[PATCH] mm: page_frag: Warn_on when frag_alloc size is bigger than PAGE_SIZE

[Chen Lin <chen45464546@163.com>]

netdev_alloc_frag->page_frag_alloc may cause memory corruption in 
the following process:

1. A netdev_alloc_frag function call need alloc 200 Bytes to build a skb.

2. Insufficient memory to alloc PAGE_FRAG_CACHE_MAX_ORDER(32K) in 
__page_frag_cache_refill to fill frag cache, then one page(eg:4K) 
is allocated, now current frag cache is 4K, alloc is success, 
nc->pagecnt_bias--.

3. Then this 200 bytes skb in step 1 is freed, page->_refcount--.

4. Another netdev_alloc_frag function call need alloc 5k, page->_refcount 
is equal to nc->pagecnt_bias, reset page count bias and offset to 
start of new frag. page_frag_alloc will return the 4K memory for a 
5K memory request.

5. The caller write on the extra 1k memory which is not actual allocated 
will cause memory corruption.

page_frag_alloc is for fragmented allocation. We should warn the caller 
to avoid memory corruption.

Signed-off-by: Chen Lin <chen45464546@163.com>
---
 mm/page_alloc.c |    5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index e008a3d..6c0db52 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5574,6 +5574,11 @@ void *page_frag_alloc_align(struct page_frag_cache *nc,
 	struct page *page;
 	int offset;
 
+	/* frag_alloc is not suitable for memory alloc which fragsz
+	 * is bigger than PAGE_SIZE, use kmalloc or alloc_pages instead.
+	 */
+	WARN_ON(fragsz > PAGE_SIZE);
+
 	if (unlikely(!nc->va)) {
 refill:
 		page = __page_frag_cache_refill(nc, gfp_mask);
-- 
1.7.9.5