but this isn't a fast path by any means
and IMO the end result makes it easier to understand the relationship between
KVM_MEM_PRIVATE and the two different structs.

E.g.

	case KVM_SET_USER_MEMORY_REGION: {
		struct kvm_user_mem_region region;
		unsigned long size;
		u32 flags;

		memset(&region, 0, sizeof(region));

		r = -EFAULT;
		if (get_user(flags, (u32 __user *)(argp + offsetof(typeof(region), flags))))
			goto out;

		if (flags & KVM_MEM_PRIVATE)
			size = sizeof(struct kvm_userspace_memory_region_ext);
		else
			size = sizeof(struct kvm_userspace_memory_region);
		if (copy_from_user(&region, argp, size))
			goto out;

		r = -EINVAL;
		if ((flags ^ region.flags) & KVM_MEM_PRIVATE)
			goto out;

		r = kvm_vm_ioctl_set_memory_region(kvm, &region);
		break;
	}

> +				goto out;
> +		}
>  
> -		r = kvm_vm_ioctl_set_memory_region(kvm, &kvm_userspace_mem);
> +		r = kvm_vm_ioctl_set_memory_region(kvm, &region_ext);
>  		break;
>  	}
>  	case KVM_GET_DIRTY_LOG: {
> -- 
> 2.17.1
>