net: fix races in page->_count manipulation

This is illegal to use atomic_set(&page->_count, ...) even if we 'own'
the page. Other entities in the kernel need to use get_page_unless_zero()
to get a reference to the page before testing page properties, so we could
loose a refcount increment.

The only case it is valid is when page->_count is 0

Fixes: 540eb7bf ("net: Update alloc frag to reduce get/put page usage and recycle pages")
Signed-off-by: NEric Dumaze <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -360,18 +360,29 @@ static void *__netdev_alloc_frag(unsigned int fragsz, gfp_t gfp_mask)
 				goto end;
 		}
 		nc->frag.size = PAGE_SIZE << order;
-recycle:
-		atomic_set(&nc->frag.page->_count, NETDEV_PAGECNT_MAX_BIAS);
+		/* Even if we own the page, we do not use atomic_set().
+		 * This would break get_page_unless_zero() users.
+		 */
+		atomic_add(NETDEV_PAGECNT_MAX_BIAS - 1,
+			   &nc->frag.page->_count);
 		nc->pagecnt_bias = NETDEV_PAGECNT_MAX_BIAS;
 		nc->frag.offset = 0;
 	}
 
 	if (nc->frag.offset + fragsz > nc->frag.size) {
-		/* avoid unnecessary locked operations if possible */
-		if ((atomic_read(&nc->frag.page->_count) == nc->pagecnt_bias) ||
-		    atomic_sub_and_test(nc->pagecnt_bias, &nc->frag.page->_count))
-			goto recycle;
-		goto refill;
+		if (atomic_read(&nc->frag.page->_count) != nc->pagecnt_bias) {
+			if (!atomic_sub_and_test(nc->pagecnt_bias,
+						 &nc->frag.page->_count))
+				goto refill;
+			/* OK, page count is 0, we can safely set it */
+			atomic_set(&nc->frag.page->_count,
+				   NETDEV_PAGECNT_MAX_BIAS);
+		} else {
+			atomic_add(NETDEV_PAGECNT_MAX_BIAS - nc->pagecnt_bias,
+				   &nc->frag.page->_count);
+		}
+		nc->pagecnt_bias = NETDEV_PAGECNT_MAX_BIAS;
+		nc->frag.offset = 0;
 	}
 
 	data = page_address(nc->frag.page) + nc->frag.offset;