[PATCH v2 03/13] KVM: Allow late setting of KVM_MEM_USERFAULT on guest_memfd memslot

[James Houghton <jthoughton@google.com>]

Currently guest_memfd memslots can only be deleted. Slightly change the
logic to allow KVM_MR_FLAGS_ONLY changes when the only flag being
changed is KVM_MEM_USERFAULT.

Signed-off-by: James Houghton <jthoughton@google.com>
---
 virt/kvm/kvm_main.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 4bceae6a6401..882c1f7b4aa8 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2015,9 +2015,6 @@ int __kvm_set_memory_region(struct kvm *kvm,
 		if ((kvm->nr_memslot_pages + npages) < kvm->nr_memslot_pages)
 			return -EINVAL;
 	} else { /* Modify an existing slot. */
-		/* Private memslots are immutable, they can only be deleted. */
-		if (mem->flags & KVM_MEM_GUEST_MEMFD)
-			return -EINVAL;
 		if ((mem->userspace_addr != old->userspace_addr) ||
 		    (npages != old->npages) ||
 		    ((mem->flags ^ old->flags) & KVM_MEM_READONLY))
@@ -2031,6 +2028,16 @@ int __kvm_set_memory_region(struct kvm *kvm,
 			return 0;
 	}
 
+	/*
+	 * Except for being able to set KVM_MEM_USERFAULT, private memslots are
+	 * immutable, they can only be deleted.
+	 */
+	if (mem->flags & KVM_MEM_GUEST_MEMFD &&
+	    !(change == KVM_MR_CREATE ||
+	      (change == KVM_MR_FLAGS_ONLY &&
+	       (mem->flags ^ old->flags) == KVM_MEM_USERFAULT)))
+		return -EINVAL;
+
 	if ((change == KVM_MR_CREATE || change == KVM_MR_MOVE) &&
 	    kvm_check_memslot_overlap(slots, id, base_gfn, base_gfn + npages))
 		return -EEXIST;
@@ -2046,7 +2053,7 @@ 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 (mem->flags & KVM_MEM_GUEST_MEMFD && change == KVM_MR_CREATE) {
 		r = kvm_gmem_bind(kvm, new, mem->guest_memfd, mem->guest_memfd_offset);
 		if (r)
 			goto out;
-- 
2.47.1.613.gc27f4b7a9f-goog