On Thu, Feb 09, 2023 at 05:48:28PM +0000, David Chen wrote:
So the problem is the check PageHead is racy because at this point we already dropped our reference to the page. So even if we came in with compound page, the page can already be freed and PageHead can return false and we will end up freeing all the tail pages causing double free.
Fixes: e320d3012d25 ("mm/page_alloc.c: fix freeing non-compound pages") Cc: Andrew Morton akpm@linux-foundation.org Cc: Matthew Wilcox (Oracle) willy@infradead.org Cc: linux-mm@kvack.org Cc: stable@vger.kernel.org Signed-off-by: Chunwei Chen david.chen@nutanix.com
Reviewed-by: Matthew Wilcox (Oracle) willy@infradead.org
That's a pretty narrow window you managed to hit! Just a few instructions wide. I guess that answers the question Andrew originally had (Does this ever happen?) I just changed it from a silent memory leak into a double-free.
mm/page_alloc.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 0745aedebb37..3bb3484563ed 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -5631,9 +5631,12 @@ EXPORT_SYMBOL(get_zeroed_page); */ void __free_pages(struct page *page, unsigned int order) {
- /* get PageHead before we drop reference */
- int head = PageHead(page);
- if (put_page_testzero(page)) free_the_page(page, order);
- else if (!PageHead(page))
- else if (!head) while (order-- > 0) free_the_page(page + (1 << order), order);
}
2.22.3