* Re: [PATCH v2 1/4] KVM: x86/mmu: Fix write-protection of PTs mapped by the TDP MMU
2022-01-13 23:30 ` [PATCH v2 1/4] KVM: x86/mmu: Fix write-protection of PTs mapped by the TDP MMU David Matlack
@ 2022-01-14 23:38 ` Sean Christopherson
0 siblings, 0 replies; 2+ messages in thread
From: Sean Christopherson @ 2022-01-14 23:38 UTC (permalink / raw)
To: David Matlack
Cc: Paolo Bonzini, Vitaly Kuznetsov, Wanpeng Li, Jim Mattson,
Joerg Roedel, Ben Gardon, kvm, stable
On Thu, Jan 13, 2022, David Matlack wrote:
> When the TDP MMU is write-protection GFNs for page table protection (as
^^^^^^^^^^^^^^^^
write-protecting
> opposed to for dirty logging, or due to the HVA not being writable), it
> checks if the SPTE is already write-protected and if so skips modifying
> the SPTE and the TLB flush.
>
> This behavior is incorrect because the SPTE may be write-protected for
> dirty logging. This implies that the SPTE could be locklessly be made
Spurious "be" between could and locklessly.
Hmm, it doesn't imply anything, the behavior of MMU-writable is quite explicit.
If the bug occurs, then _that_ implies the SPTE was write-protected for dirty
logging, otherwise MMU-Writable would have been '0' due to HOST-Writable also
being '0'.
I think what you're trying to say is:
This behavior is incorrect because it fails to check if the SPTE is
write-protected for page table protection, i.e. fails to check that
MMU-writable is '0'. If the SPTE was write-protected for dirty logging
but not page table protection, the SPTE could locklessly be made
writable, and vCPUs could still be running with writable mappings
cached in their TLB.
> writable on the next write access, and that vCPUs could still be running
> with writable SPTEs cached in their TLB.
Nit, it's technically the mapping, not the SPTE itself, that's cached in the TLB.
> Fix this by only skipping setting the SPTE if the SPTE is already
> write-protected *and* MMU-writable is already clear.
Might also be worth adding:
Note, technically checking only MMU-writable would suffice, as a SPTE
cannot be writable without MMU-writable being set, but check both to
be paranoid and because it arguably yields more readable code.
Pedantry aside,
Reviewed-by: Sean Christopherson <seanjc@google.com>
> Fixes: 46044f72c382 ("kvm: x86/mmu: Support write protection for nesting in tdp MMU")
> Cc: stable@vger.kernel.org
> Signed-off-by: David Matlack <dmatlack@google.com>
> ---
> arch/x86/kvm/mmu/tdp_mmu.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 7b1bc816b7c3..bc9e3553fba2 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -1442,12 +1442,12 @@ static bool write_protect_gfn(struct kvm *kvm, struct kvm_mmu_page *root,
> !is_last_spte(iter.old_spte, iter.level))
> continue;
>
> - if (!is_writable_pte(iter.old_spte))
> - break;
> -
> new_spte = iter.old_spte &
> ~(PT_WRITABLE_MASK | shadow_mmu_writable_mask);
>
> + if (new_spte == iter.old_spte)
> + break;
> +
> tdp_mmu_set_spte(kvm, &iter, new_spte);
> spte_set = true;
> }
>
> base-commit: fea31d1690945e6dd6c3e89ec5591490857bc3d4
> --
> 2.34.1.703.g22d0c6ccf7-goog
>
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