Linux Virtualization - Apr 2012 - [PATCH] virtio_balloon: fix handling of PAGE_SIZE != 4k

As reported by David Gibson, current code handles PAGE_SIZE != 4k
completely wrong which can lead to guest memory corruption errors.

- page_to_balloon_pfn is wrong: e.g. on system with 64K page size
 it gives the same pfn value for 16 different pages.

- we also need to convert back to linux pfns when we free.

- for each linux page we need to tell host about multiple balloon
  pages, but code only adds one pfn to the array.

Signed-off-by: David Gibson <david at gibson.dropbear.id.au>
Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
Signed-off-by: David Gibson <david at gibson.dropbear.id.au>
---
 drivers/virtio/virtio_balloon.c |   44 +++++++++++++++++++++++++++++++--------
 1 file changed, 35 insertions(+), 9 deletions(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 9e95ca6..0c0c1c3 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -43,6 +43,9 @@ struct virtio_balloon
 	struct completion acked;
 
 	/* The pages we've told the Host we're not using. */
+	/* Note that num_pages is measured in 4k balloon pages, so if
+	 * PAGE_SIZE != 4k, it will be a multiple of the actual number
+	 * of elements in the pages list */
 	unsigned int num_pages;
 	struct list_head pages;
 
@@ -60,13 +63,23 @@ static struct virtio_device_id id_table[] = {
 	{ 0 },
 };
 
+/* Balloon device works in 4K page units. So each page is
+ * pointed to by multiple balloon pages. */
+#define VIRTIO_BALLOON_PAGES_PER_PAGE (PAGE_SIZE >>
VIRTIO_BALLOON_PFN_SHIFT)
+
 static u32 page_to_balloon_pfn(struct page *page)
 {
 	unsigned long pfn = page_to_pfn(page);
 
 	BUILD_BUG_ON(PAGE_SHIFT < VIRTIO_BALLOON_PFN_SHIFT);
 	/* Convert pfn from Linux page size to balloon page size. */
-	return pfn >> (PAGE_SHIFT - VIRTIO_BALLOON_PFN_SHIFT);
+	return pfn * VIRTIO_BALLOON_PAGES_PER_PAGE;
+}
+
+static struct page *balloon_pfn_to_page(u32 pfn)
+{
+	BUG_ON(pfn % VIRTIO_BALLOON_PAGES_PER_PAGE);
+	return pfn_to_page(pfn / VIRTIO_BALLOON_PAGES_PER_PAGE);
 }
 
 static void balloon_ack(struct virtqueue *vq)
@@ -96,12 +109,23 @@ static void tell_host(struct virtio_balloon *vb, struct
virtqueue *vq)
 	wait_for_completion(&vb->acked);
 }
 
+static void set_page_pfns(u32 pfns[], struct page *page)
+{
+	unsigned int i;
+
+	/* Set balloon pfns pointing at this page.
+	 * Note that the first pfn points at start of the page. */
+	for (i = 0; i < VIRTIO_BALLOON_PAGES_PER_PAGE; i++)
+		pfns[i] = page_to_balloon_pfn(page) + i;
+}
+
 static void fill_balloon(struct virtio_balloon *vb, size_t num)
 {
 	/* We can only do one array worth at a time. */
 	num = min(num, ARRAY_SIZE(vb->pfns));
 
-	for (vb->num_pfns = 0; vb->num_pfns < num; vb->num_pfns++) {
+	for (vb->num_pfns = 0; vb->num_pfns < num;
+	     vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
 		struct page *page = alloc_page(GFP_HIGHUSER | __GFP_NORETRY |
 					__GFP_NOMEMALLOC | __GFP_NOWARN);
 		if (!page) {
@@ -113,9 +137,9 @@ static void fill_balloon(struct virtio_balloon *vb, size_t
num)
 			msleep(200);
 			break;
 		}
-		vb->pfns[vb->num_pfns] = page_to_balloon_pfn(page);
+		set_page_pfns(vb->pfns + vb->num_pfns, page);
+		vb->num_pages += VIRTIO_BALLOON_PAGES_PER_PAGE;
 		totalram_pages--;
-		vb->num_pages++;
 		list_add(&page->lru, &vb->pages);
 	}
 
@@ -130,8 +154,9 @@ static void release_pages_by_pfn(const u32 pfns[], unsigned
int num)
 {
 	unsigned int i;
 
-	for (i = 0; i < num; i++) {
-		__free_page(pfn_to_page(pfns[i]));
+	/* Find pfns pointing at start of each page, get pages and free them. */
+	for (i = 0; i < num; i += VIRTIO_BALLOON_PAGES_PER_PAGE) {
+		__free_page(balloon_pfn_to_page(pfns[i]));
 		totalram_pages++;
 	}
 }
@@ -143,11 +168,12 @@ static void leak_balloon(struct virtio_balloon *vb, size_t
num)
 	/* We can only do one array worth at a time. */
 	num = min(num, ARRAY_SIZE(vb->pfns));
 
-	for (vb->num_pfns = 0; vb->num_pfns < num; vb->num_pfns++) {
+	for (vb->num_pfns = 0; vb->num_pfns < num;
+	     vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
 		page = list_first_entry(&vb->pages, struct page, lru);
 		list_del(&page->lru);
-		vb->pfns[vb->num_pfns] = page_to_balloon_pfn(page);
-		vb->num_pages--;
+		set_page_pfns(vb->pfns + vb->num_pfns, page);
+		vb->num_pages -= VIRTIO_BALLOON_PAGES_PER_PAGE;
 	}
 
 	/*
-- 
1.7.9.5



-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson