linux-kernel - Re: [PATCH v14 055/113] KVM: TDX: TDP MMU TDX support

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Date:   Thu, 8 Jun 2023 13:55:03 -0700
From:   Isaku Yamahata <isaku.yamahata@...il.com>
To:     Erdem Aktas <erdemaktas@...gle.com>
Cc:     isaku.yamahata@...el.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, isaku.yamahata@...il.com,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Sagi Shahar <sagis@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Kai Huang <kai.huang@...el.com>,
        Zhi Wang <zhi.wang.linux@...il.com>, chen.bo@...el.com,
        Vishal Annapurve <vannapurve@...gle.com>
Subject: Re: [PATCH v14 055/113] KVM: TDX: TDP MMU TDX support

On Thu, Jun 08, 2023 at 04:29:35AM -0700,
Erdem Aktas <erdemaktas@...gle.com> wrote:

> On Sun, May 28, 2023 at 9:21 PM <isaku.yamahata@...el.com> wrote:
> >
> > From: Isaku Yamahata <isaku.yamahata@...el.com>
> >
> > +static int tdx_sept_drop_private_spte(struct kvm *kvm, gfn_t gfn,
> > +                                      enum pg_level level, kvm_pfn_t pfn)
> > +{
> > +       int tdx_level = pg_level_to_tdx_sept_level(level);
> > +       struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> > +       struct tdx_module_output out;
> > +       gpa_t gpa = gfn_to_gpa(gfn);
> > +       hpa_t hpa = pfn_to_hpa(pfn);
> > +       hpa_t hpa_with_hkid;
> > +       u64 err;
> > +
> > +       /* TODO: handle large pages. */
> > +       if (KVM_BUG_ON(level != PG_LEVEL_4K, kvm))
> > +               return -EINVAL;
> > +
> > +       if (unlikely(!is_hkid_assigned(kvm_tdx))) {
> > +               /*
> > +                * The HKID assigned to this TD was already freed and cache
> > +                * was already flushed. We don't have to flush again.
> > +                */
> > +               err = tdx_reclaim_page(hpa, false, 0);
> > +               if (KVM_BUG_ON(err, kvm))
> > +                       return -EIO;
> > +               tdx_unpin(kvm, pfn);
> > +               return 0;
> > +       }
> > +
> > +       do {
> > +               /*
> > +                * When zapping private page, write lock is held. So no race
> > +                * condition with other vcpu sept operation.  Race only with
> > +                * TDH.VP.ENTER.
> > +                */
> > +               err = tdh_mem_page_remove(kvm_tdx->tdr_pa, gpa, tdx_level, &out);
> > +       } while (unlikely(err == TDX_ERROR_SEPT_BUSY));
> > +       if (KVM_BUG_ON(err, kvm)) {
> > +               pr_tdx_error(TDH_MEM_PAGE_REMOVE, err, &out);
> > +               return -EIO;
> > +       }
> > +
> > +       hpa_with_hkid = set_hkid_to_hpa(hpa, (u16)kvm_tdx->hkid);
> > +       do {
> > +               /*
> > +                * TDX_OPERAND_BUSY can happen on locking PAMT entry.  Because
> > +                * this page was removed above, other thread shouldn't be
> > +                * repeatedly operating on this page.  Just retry loop.
> > +                */
> > +               err = tdh_phymem_page_wbinvd(hpa_with_hkid);
> > +       } while (unlikely(err == (TDX_OPERAND_BUSY | TDX_OPERAND_ID_RCX)));
> > +       if (KVM_BUG_ON(err, kvm)) {
> > +               pr_tdx_error(TDH_PHYMEM_PAGE_WBINVD, err, NULL);
> > +               return -EIO;
> > +       }
> 
> 
> It seems like when the TD is destroyed, all the TD private pages are
> removed with tdx_reclaim_page which also clears the page with
> movdir64b instruction. But when the page is removed while the TD is
> alive (in this case), the page content is never cleared with movdir64b
> which causes any poisoned cache line to be consumed by the host
> resulting in #MC exceptions in the host context.
> 
> We should clear the page before returning it back to the free pool by
> calling tdx_clear_page((unsigned long)__va(hpa)) here.

Thank you for catching it up. I'll fix tdx_sept_drop_private_spte(),
tdx_sept_free_private_spt() (and tdx_sept_merge_private_spt() for large page
support).  We should clear the page for Not only guest memory, but also
secure-ept.
-- 
Isaku Yamahata <isaku.yamahata@...il.com>