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KVM: x86/mmu: Skip the "try unsync" path iff the old SPTE was a leaf SPTE

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commit 2867eb782c upstream.

Apply make_spte()'s optimization to skip trying to unsync shadow pages if
and only if the old SPTE was a leaf SPTE, as non-leaf SPTEs in direct MMUs
are always writable, i.e. could trigger a false positive and incorrectly
lead to KVM creating a SPTE without write-protecting or marking shadow
pages unsync.

This bug only affects the TDP MMU, as the shadow MMU only overwrites a
shadow-present SPTE when synchronizing SPTEs (and only 4KiB SPTEs can be
unsync).  Specifically, mmu_set_spte() drops any non-leaf SPTEs *before*
calling make_spte(), whereas the TDP MMU can do a direct replacement of a
page table with the leaf SPTE.

Opportunistically update the comment to explain why skipping the unsync
stuff is safe, as opposed to simply saying "it's someone else's problem".

Cc: stable@vger.kernel.org
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20241010182427.1434605-5-seanjc@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Sean Christopherson
2024-10-10 11:23:06 -07:00
committed by Greg Kroah-Hartman
parent 046fb04cbc
commit d79f765b2e
1 changed files with 13 additions and 5 deletions
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18
arch/x86/kvm/mmu/spte.c
View File

@@ -206,12 +206,20 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
spte |= PT_WRITABLE_MASK | shadow_mmu_writable_mask;
/*
* Optimization: for pte sync, if spte was writable the hash
* lookup is unnecessary (and expensive). Write protection
* is responsibility of kvm_mmu_get_page / kvm_mmu_sync_roots.
* Same reasoning can be applied to dirty page accounting.
* When overwriting an existing leaf SPTE, and the old SPTE was
* writable, skip trying to unsync shadow pages as any relevant
* shadow pages must already be unsync, i.e. the hash lookup is
* unnecessary (and expensive).
*
* The same reasoning applies to dirty page/folio accounting;
* KVM will mark the folio dirty using the old SPTE, thus
* there's no need to immediately mark the new SPTE as dirty.
*
* Note, both cases rely on KVM not changing PFNs without first
* zapping the old SPTE, which is guaranteed by both the shadow
* MMU and the TDP MMU.
*/
if (is_writable_pte(old_spte))
if (is_last_spte(old_spte, level) && is_writable_pte(old_spte))
goto out;
/*