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AFNElJ918giFPWNtgaZTVJOuIvT4yCsD8RwLgNl2sUUamDmVLjq1bzHUrUOXn6WN3JIFsrj3iDNMxag=@lists.linux.dev X-Gm-Message-State: AOJu0Yycv0e3qDfSgzM+Mtm31naX0gb5m2Q4RjgvzYCOiMbIofOfPjF3 bDxfC3zm91d5MfzYuTIYshcEMkrsPnQ4FZr4Bukgr0rtY5RhwPueeQCWdfft+tGFWMMnELH87u3 Ni9DS6g== X-Received: from pgaf19.prod.google.com ([2002:a63:3813:0:b0:c80:2656:4e70]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6a20:3c89:b0:3b3:241f:66c6 with SMTP id adf61e73a8af0-3b7e4cf86b3mr95295637.26.1781545635135; Mon, 15 Jun 2026 10:47:15 -0700 (PDT) Date: Mon, 15 Jun 2026 10:47:14 -0700 In-Reply-To: <20260615160901.9A1A61F000E9@smtp.kernel.org> Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20260615155244.183044-1-alexandru.elisei@arm.com> <20260615160901.9A1A61F000E9@smtp.kernel.org> Message-ID: Subject: Re: [RFC PATCH] KVM: Ignore MMU notifiers for guest_memfd-only memslots From: Sean Christopherson To: sashiko-reviews@lists.linux.dev Cc: Alexandru Elisei , Marc Zyngier , kvm@vger.kernel.org, kvmarm@lists.linux.dev, Oliver Upton Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable On Mon, Jun 15, 2026, sashiko-bot@kernel.org wrote: > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > > --- a/virt/kvm/kvm_main.c > > +++ b/virt/kvm/kvm_main.c > [ ... ] > > @@ -592,6 +592,10 @@ static __always_inline kvm_mn_ret_t kvm_handle_hva= _range(struct kvm *kvm, > > unsigned long hva_start, hva_end; > > =20 > > slot =3D container_of(node, struct kvm_memory_slot, hva_node[slots-= >node_idx]); > > + > > + if (kvm_slot_has_gmem(slot) && kvm_memslot_is_gmem_only(slot)) > > + continue; > > + >=20 > [Severity: Critical] > Does this change inadvertently introduce a regression in the pfncache ret= ry > protocol? >=20 > Looking at the pfncache framework, it maps guest memory into kernel space= and > explicitly drops the page reference after mapping it: >=20 > virt/kvm/pfncache.c:hva_to_pfn_retry() { > ... > kvm_release_page_clean(page); > ... > } >=20 > It appears to rely entirely on KVM's MMU notifiers (kvm->mmu_invalidate_s= eq) > to invalidate the cache when the page is unmapped by the host. >=20 > If a VMM defines a guest_memfd-backed memslot with KVM_MEMSLOT_GMEM_ONLY > but still provides a valid anonymous user mapping as its userspace_addr, > could this regression lead to a use-after-free? Sadly, yes. To land this, we would need to first teach the gfn_to_pfn_cach= e code to be able to pull directly from guest_memfd. I forget if anyone is workin= g on that. > By unmapping the anonymous memory, the host would trigger MMU notifiers, = but > this new check skips the memslot. As a result, kvm->mmu_invalidate_seq > wouldn't increment, and KVM might retain a kernel mapping to a freed phys= ical > page. >=20 > Could this allow the guest to read or write arbitrary host physical memor= y? >=20 > > hva_start =3D max_t(unsigned long, range->start, slot->userspace_ad= dr); > > hva_end =3D min_t(unsigned long, range->end, > > slot->userspace_addr + (slot->npages << PAGE_SHIFT)); >=20 > --=20 > Sashiko AI review =C2=B7 https://sashiko.dev/#/patchset/20260615155244.18= 3044-1-alexandru.elisei@arm.com?part=3D1