linux-kernel - [PATCH 2/2] KVM: Harden and prepare for modifying existing guest

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20251202020334.1171351-3-seanjc@google.com>
Date: Mon,  1 Dec 2025 18:03:34 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Alexander Potapenko <glider@...gle.com>, Sean Christopherson <seanjc@...gle.com>
Subject: [PATCH 2/2] 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.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 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 8891df136416..f822d3e389b0 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
-- 
2.52.0.107.ga0afd4fd5b-goog