linux-kernel - Re: [PATCH Part2 v5 26/45] KVM: SVM: Mark the private vma unmerable for SEV-SNP guests

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Date:   Tue, 12 Oct 2021 18:46:57 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>, tony.luck@...el.com,
        marcorr@...gle.com, sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH Part2 v5 26/45] KVM: SVM: Mark the private vma unmerable
 for SEV-SNP guests

On Fri, Aug 20, 2021, Brijesh Singh wrote:
> 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.

To be safe from what?  Does the !PRESENT #PF crash the kernel?

> Signed-off-by: Brijesh Singh <brijesh.singh@....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 4b126598b7aa..dcef0ae5f8e4 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -18,11 +18,13 @@
>  #include <linux/processor.h>
>  #include <linux/trace_events.h>
>  #include <linux/sev.h>
> +#include <linux/ksm.h>
>  #include <asm/fpu/internal.h>
>  
>  #include <asm/pkru.h>
>  #include <asm/trapnr.h>
>  #include <asm/sev.h>
> +#include <asm/mman.h>
>  
>  #include "x86.h"
>  #include "svm.h"
> @@ -1683,6 +1685,30 @@ 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;
> @@ -1707,6 +1733,12 @@ 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);

This does not, and practically speaking cannot, work.  There are multiple TOCTOU
bugs, here and in __snp_handle_page_state_change().  Userspace can madvise() the
range at any later point, munmap()/mmap() the entire range, mess with the memslots
in the PSC case, and so on and so forth.  Relying on MADV_UNMERGEABLE for functional
correctness simply cannot work in KVM, barring mmu_notifier and a big pile of code.

> +	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
> -- 
> 2.17.1
>