Re: [PATCH v8 21/46] KVM: guest_memfd: Zero page while getting pfn

From: Ackerley Tng <redacted>

Move the folio initialization logic from kvm_gmem_get_pfn() into
__kvm_gmem_get_pfn() to also zero pages if the page is to be used in
kvm_gmem_populate().

With in-place conversion, the existing data in a guest_memfd page can be
populated into guest memory through platform-specific ioctls.

Without first zeroing the page obtained using __kvm_gmem_get_pfn(), it
might contain uninitialized host memory, which would leak to the guest if
the populate completes.

guest_memfd pages are zeroed at most once in the page's entire lifetime
with guest_memfd, and that is tracked using the uptodate flag.

Zeroing the page in __kvm_gmem_get_pfn() is chosen over zeroing in
kvm_gmem_get_folio() since other flows, such as a future write() syscall,
can get a page, write to the page and then set page uptodate without
zeroing.

This aligns with the concept of zeroing before first use - the other place
where zeroing happens is in kvm_gmem_fault_user_mapping().

Signed-off-by: Ackerley Tng <redacted>
---
 virt/kvm/guest_memfd.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 90bc1a26512b6..86c9f5b0863cb 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c

@@ -1137,6 +1137,11 @@ static struct folio *__kvm_gmem_get_pfn(struct file *file,
 		return ERR_PTR(-EHWPOISON);
 	}
 
+	if (!folio_test_uptodate(folio)) {
+		clear_highpage(folio_page(folio, 0));
+		folio_mark_uptodate(folio);
+	}

 	*pfn = folio_file_pfn(folio, index);
 	if (max_order)
 		*max_order = 0;

@@ -1166,11 +1171,6 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
 		goto out;
 	}
 
-	if (!folio_test_uptodate(folio)) {
-		clear_highpage(folio_page(folio, 0));
-		folio_mark_uptodate(folio);
-	}
-
 	if (kvm_gmem_is_private_mem(inode, index))
 		r = kvm_gmem_prepare_folio(kvm, slot, gfn, folio);