linux-kernel - Re: [PATCH Part2 RFC v4 25/40] KVM: SVM: Reclaim the guest pages when SEV-SNP VM terminates

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Date:   Fri, 16 Jul 2021 20:09:19 +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-efi@...r.kernel.org, platform-driver-x86@...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>, tony.luck@...el.com,
        npmccallum@...hat.com, brijesh.ksingh@...il.com
Subject: Re: [PATCH Part2 RFC v4 25/40] KVM: SVM: Reclaim the guest pages
 when SEV-SNP VM terminates

On Wed, Jul 07, 2021, Brijesh Singh wrote:
> The guest pages of the SEV-SNP VM maybe added as a private page in the
> RMP entry (assigned bit is set). The guest private pages must be
> transitioned to the hypervisor state before its freed.

Isn't this patch needed much earlier in the series, i.e. when the first RMPUPDATE
usage goes in?

> Signed-off-by: Brijesh Singh <brijesh.singh@....com>
> ---
>  arch/x86/kvm/svm/sev.c | 39 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 39 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 1f0635ac9ff9..4468995dd209 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -1940,6 +1940,45 @@ find_enc_region(struct kvm *kvm, struct kvm_enc_region *range)
>  static void __unregister_enc_region_locked(struct kvm *kvm,
>  					   struct enc_region *region)
>  {
> +	struct rmpupdate val = {};
> +	unsigned long i, pfn;
> +	struct rmpentry *e;
> +	int level, rc;
> +
> +	/*
> +	 * The guest memory pages are assigned in the RMP table. Unassign it
> +	 * before releasing the memory.
> +	 */
> +	if (sev_snp_guest(kvm)) {
> +		for (i = 0; i < region->npages; i++) {
> +			pfn = page_to_pfn(region->pages[i]);
> +
> +			if (need_resched())
> +				schedule();

This can simply be "cond_resched();"

> +
> +			e = snp_lookup_page_in_rmptable(region->pages[i], &level);
> +			if (unlikely(!e))
> +				continue;
> +
> +			/* If its not a guest assigned page then skip it. */
> +			if (!rmpentry_assigned(e))
> +				continue;
> +
> +			/* Is the page part of a 2MB RMP entry? */
> +			if (level == PG_LEVEL_2M) {
> +				val.pagesize = RMP_PG_SIZE_2M;
> +				pfn &= ~(KVM_PAGES_PER_HPAGE(PG_LEVEL_2M) - 1);
> +			} else {
> +				val.pagesize = RMP_PG_SIZE_4K;

This raises yet more questions (for me) as to the interaction between Page-Size
and Hyperivsor-Owned flags in the RMP.  It also raises questions on the correctness
of zeroing the RMP entry if KVM_SEV_SNP_LAUNCH_START (in the previous patch).

> +			}
> +
> +			/* Transition the page to hypervisor owned. */
> +			rc = rmpupdate(pfn_to_page(pfn), &val);
> +			if (rc)
> +				pr_err("Failed to release pfn 0x%lx ret=%d\n", pfn, rc);

This is not robust, e.g. KVM will unpin the memory and release it back to the
kernel with a stale RMP entry.  Shouldn't this be a WARN+leak situation?

> +		}
> +	}
> +
>  	sev_unpin_memory(kvm, region->pages, region->npages);
>  	list_del(&region->list);
>  	kfree(region);
> -- 
> 2.17.1
>