From af62fe2494da84eb01752282c8228c9bb3fe9f67 Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Mon, 1 Dec 2025 18:03:34 -0800 Subject: [PATCH] KVM: Harden and prepare for modifying existing guest_memfd memslots Unbind guest_memfd memslots if KVM commits a MOVE or FLAGS_ONLY memslot change to harden against use-after-free, and to prepare for eventually supporting dirty logging on guest_memfd memslots, at which point FLAGS_ONLY changes will be expected/supported. Add two separate WARNs, once to yell if a guest_memfd memslot is moved (which KVM is never expected to allow/support), and again if the unbind() is triggered, to help detect uAPI goofs prior to deliberately allowing FLAGS_ONLY changes. Link: https://patch.msgid.link/20251202020334.1171351-3-seanjc@google.com Signed-off-by: Sean Christopherson --- virt/kvm/kvm_main.c | 15 +++++++++++++++ 1 file changed, 15 insertions(+) diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index 7fea6ba91c1e..32b6c6209b63 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -1748,6 +1748,12 @@ static void kvm_commit_memory_region(struct kvm *kvm, kvm_free_memslot(kvm, old); break; case KVM_MR_MOVE: + /* + * Moving a guest_memfd memslot isn't supported, and will never + * be supported. + */ + WARN_ON_ONCE(old->flags & KVM_MEM_GUEST_MEMFD); + fallthrough; case KVM_MR_FLAGS_ONLY: /* * Free the dirty bitmap as needed; the below check encompasses @@ -1756,6 +1762,15 @@ static void kvm_commit_memory_region(struct kvm *kvm, if (old->dirty_bitmap && !new->dirty_bitmap) kvm_destroy_dirty_bitmap(old); + /* + * Unbind the guest_memfd instance as needed; the @new slot has + * already created its own binding. TODO: Drop the WARN when + * dirty logging guest_memfd memslots is supported. Until then, + * flags-only changes on guest_memfd slots should be impossible. + */ + if (WARN_ON_ONCE(old->flags & KVM_MEM_GUEST_MEMFD)) + kvm_gmem_unbind(old); + /* * The final quirk. Free the detached, old slot, but only its * memory, not any metadata. Metadata, including arch specific -- 2.47.3