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Thu, 10 Oct 2024 11:25:00 -0700 (PDT) Reply-To: Sean Christopherson Date: Thu, 10 Oct 2024 11:23:07 -0700 In-Reply-To: <20241010182427.1434605-1-seanjc@google.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20241010182427.1434605-1-seanjc@google.com> X-Mailer: git-send-email 2.47.0.rc1.288.g06298d1525-goog Message-ID: <20241010182427.1434605-6-seanjc@google.com> Subject: [PATCH v13 05/85] KVM: x86/mmu: Don't overwrite shadow-present MMU SPTEs when prefaulting From: Sean Christopherson To: Paolo Bonzini , Marc Zyngier , Oliver Upton , Tianrui Zhao , Bibo Mao , Huacai Chen , Michael Ellerman , Anup Patel , Paul Walmsley , Palmer Dabbelt , Albert Ou , Christian Borntraeger , Janosch Frank , Claudio Imbrenda , Sean Christopherson Cc: kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, loongarch@lists.linux.dev, linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, " =?utf-8?q?Alex_Benn=C3=A9e?= " , Yan Zhao , David Matlack , David Stevens , Andrew Jones Treat attempts to prefetch/prefault MMU SPTEs as spurious if there's an existing shadow-present SPTE, as overwriting a SPTE that may have been create by a "real" fault is at best confusing, and at worst potentially harmful. E.g. mmu_try_to_unsync_pages() doesn't unsync when prefetching, which creates a scenario where KVM could try to replace a Writable SPTE with a !Writable SPTE, as sp->unsync is checked prior to acquiring mmu_unsync_pages_lock. Note, this applies to three of the four flavors of "prefetch" in KVM: - KVM_PRE_FAULT_MEMORY - Async #PF (host or PV) - Prefetching The fourth flavor, SPTE synchronization, i.e. FNAME(sync_spte), _only_ overwrites shadow-present SPTEs when calling make_spte(). But SPTE synchronization specifically uses mmu_spte_update(), and so naturally avoids the @prefetch check in mmu_set_spte(). Signed-off-by: Sean Christopherson --- arch/x86/kvm/mmu/mmu.c | 3 +++ arch/x86/kvm/mmu/tdp_mmu.c | 3 +++ 2 files changed, 6 insertions(+) diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index a9a23e058555..a8c64069aa89 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -2919,6 +2919,9 @@ static int mmu_set_spte(struct kvm_vcpu *vcpu, struct kvm_memory_slot *slot, } if (is_shadow_present_pte(*sptep)) { + if (prefetch) + return RET_PF_SPURIOUS; + /* * If we overwrite a PTE page pointer with a 2MB PMD, unlink * the parent of the now unreachable PTE. diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c index 3b996c1fdaab..3c6583468742 100644 --- a/arch/x86/kvm/mmu/tdp_mmu.c +++ b/arch/x86/kvm/mmu/tdp_mmu.c @@ -1026,6 +1026,9 @@ static int tdp_mmu_map_handle_target_level(struct kvm_vcpu *vcpu, if (WARN_ON_ONCE(sp->role.level != fault->goal_level)) return RET_PF_RETRY; + if (fault->prefetch && is_shadow_present_pte(iter->old_spte)) + return RET_PF_SPURIOUS; + if (unlikely(!fault->slot)) new_spte = make_mmio_spte(vcpu, iter->gfn, ACC_ALL); else