Yan Zhao [off-list ref] writes:

On Thu, Jun 18, 2026 at 05:31:58PM -0700, Ackerley Tng via B4 Relay wrote:

quoted

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);
+	}

Note:
In the __kvm_gmem_populate() path, this folio_mark_uptodate() call makes the
later one after post_populate() pointless.

__kvm_gmem_populate
    |1.__kvm_gmem_get_pfn
    |     |->folio = kvm_gmem_get_folio()
    |     |  if (!folio_test_uptodate(folio))
    |     |     folio_mark_uptodate(folio);
    |2. ret = post_populate()
    |3. if (!ret)
    |       folio_mark_uptodate(folio);

Good point! I'll remove the folio_mark_uptodate() in the populate path
then. Thanks!

quoted

[...snip...]