Minchan Kim noticed that balloon_page_dequeue walks the pages list
without holding the pages_lock. This can race e.g. with isolation, which
has been reported to cause list corruption and crashes in leak_balloon.
Page can also in theory get freed before it's locked, corrupting memory.
To fix, make sure list accesses are done under lock, and
always take a page reference before trying to lock it.
Reported-by: Minchan Kim <minc...@kernel.org>
Cc: <sta...@vger.kernel.org>
Cc: Rafael Aquini <aqu...@redhat.com>
Signed-off-by: Michael S. Tsirkin <m...@redhat.com>
---
This is an alternative to patch
virtio_balloon: fix race between migration and ballooning
by Minchan Kim in -mm.
Untested - Minchan, could you pls confirm this fixes the issue for you?
mm/balloon_compaction.c | 34 ++++++++++++++++++++++++++++++++--
1 file changed, 32 insertions(+), 2 deletions(-)
diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
index d3116be..66d69c5 100644
--- a/mm/balloon_compaction.c
+++ b/mm/balloon_compaction.c
@@ -56,12 +56,34 @@ EXPORT_SYMBOL_GPL(balloon_page_enqueue);
*/
struct page *balloon_page_dequeue(struct balloon_dev_info *b_dev_info)
{
- struct page *page, *tmp;
+ struct page *page;
unsigned long flags;
bool dequeued_page;
+ LIST_HEAD(processed); /* protected by b_dev_info->pages_lock */
dequeued_page = false;
- list_for_each_entry_safe(page, tmp, &b_dev_info->pages, lru) {
+ /*
+ * We need to go over b_dev_info->pages and lock each page,
+ * but b_dev_info->pages_lock must nest within page lock.
+ *
+ * To make this safe, remove each page from b_dev_info->pages list
+ * under b_dev_info->pages_lock, then drop this lock. Once list is
+ * empty, re-add them also under b_dev_info->pages_lock.
+ */
+ spin_lock_irqsave(&b_dev_info->pages_lock, flags);
+ while (!list_empty(&b_dev_info->pages)) {
+ page = list_first_entry(&b_dev_info->pages, typeof(*page), lru);
+ /* move to processed list to avoid going over it another time */
+ list_move(&page->lru, &processed);
+
+ if (!get_page_unless_zero(page))
+ continue;
+ /*
+ * pages_lock nests within page lock,
+ * so drop it before trylock_page
+ */
+ spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
+
/*
* Block others from accessing the 'page' while we get around
* establishing additional references and preparing the 'page'
@@ -72,6 +94,7 @@ struct page *balloon_page_dequeue(struct balloon_dev_info
*b_dev_info)
if (!PagePrivate(page)) {
/* raced with isolation */
unlock_page(page);
+ put_page(page);
continue;
}
#endif
@@ -80,11 +103,18 @@ struct page *balloon_page_dequeue(struct balloon_dev_info
*b_dev_info)
__count_vm_event(BALLOON_DEFLATE);
spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
unlock_page(page);
+ put_page(page);
dequeued_page = true;
break;
}
+ put_page(page);
+ spin_lock_irqsave(&b_dev_info->pages_lock, flags);
}
+ /* re-add remaining entries */
+ list_splice(&processed, &b_dev_info->pages);
+ spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
+
if (!dequeued_page) {
/*
* If we are unable to dequeue a balloon page because the page
--
MST
--
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