linux-kernel - Re: [PATCH v3 14/25] KVM: TDX: Bug the VM if extended the initial measurement fails

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Message-ID: <aP876BCemBr8Gm7J@yzhao56-desk.sh.intel.com>
Date: Mon, 27 Oct 2025 17:31:20 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: Kai Huang <kai.huang@...el.com>, "chenhuacai@...nel.org"
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Subject: Re: [PATCH v3 14/25] KVM: TDX: Bug the VM if extended the initial
 measurement fails

On Fri, Oct 24, 2025 at 09:35:43AM -0700, Sean Christopherson wrote:
> On Thu, Oct 23, 2025, Kai Huang wrote:
> > On Thu, 2025-10-16 at 17:32 -0700, Sean Christopherson wrote:
> > > WARN and terminate the VM if TDH_MR_EXTEND fails, as extending the
> > > measurement should fail if and only if there is a KVM bug, or if the S-EPT
> > > mapping is invalid, and it should be impossible for the S-EPT mappings to
> > > be removed between kvm_tdp_mmu_map_private_pfn() and tdh_mr_extend().
> > > 
> > > Holding slots_lock prevents zaps due to memslot updates,
> > > filemap_invalidate_lock() prevents zaps due to guest_memfd PUNCH_HOLE,
> > > and all usage of kvm_zap_gfn_range() is mutually exclusive with S-EPT
> > > entries that can be used for the initial image.  The call from sev.c is
> > > obviously mutually exclusive, TDX disallows KVM_X86_QUIRK_IGNORE_GUEST_PAT
> > > so same goes for kvm_noncoherent_dma_assignment_start_or_stop, and while
> > > __kvm_set_or_clear_apicv_inhibit() can likely be tripped while building the
> > > image, the APIC page has its own non-guest_memfd memslot and so can't be
> > > used for the initial image, which means that too is mutually exclusive.
> > > 
> > > Opportunistically switch to "goto" to jump around the measurement code,
> > > partly to make it clear that KVM needs to bail entirely if extending the
> > > measurement fails, partly in anticipation of reworking how and when
> > > TDH_MEM_PAGE_ADD is done.
> > > 
> > > Fixes: d789fa6efac9 ("KVM: TDX: Handle vCPU dissociation")
Maybe this tag is for patch 25?

> > So IIUC this patch only adds a KVM_BUG_ON() when TDH.MR.EXTEND fails.  How
> > does this fix anything?
> 
> Hmm, yeah, I'll drop the Fixes.  It made more sense when I thought it was
> impossible for tdh_mr_extend() to fail, as returning an error and not explicitly
> terminating the VM was "wrong".  But I agree it does far more harm than good,
> even when relocated to the end of the series.