[PATCH v4 3/5] KVM: Only bind memslot to guest_memfd instance for CREATE operations

[Sean Christopherson <seanjc@google.com>]

For additional defense-in-depth, and to avoid having to handle impossible
unwind scenarios when binding to a memslot fails, bind a memslot to a gmem
instance only when for CREATE operations, i.e. don't attempt to establish a
binding for MOVE and FLAGS_ONLY operations.  And when FLAGS_ONLY operations
are eventually supported (this is currently all dead code), creating a new
binding would be incorrect; KVM instead needs to do a 1:1 replacement of
the existing binding, i.e. FLAGS_ONLY will need its own dedicated handling.

Update the relevant TODO to make a better guess as to what needs to be done
to support toggling dirty logging for guest_memfd memslots.

Because it's dead code, no functional change intended.

Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 virt/kvm/kvm_main.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 24cf96840827..b417b1f7095f 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1752,10 +1752,10 @@ static void kvm_commit_memory_region(struct kvm *kvm,
 			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.
+		 * TODO: Drop the WARN and do the unbind() call only for MOVE
+		 * when dirty logging guest_memfd memslots is supported.  Until
+		 * then, flags-only changes on guest_memfd slots should also be
+		 * impossible; unbind the old memslot for defense-in-depth.
 		 */
 		if (WARN_ON_ONCE(old->flags & KVM_MEM_GUEST_MEMFD))
 			kvm_gmem_unbind(old);
@@ -2116,7 +2116,7 @@ static int kvm_set_memory_region(struct kvm *kvm,
 	new->npages = npages;
 	new->flags = mem->flags;
 	new->userspace_addr = mem->userspace_addr;
-	if (mem->flags & KVM_MEM_GUEST_MEMFD) {
+	if (change == KVM_MR_CREATE && (mem->flags & KVM_MEM_GUEST_MEMFD)) {
 		r = kvm_gmem_bind(kvm, new, mem->guest_memfd, mem->guest_memfd_offset);
 		if (r)
 			goto out;
-- 
2.55.0.1082.g2b9226bbc0-goog