vmm: check correct buffer size during import

When we import a page, we have a page with
some data or empty, empty does not mean there is no data,
it rather means it's full of zeros. We can skip writing the
data as guest memory of the page is already zeroed.

A page could be partially filled and the rest of the content is zero.
Our IGVM generation tool only fills data here if there is some data
without zeros. Rest of them are padded. We only write data
without padding and compare whether we complete  writing
the buffer content. Still it's a full page and update the variable
with length of the full page.

Signed-off-by: Muminul Islam <muislam@microsoft.com>

vmm/src/igvm/loader.rs

@@ -82,6 +82,15 @@ impl Loader {
         acceptance: BootPageAcceptance,
         data: &[u8],
     ) -> Result<(), Error> {
+        // Once we are here at this point, we have a page with
+        // some data or empty, empty does not mean there is no data,
+        // it rather means it's full of zeros. We can skip writing the
+        // data as the guest page is already zeroed. So we return with
+        // updating the bytes_written variable
+        if data.is_empty() {
+            self.bytes_written += page_count * HV_PAGE_SIZE;
+            return Ok(());
+        }
         // Page count must be larger or equal to data.
         if page_count * HV_PAGE_SIZE < data.len() as u64 {
             return Err(Error::DataTooLarge);
@@ -98,10 +107,16 @@ impl Loader {
                 debug!("Importing pages failed due to MemoryError");
                 Error::MemoryUnavailable
             })?;
-        if bytes_written != (page_count * HV_PAGE_SIZE) as usize {
+
+        // A page could be partially filled and the rest of the content is zero.
+        // Our IGVM generation tool only fills data here if there is some data without zeros.
+        // Rest of them are padded. We only write data without padding and compare whether we
+        // complete writing the buffer content. Still it's a full page and update the variable
+        // with length of the page.
+        if bytes_written != data.len() {
             return Err(Error::ImportPagesFailed);
         }
-        self.bytes_written += bytes_written as u64;
+        self.bytes_written += page_count * HV_PAGE_SIZE;
         Ok(())
     }