[RFC PATCH 15/18] KVM: guest_memfd: Add KVM Userfault support

[James Houghton]

We now have to pass our struct kvm into __kvm_gmem_get_pfn to know if a
gfn is userfault-enabled or not.

For faults on userfault-enabled gfns, indicate this to the caller by
setting *pfn to KVM_PFN_ERR_USERFAULT. Architectures may use this to
know to return a userfault to userspace, though they should be careful
to set a value for *pfn before calling (e.g. KVM_PFN_ERR_FAULT).

While we're at it, set *pfn to KVM_PFN_ERR_HWPOISON for accesses to
poisoned gfns.

Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx>
---
 virt/kvm/guest_memfd.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 9148b9679bb1..ba7a981e3396 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -542,8 +542,9 @@ void kvm_gmem_unbind(struct kvm_memory_slot *slot)
 	fput(file);
 }
 
-static int __kvm_gmem_get_pfn(struct file *file, struct kvm_memory_slot *slot,
-		       gfn_t gfn, kvm_pfn_t *pfn, int *max_order, bool prepare)
+static int __kvm_gmem_get_pfn(struct kvm *kvm, struct file *file,
+		       struct kvm_memory_slot *slot, gfn_t gfn, kvm_pfn_t *pfn,
+		       int *max_order, bool prepare)
 {
 	pgoff_t index = gfn - slot->base_gfn + slot->gmem.pgoff;
 	struct kvm_gmem *gmem = file->private_data;
@@ -551,6 +552,11 @@ static int __kvm_gmem_get_pfn(struct file *file, struct kvm_memory_slot *slot,
 	struct page *page;
 	int r;
 
+	if (gfn_has_userfault(kvm, gfn)) {
+		*pfn = KVM_PFN_ERR_USERFAULT;
+		return -EFAULT;
+	}
+
 	if (file != slot->gmem.file) {
 		WARN_ON_ONCE(slot->gmem.file);
 		return -EFAULT;
@@ -567,6 +573,7 @@ static int __kvm_gmem_get_pfn(struct file *file, struct kvm_memory_slot *slot,
 		return PTR_ERR(folio);
 
 	if (folio_test_hwpoison(folio)) {
+		*pfn = KVM_PFN_ERR_HWPOISON;
 		folio_unlock(folio);
 		folio_put(folio);
 		return -EHWPOISON;
@@ -594,7 +601,7 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
 	if (!file)
 		return -EFAULT;
 
-	r = __kvm_gmem_get_pfn(file, slot, gfn, pfn, max_order, true);
+	r = __kvm_gmem_get_pfn(kvm, file, slot, gfn, pfn, max_order, true);
 	fput(file);
 	return r;
 }
@@ -634,7 +641,8 @@ long kvm_gmem_populate(struct kvm *kvm, gfn_t start_gfn, void __user *src, long
 			break;
 		}
 
-		ret = __kvm_gmem_get_pfn(file, slot, gfn, &pfn, &max_order, false);
+		ret = __kvm_gmem_get_pfn(kvm, file, slot, gfn, &pfn,
+					 &max_order, false);
 		if (ret)
 			break;
 
-- 
2.45.2.993.g49e7a77208-goog