[PATCH v2] 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.

When fragsz is larger than one page, we report the failure and return.
I don't think it is a good idea to make efforts to support the
allocation of more than one page in this function because the total
frag cache size(PAGE_FRAG_CACHE_MAX_SIZE 32768) is relatively small.
When the request is larger than one page, the caller should switch to
use other kernel interfaces, such as kmalloc and alloc_Pages.

This bug is mainly caused by the reuse of the previously allocated
frag cache memory by the following LARGER allocations. This bug existed
before page_frag_alloc was ported from __netdev_alloc_frag in 
net/core/skbuff.c, so most Linux versions have this problem.

Signed-off-by: Chen Lin <chen45464546@163.com>
---
 mm/page_alloc.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index e008a3d..1e9e2c4 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5574,6 +5574,16 @@ 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.
+	 */
+	if (unlikely(fragsz > PAGE_SIZE)) {
+		WARN(1, "alloc fragsz(%d) > PAGE_SIZE(%ld) not supported,
+			alloc fail\n", fragsz, PAGE_SIZE);
+
+		return NULL;
+	}
+
 	if (unlikely(!nc->va)) {
 refill:
 		page = __page_frag_cache_refill(nc, gfp_mask);
-- 
1.7.9.5