[PATCH] virt: guest_memfd: fix reference leak on hwpoisoned page

[Paolo Bonzini <pbonzini@xxxxxxxxxx>]

If __kvm_gmem_get_pfn() detects an hwpoisoned page, it returns -EHWPOISON
but it does not put back the reference that kvm_gmem_get_folio() had
grabbed.  Move the whole check to kvm_gmem_get_folio(), via an __-prefixed
function.  This removes a "goto" and simplifies the code.

Now even fallocate() is prevented from picking an hwpoisoned page successfully.
This is temporary until the page allocation flow is cleaned up.

Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
 virt/kvm/guest_memfd.c | 27 +++++++++++++++------------
 1 file changed, 15 insertions(+), 12 deletions(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 9714add38852..53742ec34a31 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -49,7 +49,7 @@ static int kvm_gmem_prepare_folio(struct inode *inode, pgoff_t index, struct fol
 	return 0;
 }
 
-static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t index, bool prepare)
+static struct folio *__kvm_gmem_get_folio(struct inode *inode, pgoff_t index)
 {
 	struct folio *folio;
 
@@ -58,6 +58,19 @@ static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t index, bool
 	if (IS_ERR(folio))
 		return folio;
 
+	if (folio_test_hwpoison(folio)) {
+		folio_unlock(folio);
+		folio_put(folio);
+		folio = ERR_PTR(-EHWPOISON);
+	}
+
+	return folio;
+}
+
+static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t index, bool prepare)
+{
+	struct folio *folio = __kvm_gmem_get_folio(inode, index);
+
 	/*
 	 * Use the up-to-date flag to track whether or not the memory has been
 	 * zeroed before being handed off to the guest.  There is no backing
@@ -549,7 +562,6 @@ static int __kvm_gmem_get_pfn(struct file *file, struct kvm_memory_slot *slot,
 	struct kvm_gmem *gmem = file->private_data;
 	struct folio *folio;
 	struct page *page;
-	int r;
 
 	if (file != slot->gmem.file) {
 		WARN_ON_ONCE(slot->gmem.file);
@@ -566,23 +578,14 @@ static int __kvm_gmem_get_pfn(struct file *file, struct kvm_memory_slot *slot,
 	if (IS_ERR(folio))
 		return PTR_ERR(folio);
 
-	if (folio_test_hwpoison(folio)) {
-		r = -EHWPOISON;
-		goto out_unlock;
-	}
-
 	page = folio_file_page(folio, index);
 
 	*pfn = page_to_pfn(page);
 	if (max_order)
 		*max_order = 0;
 
-	r = 0;
-
-out_unlock:
 	folio_unlock(folio);
-
-	return r;
+	return 0;
 }
 
 int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
-- 
2.43.0