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Date:   Wed, 22 Jun 2022 11:29:40 +0100
From:   "Dr. David Alan Gilbert" <dgilbert@redhat.com>
To:     Ashish Kalra <Ashish.Kalra@amd.com>
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Subject: Re: [PATCH Part2 v6 27/49] KVM: SVM: Mark the private vma unmerable
 for SEV-SNP guests
Message-ID: <YrLvFB+cyBbXMbrB@work-vm>
References: <cover.1655761627.git.ashish.kalra@amd.com>
 <bb10f0a4c5eb13a5338f77ef34f08f1190d4ae30.1655761627.git.ashish.kalra@amd.com>
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* Ashish Kalra (Ashish.Kalra@amd.com) wrote:
> From: Brijesh Singh <brijesh.singh@amd.com>
> 
> When SEV-SNP is enabled, the guest private pages are added in the RMP
> table; while adding the pages, the rmp_make_private() unmaps the pages
> from the direct map. If KSM attempts to access those unmapped pages then
> it will trigger #PF (page-not-present).
> 
> Encrypted guest pages cannot be shared between the process, so an
> userspace should not mark the region mergeable but to be safe, mark the
> process vma unmerable before adding the pages in the RMP table.
                   ^
                   Typo 'unmergable' (also in title)

> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> ---
>  arch/x86/kvm/svm/sev.c | 32 ++++++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index b5f0707d7ed6..a9461d352eda 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -19,11 +19,13 @@
>  #include <linux/trace_events.h>
>  #include <linux/hugetlb.h>
>  #include <linux/sev.h>
> +#include <linux/ksm.h>
>  
>  #include <asm/pkru.h>
>  #include <asm/trapnr.h>
>  #include <asm/fpu/xcr.h>
>  #include <asm/sev.h>
> +#include <asm/mman.h>
>  
>  #include "x86.h"
>  #include "svm.h"
> @@ -1965,6 +1967,30 @@ static bool is_hva_registered(struct kvm *kvm, hva_t hva, size_t len)
>  	return false;
>  }
>  
> +static int snp_mark_unmergable(struct kvm *kvm, u64 start, u64 size)
> +{
> +	struct vm_area_struct *vma;
> +	u64 end = start + size;
> +	int ret;
> +
> +	do {
> +		vma = find_vma_intersection(kvm->mm, start, end);
> +		if (!vma) {
> +			ret = -EINVAL;
> +			break;
> +		}
> +
> +		ret = ksm_madvise(vma, vma->vm_start, vma->vm_end,
> +				  MADV_UNMERGEABLE, &vma->vm_flags);
> +		if (ret)
> +			break;
> +
> +		start = vma->vm_end;
> +	} while (end > vma->vm_end);
> +
> +	return ret;
> +}
> +
>  static int snp_launch_update(struct kvm *kvm, struct kvm_sev_cmd *argp)
>  {
>  	struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> @@ -1989,6 +2015,12 @@ static int snp_launch_update(struct kvm *kvm, struct kvm_sev_cmd *argp)
>  	if (!is_hva_registered(kvm, params.uaddr, params.len))
>  		return -EINVAL;
>  
> +	mmap_write_lock(kvm->mm);
> +	ret = snp_mark_unmergable(kvm, params.uaddr, params.len);
> +	mmap_write_unlock(kvm->mm);
> +	if (ret)
> +		return -EFAULT;
> +
>  	/*
>  	 * The userspace memory is already locked so technically we don't
>  	 * need to lock it again. Later part of the function needs to know
> -- 
> 2.25.1
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK