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: 540eb7bf0bbed ("net: Update alloc frag to reduce get/put page usage and recycle pages")
Signed-off-by: Eric Dumaze <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 18 insertions, 7 deletions

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index a30d750647e7..829d013745ab 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -360,18 +360,29 @@ refill:
                                 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;